Stockholders' Equity Note (Details) (USD $)	1 Months Ended				12 Months Ended
	Aug. 31, 2011	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stockholders' Equity Note [Abstract]
Number of votes per share of common stock					1
Number of shares of common stock available for issuance					8,200,000
Amount of common stock repurchases authorized	$ 400,000,000			$ 100,000,000
Increase in stock repurchase authorized amount	250,000,000	150,000,000	150,000,000
Value of shares repurchased during period					324,733,000	91,960,000	66,301,000
Remaining amount available for future purchases of shares under approved repurchase program.					$ 129,700,000
Common Stock [Member]
Statement [Line Items]
Treasury Stock, Shares, Acquired					12,276,878	2,452,595	3,327,146