Quarterly Financial Information (Unaudited) - Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenues	$ 360,565	$ 292,249	$ 333,934	$ 288,370	$ 345,175	$ 264,724	$ 298,778	$ 276,042	$ 1,275,118	$ 1,184,719	$ 1,149,207
Depreciation and amortization	31,221	30,994	29,995	28,666	28,097	28,546	27,679	27,637	120,876	111,959	109,816
Operating income	51,526	40,100	76,699	45,944	36,788	37,510	64,644	46,975	214,269	185,917	215,442
Income before income taxes	161,191	24,454	61,222	29,548	21,141	24,400	48,191	33,213	276,415	126,945	162,766
(Provision) benefit for income taxes	(1,997)	(1,863)	(5,676)	(2,209)	51,177	(530)	(899)	(593)	(11,745)	49,155	(3,400)
Net income	$ 159,194	$ 22,591	$ 55,546	$ 27,339	$ 72,318	$ 23,870	$ 47,292	$ 32,620	$ 264,670	$ 176,100	$ 159,366
Net income, per share (in dollars per share)	$ 3.10	$ 0.44	$ 1.08	$ 0.53	$ 1.41	$ 0.47	$ 0.92	$ 0.64	$ 5.16	$ 3.44	$ 3.12
Net income, per share - assuming dilution (in dollars per share)	$ 3.09	$ 0.44	$ 1.08	$ 0.53	$ 1.41	$ 0.46	$ 0.92	$ 0.63	$ 5.14	$ 3.43	$ 3.11