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	442				442
Other comprehensive income	6					6
Contributions	100			100
Preferred stock dividends declared	(2)				(2)
Balance at Sep. 30, 2010	7,194	41	0	4,479	2,674	0	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	425				425
Contributions	0
Preferred stock dividends declared	(2)				(2)
Common stock dividends declared	(550)				(550)
Balance at Sep. 30, 2011	$ 7,184	$ 41	$ 0	$ 4,479	$ 2,671	$ (7)	$ 0