Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Preferred Stock [Member]
Preferred Stock
Declaration Date	Nov. 15, 2012	Aug. 15, 2012	May 15, 2012	Mar. 05, 2012	Nov. 15, 2011	Aug. 15, 2011	May 16, 2011	Mar. 07, 2011	Nov. 15, 2010	Aug. 16, 2010	May 17, 2010	Mar. 05, 2010
Record Date	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2011	Feb. 28, 2011	Nov. 30, 2010	Aug. 31, 2010	May 31, 2010	Feb. 28, 2010
Series A Preferred Stock [Member]
Preferred Stock
Preferred stock, dividend rate	$ 0.253	$ 0.256	$ 0.256	$ 0.253	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.250
Preferred stock, dividends	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 26	$ 26	$ 26
Series B Preferred Stock [Member]
Preferred Stock
Preferred stock, dividend rate	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406
Preferred stock, dividends	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 96	$ 96	$ 96