Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) item $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares
Numerator
Net income | $	$ 100,925	$ 91,803	$ 185,862	$ 184,661
Denominator
Weighted-average common shares outstanding	103,180,769	102,837,443	103,064,834	102,758,427
Basic net income per share (in dollars per share) | $ / shares	$ 0.98	$ 0.89	$ 1.80	$ 1.80
Numerator
Net income | $	$ 100,925	$ 91,803	$ 185,862	$ 184,661
Assumed conversion of convertible senior notes | $	1,431	1,455	2,847	2,909
Net income plus assumed conversions | $	$ 102,356	$ 93,258	$ 188,709	$ 187,570
Denominator
Number of shares used in basic computation	103,180,769	102,837,443	103,064,834	102,758,427
Weighted-average effect of dilutive securities (in shares)	8,383,714	8,001,737	8,425,849	7,951,747
Number of shares used in per share computation	111,564,483	110,839,180	111,490,683	110,710,174
Diluted net income per share (in dollars per share) | $ / shares	$ 0.92	$ 0.84	$ 1.69	$ 1.69
Number of classes of common stock | item			2
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)			1,086,653	1,006,647