Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic Net Income per Share
Net income	$ 22,344	$ 28,110	$ 45,621	$ 52,303
Less: Income allocated to participating securities	(37)	(128)	(72)	(227)
Net income allocated to common stockholders	22,307	27,982	45,549	52,076
Weighted average common shares outstanding - basic	47,884	48,902	47,873	49,066
Net income per share - Basic	$ 0.47	$ 0.57	$ 0.95	$ 1.06
Diluted Net Income per Share
Net income	22,344	28,110	45,621	52,303
Less: Income allocated to participating securities	(36)	(126)	(70)	(225)
Net income allocated to common stockholders	$ 22,308	$ 27,984	$ 45,551	$ 52,078
Weighted average common shares outstanding - basic	47,884	48,902	47,873	49,066
Dilutive securities	1,454	464	1,323	483
Weighted average common shares outstanding - diluted	49,338	49,366	49,196	49,549
Net income per share - diluted	$ 0.45	$ 0.57	$ 0.93	$ 1.05