Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Details) (USD $) In Thousands, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 19,654	$ 39,761	$ 62,522	$ 85,405
Weighted average number of shares outstanding, basic (net of Treasury)	83,534	84,275	83,795	85,924
Less: Unvested restricted stock	(129)	(329)	(155)	(433)
Denominator for basic calculation	83,405	83,946	83,640	85,491
Denominator for diluted calculation	84,791	85,496	84,764	87,503
Net income per share-basic	$ 0.24	$ 0.47	$ 0.75	$ 1.00
Net income per share-diluted	$ 0.23	$ 0.47	$ 0.74	$ 0.98
Employee Stock Options and Stock Appreciation Rights [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Incremental common shares attributable to share-based payment arrangements	1	319	12	776
Restricted Stock Awards and Units [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Incremental common shares attributable to share-based payment arrangements	1,385	1,231	1,112	1,236