Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares	Mar. 31, 2015 USD ($) $ / shares	Dec. 31, 2016 USD ($) item $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Numerator
Net income | $	$ 96,988	$ 93,276	$ 91,803	$ 92,858	$ 80,899	$ 77,042	$ 76,118	$ 19,332	$ 374,925	$ 253,391	$ 255,998
Denominator
Weighted-average common shares outstanding									102,801,161	102,547,774	102,142,828
Basic net income per share (in dollars per share) | $ / shares	$ 0.94	$ 0.91	$ 0.89	$ 0.90	$ 0.79	$ 0.75	$ 0.74	$ 0.19	$ 3.65	$ 2.47	$ 2.51
Numerator
Net income | $	$ 96,988	$ 93,276	$ 91,803	$ 92,858	$ 80,899	$ 77,042	$ 76,118	$ 19,332	$ 374,925	$ 253,391	$ 255,998
Assumed conversion of convertible senior notes | $									5,780	5,806	5,811
Net income plus assumed conversions | $									$ 380,705	$ 259,197	$ 261,809
Denominator
Number of shares used in basic computation									102,801,161	102,547,774	102,142,828
Weighted-average effect of dilutive securities (in shares)									7,997,566	8,081,091	8,049,943
Number of shares used in per share computation									110,798,727	110,628,865	110,192,771
Diluted net income per share (in dollars per share) | $ / shares	$ 0.89	$ 0.86	$ 0.84	$ 0.85	$ 0.74	$ 0.71	$ 0.70	$ 0.19	$ 3.44	$ 2.34	$ 2.38
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)									1,005,697	993,092	969,225