Consolidated Statements of Changes in Equity (USD $) In Millions, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]
Balance at Dec. 31, 2008	$ 10,477	$ 0	$ 5,455	$ 5,631	$ (879)	$ 270
Balance (shares) at Dec. 31, 2008		75
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,188			1,157		31
Other comprehensive income (loss)	1,214				1,214	0
Stock-based compensation	123		123
Exercise of common stock options (shares)		1
Exercise of common stock options	25		25
Common stock purchases (shares)		(1)
Common stock purchases	(148)		(148)	0
Contributions	28					28
Distributions	(73)					(73)
Other equity transactions	9	0	(2)	0	0	11
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	1,310			1,238		72
Other comprehensive income (loss)	(509)				(509)	0
Common stock purchases (shares)		0
Common stock purchases	(56)		(9)	(47)
Purchase of noncontrolling interest	(57)		(13)			(44)
Distributions	(34)					(34)
Other equity transactions	(5)	0	(4)	0	0	(1)
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	1,352			1,331		21
Other comprehensive income (loss)	(467)				(467)	0
Distributions	(25)					(25)
Other equity transactions	(3)	0	(4)	0	0	1
Balance at Dec. 31, 2011	$ 14,265	$ 0	$ 5,423	$ 9,310	$ (641)	$ 173
Balance (shares) at Dec. 31, 2011	75	75