Stockholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Common Stock [Member]	Mar. 31, 2011 Common Stock [Member]	Jan. 31, 2012 Maximum [Member]
Stockholders' Equity [Line Items]
Repurchase of common stock, authorized value					$ 500
Repurchase of common stock, shares			12.1	0.7
Total purchase price of common stock	$ 435
Shares repurchased under stock repurchase program, average price per share	$ 36.00	$ 33.49