Net Income per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income	$ 28,058	$ 20,790
Denominator:
Weighted average number of common shares outstanding-basic	27,110,000	27,199,000
Common stock equivalents-stock options (in shares)	467,000	432,000
Common stock equivalents-restricted stock (in shares)	17,000	13,000
Common stock equivalents-restricted stock units (in shares)	52,000	102,000
Common stock equivalents-employee stock purchase plan (in shares)	2,000	1,000
Weighted average number of common shares outstanding-diluted	27,648,000	27,747,000
Net income per common share-basic (in dollars per share)	$ 1.03	$ 0.76
Net income per common share-diluted (in dollars per share)	$ 1.01	$ 0.75
Potential dilutive securities excluded from computation of dilutive securities (in shares)	1,700,000	1,600,000