Stockholders' Equity - Changes in Stockholders' Equity (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Changes in Stockholders' Equity
Beginning Balance	$ 19,004,000,000		$ 18,285,000,000		$ 18,285,000,000
Net Income After Dividends on Preferred and Preference Stock	81,000,000	[1],[2]	368,000,000	[1]
Other comprehensive income (loss)	4,000,000		6,000,000
Stock issued	99,000,000		157,000,000
Stock Repurchased During Period, Shares	0				9,000,000
Stock Repurchased During Period, Value	(18,000,000)
Cash dividends on common stock	(426,000,000)		(410,000,000)
Other	0		0
Ending Balance	18,744,000,000		18,406,000,000		19,004,000,000
Stock Issued During Period, Value, Employee Stock Purchase Plan	430,000,000
Number of Common shares Issued [Member]
Changes in Stockholders' Equity
Beginning Balance, Shares	877,803,000		865,664,000		865,664,000
Stock issued, shares	2,638,000		3,571,000
Stock Repurchased During Period, Shares	0
Ending Balance, Shares	880,441,000		869,235,000
Number of Common shares Treasury [Member]
Changes in Stockholders' Equity
Beginning Balance, Shares	10,035,000		539,000		539,000
Stock Repurchased During Period, Shares	0
Other, Shares	(15,000)		(6,000)
Ending Balance, Shares	10,050,000		545,000
Common Stockholders' Equity [Member]
Changes in Stockholders' Equity
Beginning Balance	18,297,000,000		17,578,000,000		17,578,000,000
Net Income After Dividends on Preferred and Preference Stock	81,000,000		368,000,000
Other comprehensive income (loss)	4,000,000		6,000,000
Stock issued	99,000,000		157,000,000
Stock Repurchased During Period, Value	(18,000,000)
Cash dividends on common stock	(426,000,000)		(410,000,000)
Other	0		0
Ending Balance	18,037,000,000		17,699,000,000
Preferred and Preference Stock of Subsidiaries [Member]
Changes in Stockholders' Equity
Beginning Balance	707,000,000		707,000,000		707,000,000
Net Income After Dividends on Preferred and Preference Stock	0		0
Other comprehensive income (loss)	0		0
Stock issued	0		0
Stock Repurchased During Period, Value	0
Cash dividends on common stock	0		0
Other	0		0
Ending Balance	$ 707,000,000		$ 707,000,000

[1] After dividends on preferred and preference stock of subsidiaries

[2] Segment net income (loss) for the three months ended March 31, 2013 includes a $540.0 million pre-tax charge ($333.5 million after tax) for an estimated probable loss on the Kemper IGCC. See Note (B) under "Integrated Coal Gasification Combined Cycle" herein for additional information.