Stockholders' Equity (Summary of Follow-on Public Offerings of Common Stock) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended									12 Months Ended
	Jul. 31, 2012	Mar. 31, 2012	Nov. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jan. 31, 2011	Dec. 31, 2010	Oct. 31, 2010	May 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Preferred Stock, Shares Authorized										3.1
Public offering price per share	$ 33.70	$ 29.00	$ 27.36	$ 27.56	$ 27.72	$ 28.00	$ 27.44	$ 26.00	$ 25.75			$ 27.44
Public offering of common stock	36.8	71.2	40.5	49.7	32.2	26.9	8.3	13.2	6.9	108.0	149.3	28.4
Proceeds from Issuance of Common Stock	$ 1,240	$ 2,063	$ 1,108	$ 1,369	$ 892	$ 719	$ 227	$ 328	$ 169	$ 3,601	$ 4,377	$ 1,055
At the Market Offering Program [Member]
Common Stock, Capital Shares Reserved for Future Issuance										16.7
Public offering of common stock										9.5	9.4	4.4
Common Stock [Member]
Public offering of common stock										117.4	159.3	40.6
Proceeds from Issuance of Common Stock										$ 3,303	$ 4,088	$ 724