Equity (Preferred Stock) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011
Preferred Stock [Member]
Preferred Stock
Declaration Date	Nov. 15, 2013	Aug. 15, 2013	May 15, 2013	Mar. 05, 2013	Nov. 15, 2012	Aug. 15, 2012	May 15, 2012	Mar. 05, 2012	Nov. 15, 2011	Aug. 15, 2011	May 16, 2011	Mar. 07, 2011
Record Date	Nov. 30, 2013	Aug. 31, 2013	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2011	Feb. 28, 2011
Series A Preferred Stock [Member]
Preferred Stock
Preferred stock, dividend rate	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.253	$ 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