Consolidated Statements of Changes in Equity (USD $) In Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]	Noncontrolling Interests [Member]
Balance at Dec. 31, 2009	$ 12,843	$ 0	$ 5,453	$ 6,788	$ 335	$ 267
Balance (shares) at Dec. 31, 2009		75
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Deconsolidation of Bridger Coal	(84)					(84)
Net income	587			495		92
Other comprehensive loss	(348)				(348)	0
Common stock purchases (shares)		0
Common stock purchases	(56)	0	(9)	(47)
Distributions	(13)					(13)
Other equity transactions	(38)	0	0	0	0	(38)
Balance at Jun. 30, 2010	12,891	0	5,444	7,236	(13)	224
Balance (shares) at Jun. 30, 2010		75
Balance at Dec. 31, 2010	13,408	0	5,427	7,979	(174)	176
Balance (shares) at Dec. 31, 2010	75	75
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	575			567		8
Other comprehensive loss	(181)				(181)	0
Distributions	(13)					(13)
Other equity transactions	(2)	0	(4)	0	0	2
Balance at Jun. 30, 2011	$ 13,787	$ 0	$ 5,423	$ 8,546	$ (355)	$ 173
Balance (shares) at Jun. 30, 2011	75	75