Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares	Mar. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($) $ / shares	Sep. 30, 2014 USD ($) $ / shares	Jun. 30, 2014 USD ($) $ / shares	Mar. 31, 2014 USD ($) $ / shares	Dec. 31, 2015 USD ($) item $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares
Numerator
Net income | $	$ 80,899	$ 77,042	$ 76,118	$ 19,332	$ 70,131	$ 62,433	$ 62,037	$ 61,397	$ 253,391	$ 255,998	$ 190,411
Denominator
Weighted-average common shares outstanding									102,547,774	102,142,828	101,529,137
Basic net income per share (in dollars per share) | $ / shares	$ 0.79	$ 0.75	$ 0.74	$ 0.19	$ 0.68	$ 0.61	$ 0.61	$ 0.60	$ 2.47	$ 2.51	$ 1.88
Numerator
Net income | $	$ 80,899	$ 77,042	$ 76,118	$ 19,332	$ 70,131	$ 62,433	$ 62,037	$ 61,397	$ 253,391	$ 255,998	$ 190,411
Assumed conversion of convertible senior notes | $									5,806	5,811	5,783
Net income plus assumed conversions | $									$ 259,197	$ 261,809	$ 196,194
Denominator
Number of shares used in basic computation									102,547,774	102,142,828	101,529,137
Weighted-average effect of dilutive securities (in shares)									8,081,091	8,049,943	7,434,413
Number of shares used in per share computation									110,628,865	110,192,771	108,963,550
Diluted net income per share (in dollars per share) | $ / shares	$ 0.74	$ 0.71	$ 0.70	$ 0.19	$ 0.65	$ 0.58	$ 0.58	$ 0.57	$ 2.34	$ 2.38	$ 1.80
Number of classes of common stock | item									2
Employee and Directors Stock Options
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)										150,000
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)									993,092	969,225	1,569,005