Consolidated Statements of Changes in Equity Statement (USD $) In Millions	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net[Member]	Noncontrolling Interest [Member]
Balance at Dec. 31, 2009	$ 6,732	$ 41	$ 0	$ 4,379	$ 2,234	$ (6)	$ 84
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Deconsolidation of Bridger Coal Company	(84)						(84)
Net income	286				286
Other comprehensive income	5					5
Contributions	100			100
Preferred stock dividends declared	(1)				(1)
Balance at Jun. 30, 2010	7,038	41	0	4,479	2,519	(1)	0
Balance at Dec. 31, 2010	7,311	41	0	4,479	2,798	(7)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	256				256
Contributions	0
Preferred stock dividends declared	(1)				(1)
Common stock dividends declared	(550)				(550)
Balance at Jun. 30, 2011	$ 7,016	$ 41	$ 0	$ 4,479	$ 2,503	$ (7)	$ 0