Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) item $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares
Net Earnings Per Share
Number of classes of common stock | item			2
Numerator
Net income | $	$ 76,118	$ 62,037	$ 95,450	$ 123,434
Denominator
Weighted-average common shares outstanding	102,571,600	101,934,815	102,513,642	101,896,210
Basic net income per share (in dollars per share) | $ / shares	$ 0.74	$ 0.61	$ 0.93	$ 1.21
Numerator
Net income | $	$ 76,118	$ 62,037	$ 95,450	$ 123,434
Assumed conversion of convertible senior notes | $	1,446	1,447	2,877	2,880
Net income plus assumed conversions | $	$ 77,564	$ 63,484	$ 98,327	$ 126,314
Denominator
Weighted-average common shares outstanding	102,571,600	101,934,815	102,513,642	101,896,210
Weighted-average effect of dilutive securities (in shares)	8,166,244	8,121,810	8,131,672	8,070,989
Number of shares used in per share computation	110,737,844	110,056,625	110,645,314	109,967,199
Diluted net income per share (in dollars per share) | $ / shares	$ 0.70	$ 0.58	$ 0.89	$ 1.15
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)			982,769	953,865