Net Income per Common Share (Details) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator:
Net income	$ 31,355	$ 44,784	$ 99,884	$ 105,731
Denominator:
Weighted average number of common shares outstanding-basic (in shares)	29,900,000	33,450,000	31,406,000	33,715,000
Common stock equivalents-stock options (in shares)	446,000	475,000	487,000	389,000
Common stock equivalents-warrants (in shares)		157,000		150,000
Common stock equivalents-restricted stock (in shares)	4,000	6,000	10,000	13,000
Common stock equivalents-restricted stock units (in shares)	88,000	83,000	85,000	78,000
Weighted average number of common shares outstanding-diluted (in shares)	30,438,000	34,171,000	31,988,000	34,345,000
Net income per common share-basic (in dollars per share)	$ 1.05	$ 1.34	$ 3.18	$ 3.14
Net income per common share-diluted (in dollars per share)	$ 1.03	$ 1.31	$ 3.12	$ 3.08
Potential dilutive securities excluded from computation of dilutive securities (in shares)	1,100,000	1,200,000	900,000	1,900,000