Stockholders' Equity (Schedule of Treasury Stock) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		50 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012
Stockholders' Equity Note [Abstract]
Shares repurchased	444,901	69,400	12,527,709
Average purchase price per share	$ 46.34	$ 43.42	$ 38.07
Amount spent on repurchased shares	$ 20	$ 3	$ 476