Selected Quarterly Financial Data - (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 208,195	$ 267,577	$ 241,751	$ 207,118	$ 210,951	$ 241,526	$ 232,156	$ 175,245	$ 924,641	$ 859,878	$ 757,970
Operating income	52,462	78,618	64,942	42,873	47,195	73,803	62,917	41,404	238,895	225,319	214,568
Income before income taxes	43,792	70,200	55,610	30,378	37,804	62,268	52,879	31,034	199,980	183,985	173,758
Net income	$ 31,821	$ 47,565	$ 38,822	$ 21,163	$ 29,203	$ 41,419	$ 35,813	$ 21,594	$ 139,371	$ 128,029	$ 123,160
Earnings per share:
Basic earnings per share, Continuing operations (in dollars per share)	$ 0.57	$ 0.85	$ 0.69	$ 0.38	$ 0.52	$ 0.72	$ 0.62	$ 0.38	$ 2.48	$ 2.24	$ 2.08
Diluted earnings per share, Continuing operations (in dollars per share)	$ 0.56	$ 0.84	$ 0.68	$ 0.37	$ 0.51	$ 0.72	$ 0.62	$ 0.37	$ 2.46	$ 2.22	$ 2.07
Increase (Decrease) in income taxes				$ 1,600					$ (5,775)	$ (808)	$ (139)