Net Income per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator:
Net income	$ 26,973	$ 34,231	$ 47,763	$ 68,529
Denominator:
Weighted average number of common shares outstanding-basic	27,317,000	31,301,000	27,258,000	32,171,000
Common stock equivalents-stock options (in shares)	328,000	523,000	380,000	508,000
Common stock equivalents-restricted stock (in shares)	10,000	10,000	12,000	13,000
Common stock equivalents-restricted stock units (in shares)	61,000	69,000	81,000	83,000
Common stock equivalents-employee stock purchase plan (in shares)	1,000		1,000
Weighted average number of common shares outstanding-diluted	27,717,000	31,903,000	27,732,000	32,775,000
Net income per common share-basic (in dollars per share)	$ 0.99	$ 1.09	$ 1.75	$ 2.13
Net income per common share-diluted	$ 0.97	$ 1.07	$ 1.72	$ 2.09
Potential dilutive securities excluded from computation of dilutive securities (in shares)	2,700,000	1,100,000	2,100,000	800,000