Consolidated Statements of Changes in Equity (USD $) In Millions, unless otherwise specified	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive 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	566				566
Other comprehensive loss	(1)					(1)
Contributions	100			100
Preferred stock dividends declared	(2)				(2)
Balance at Dec. 31, 2010	7,311	41	0	4,479	2,798	(7)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	555				555
Other comprehensive loss	(2)					(2)
Contributions	0
Preferred stock dividends declared	(2)				(2)
Common stock dividends declared	(550)				(550)
Balance at Dec. 31, 2011	7,312	41	0	4,479	2,801	(9)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	537				537
Other comprehensive loss	(3)					(3)
Contributions	0
Preferred stock dividends declared	(2)				(2)
Common stock dividends declared	(200)				(200)
Balance at Dec. 31, 2012	$ 7,644	$ 41	$ 0	$ 4,479	$ 3,136	$ (12)	$ 0