Net Income Per Share (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of weighted-average shares used in computing net income per share
Shares used in computing basic net income per share	56,490,000	56,642,000	56,537,000	56,388,000	56,908,000	57,752,000	58,303,000	58,270,000	56,508,000	57,803,000	59,623,000
Dilutive effect of stock options and restricted stock units									1,030,000	703,000	665,000
Shares used in computing diluted net income per share	57,710,000	57,708,000	57,448,000	57,208,000	57,546,000	58,440,000	59,007,000	59,030,000	57,538,000	58,506,000	60,288,000
Stock options excluded from the weighted-average number of common shares outstanding for the diluted net income per share computations as they were anti-dilutive									2,700,000	5,400,000	5,200,000