Income Taxes - Components of the Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current provision for income taxes:
Federal									$ 36,862	$ 67,253	$ 43,706
State									3,645	10,529	5,500
Foreign									2,395	2,016	1,588
Total current									42,902	79,798	50,794
Deferred taxes:
Federal									12,795	(18,579)	(23,896)
State									(3,404)	(3,564)	(2,300)
Foreign									(734)	381	309
Deferred Income Taxes and Tax Credits									8,657	(21,762)	(25,887)
Total provision for income taxes	$ 37,802	$ 8,545	$ 14,445	$ (9,233)	$ 18,354	$ 11,668	$ 13,938	$ 14,076	$ 51,559	$ 58,036	$ 24,907