Net Income per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income	$ 47,232	$ 66,262	$ 106,770	$ 114,025
Denominator:
Weighted average number of common shares outstanding-basic	26,990,000	27,521,000	26,976,000	27,346,000
Common stock equivalents-stock options (in shares)	655,000	426,000	539,000	395,000
Common stock equivalents-restricted stock (in shares)	4,000	7,000	11,000	10,000
Common stock equivalents-restricted stock units (in shares)	54,000	87,000	35,000	83,000
Common stock equivalents-employee stock purchase plan (in shares)	1,000	1,000	2,000	1,000
Weighted average number of common shares outstanding-diluted	27,704,000	28,042,000	27,563,000	27,835,000
Net income per common share-basic (in dollars per share)	$ 1.75	$ 2.41	$ 3.96	$ 4.17
Net income per common share-diluted (in dollars per share)	$ 1.70	$ 2.36	$ 3.87	$ 4.10
Potential dilutive securities excluded from computation of dilutive securities (in shares)	900,000	2,300,000	900,000	2,200,000