Stockholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended		3 Months Ended			9 Months Ended			9 Months Ended		1 Months Ended
	Jul. 31, 2012	Mar. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 At-the-Market Offering Program [Member]	Sep. 30, 2012 Dividend Reinvestment And Direct Stock Purchase Plan [Member]	Apr. 30, 2012 Issuance of Equity [Member]
Preferred stock, shares issued			6.9			6.9		0			6.9
Public offering, average price per share									$ 31.41
Issuance of common stock, value			$ 1,240	$ 155	$ 2,205				$ 298
Public offering of common stock	36.8	71.2							9.5
Shares available for issuance									16.7	4.7
Preferred Stock, Par or Stated Value Per Share			$ 0.01			$ 0.01		$ 0.01			$ 24.21
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 167		$ 167					$ 167
Preferred Stock, Dividend Rate, Percentage											8.00%
Preferred Stock, Liquidation Preference Per Share											$ 25.00