Stockholders' Equity (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Purchases of common stock, value	$ 303,414,000	$ 159,639,000	$ 2,900,000
Non-Employee Director [Member]
Shares issued to non-employee directors	29,213	34,428	59,500
Stock-based compensation expense	$ 1,300,000	$ 1,300,000	$ 1,000,000
Common Stock
Purchases of common stock, shares	7,377,803	4,133,858	115,875