Stockholders' Equity (Summary of Follow-on Public Offerings of Common Stock) (Details) (USD $) In Billions, except Share data in Millions, unless otherwise specified	1 Months Ended			3 Months Ended
	Jul. 31, 2012	Mar. 31, 2012	Sep. 30, 2012	Sep. 30, 2012 Common Stock [Member]	Jun. 30, 2012 Common Stock [Member]	Mar. 31, 2012 Common Stock [Member]
Preferred Stock, Shares Authorized			3.1
Public offering price per share		$ 29.00	$ 33.70
Public offering of common stock	36.8	71.2		36.8	4.8	75.9
Public offering net proceeds	$ 1.2	$ 2.1