Subsequent Event (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended											3 Months Ended								12 Months Ended
	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 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	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Subsequent Event [Line Items]
Stock Repurchase Program, Authorized Amount	$ 500
Issuance of common stock, shares		36.8		71.2	40.5	49.7	32.2	26.9	8.3	13.2	6.9									108.0	149.3	28.4
Preferred stock, shares issued												6.9				0				6.9	0
Dividends declared per common share												$ 1.25	$ 1.25	$ 1.25	$ 1.25	$ 1.40	$ 1.40	$ 1.40	$ 1.40	$ 5.00	$ 5.60	$ 5.60
Preferred Stock, Par or Stated Value Per Share												$ 0.01				$ 0.01				$ 0.01	$ 0.01
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 167																	$ 167	$ 0	$ 0
Preferred Stock, Dividend Rate, Percentage			8.00%
Preferred Stock, Liquidation Preference Per Share			$ 25.00
Sale of Stock, Price Per Share		$ 33.70		$ 29.00	$ 27.36	$ 27.56	$ 27.72	$ 28.00	$ 27.44	$ 26.00	$ 25.75				$ 29.00			$ 27.56	$ 27.72			$ 27.44
Issuance of Equity [Member]
Subsequent Event [Line Items]
Preferred Stock, Par or Stated Value Per Share			$ 24.21