SCHEDULE 13G

Amendment No. 0
Alpha Natural Resources Inc.
Common Stock
Cusip #02076X102


Cusip #02076X102
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	2,037,203
Item 6:	0
Item 7:	7,119,000
Item 8:	0
Item 9:	7,119,000
Item 11:	10.958%
Item 12:	    HC


Cusip #02076X102
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	7,119,000
Item 8:	0
Item 9:	7,119,000
Item 11:	10.958%
Item 12:	IN




	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)




Item 1(a).	Name of Issuer:

		Alpha Natural Resources Inc.

Item 1(b).	Name of Issuer's Principal Executive Offices:

		One Alpha Place
		P.O. BOX 2345
		Abingdon, VA  24212


Item 2(a).	Name of Person Filing:

		FMR Corp.

Item 2(b).	Address or Principal Business Office or, if None,
Residence:

		82 Devonshire Street, Boston,
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock

Item 2(e).	CUSIP Number:

		02076X102

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b)
and the person filing, FMR Corp., is a parent holding company
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	7,119,000

	(b)	Percent of Class:	10.958%

	(c)	Number of shares as to which such
person has:

	(i)	sole power to vote or to direct
the vote:	2,037,203

	(ii)	shared power to vote or to
direct the vote:	0

	(iii)	sole power to dispose or to
direct the disposition of:	7,119,000

	(iv)	shared power to dispose or to
direct the disposition of:	0



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another
Person.

	Various persons have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the
sale of, the Common Stock of Alpha Natural Resources Inc..
No one person's interest in the Common Stock of Alpha
Natural Resources Inc. is more than five percent of the total
outstanding Common Stock.

Item 7.	Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.

	See attached Exhibit A.

Item 8.	Identification and Classification of Members of
the Group.

	Not applicable.  See attached Exhibit A.

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge
and belief, the securities referred to above were acquired in the
ordinary course of business and were not acquired for the
purpose of and do not have the effect of changing or
influencing the control of the issuer of such securities and
were not acquired in connection with or as a participant in any
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and
belief, I certify that the information set forth in this statement
is true, complete and correct.

June 12, 2006
Date

/s/Eric D. Roiter
Signature

Eric D, Roiter
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf of FMR Corp. and
its direct and indirect subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 13G,
Fidelity Management & Research Company ("Fidelity"), 82
Devonshire Street, Boston, Massachusetts 02109, a wholly-
owned subsidiary of FMR Corp. and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 5,119,997 shares or 7.881%
of the Common Stock outstanding of Alpha Natural Resources
Inc.  ("the Company") as a result of acting as investment
adviser to various investment companies registered under
Section 8 of the Investment Company Act of 1940.

	Edward C. Johnson 3d and FMR Corp., through its
control of Fidelity, and the funds each has sole power to
dispose of the 5,119,997 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d,
Chairman of FMR Corp., are the predominant owners, directly
or through trusts, of Series B shares of common stock of FMR
Corp., representing 49% of the voting power of FMR Corp.
The Johnson family group and all other Series B shareholders
have entered into a shareholders' voting agreement under
which all Series B shares will be voted in accordance with the
majority vote of Series B shares.  Accordingly, through their
ownership of voting common stock and the execution of the
shareholders' voting agreement, members of the Johnson
family may be deemed, under the Investment Company Act of
1940, to form a controlling group with respect to FMR Corp.

	Neither FMR Corp. nor Edward C. Johnson 3d,
Chairman of FMR Corp., has the sole power to vote or direct
the voting of the shares owned directly by the Fidelity Funds,
which power resides with the Funds' Boards of Trustees.
Fidelity carries out the voting of the shares under written
guidelines established by the Funds' Boards of Trustees.

	Fidelity Management Trust Company, 82 Devonshire
Street, Boston, Massachusetts 02109, a wholly-owned
subsidiary of FMR Corp. and a bank as defined in Section
3(a)(6) of the Securities Exchange Act of 1934, is the
beneficial owner of  785,600 shares or 1.209% of the
Common Stock outstanding of the Company as a result of its
serving as investment manager of the institutional account(s).

	Edward C. Johnson 3d and FMR Corp., through its
control of Fidelity Management Trust Company, each has sole
dispositive power over 785,600 shares and sole power to vote
or to direct the voting of 785,600 shares of Common Stock
owned by the institutional account(s) as reported above.

	Fidelity International Limited ("FIL"), Pembroke Hall,
42 Crow Lane, Hamilton, Bermuda, and various foreign-based
subsidiaries provide investment advisory and management
services to a number of non-U.S. investment companies and
certain institutional investors.  FIL, which is a qualified
institution under section 240.13d-1(b)(1) pursuant to an SEC
No-Action letter dated October 5, 2000, is the beneficial
owner of 1,213,403 shares or 1.868% of the Common Stock
outstanding of the Company.

	A partnership controlled predominantly by members of
the family of Edward C. Johnson 3d, Chairman of FMR Corp.
and FIL, or trusts for their benefit, owns shares of FIL voting
stock with the right to cast approximately 38% of the total
votes which may be cast by all holders of FIL voting stock.
FMR Corp. and FIL are separate and independent corporate
entities, and their Boards of Directors are generally composed
of different individuals.

	FMR Corp. and FIL are of the view that they are not
acting as a "group" for purposes of Section 13(d) under the
Securities Exchange Act of 1934 (the "1934" Act) and that
they are not otherwise required to attribute to each other the
"beneficial ownership" of securities "beneficially owned" by
the other corporation within the meaning of Rule 13d-3
promulgated under the 1934 Act.  Therefore, they are of the
view that the shares held by the other corporation need not be
aggregated for purposes of Section 13(d). However, FMR
Corp. is making this filing on a voluntary basis as if all of the
shares are beneficially owned by FMR Corp. and FIL on a
joint basis.



	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on June 12, 2006, agree and
consent to the joint filing on their behalf of this Schedule 13G
in connection with their beneficial ownership of the Common
Stock of Alpha Natural Resources Inc. at May 31, 2006.

	FMR Corp.

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney dated
December 30, 1997, by and on behalf of FMR Corp. and its
direct and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney dated
December 30, 1997, by and on behalf of Edward C. Johnson
3d

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Senior V.P. and General Counsel