Stockholders' Equity (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Numerator:
Net income	$ 29,739	$ 31,355	$ 34,231	$ 34,298	$ 32,928	$ 44,784	$ 35,428	$ 25,519	$ 129,623	$ 138,659	$ 106,671
Denominator:
Weighted average number of common shares outstanding-basic	27,724,000	29,900,000	31,301,000	33,051,000	33,971,000	33,450,000	33,323,000	34,382,000	30,478,000	33,779,000	35,248,000
Common stock equivalents-stock options (in shares)									480,000	448,000	46,000
Common stock equivalents-warrants (in shares)										116,000	52,000
Common stock equivalents-restricted stock (in shares)									9,000	12,000	29,000
Common stock equivalents-restricted stock units (in shares)									91,000	86,000	41,000
Weighted average number of common shares outstanding-diluted	28,300,000	30,438,000	31,903,000	33,656,000	34,730,000	34,171,000	33,800,000	35,074,000	31,058,000	34,441,000	35,416,000
Net income per common share-basic (in dollars per share)	$ 1.07	$ 1.05	$ 1.09	$ 1.04	$ 0.97	$ 1.34	$ 1.06	$ 0.74	$ 4.25	$ 4.10	$ 3.03
Net income per common share-diluted (in dollars per share)	$ 1.05	$ 1.03	$ 1.07	$ 1.02	$ 0.95	$ 1.31	$ 1.05	$ 0.73	$ 4.17	$ 4.03	$ 3.01
Potential dilutive securities excluded from computation of dilutive securities (in shares)									1,000,000	2,000,000	5,200,000