Quarterly Financial Information (Unaudited) - Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 345,175	$ 264,724	$ 298,778	$ 276,042	$ 319,775	$ 271,720	$ 296,215	$ 261,497	$ 1,184,719	$ 1,149,207	$ 1,092,124
Depreciation and amortization	28,097	28,546	27,679	27,637	27,928	26,706	26,409	28,773	111,959	109,816	114,383
Operating income	36,490	36,409	64,693	47,060	61,499	46,567	66,945	38,794	184,652	213,805	162,062
Income before income taxes	21,141	24,400	48,191	33,213	49,144	35,415	52,746	25,461	126,945	162,766	99,656
(Provision) benefit for income taxes	51,177	(530)	(899)	(593)	(1,048)	(1,822)	(1,415)	885	49,155	(3,400)	11,855
Net income	$ 72,318	$ 23,870	$ 47,292	$ 32,620	$ 48,096	$ 33,593	$ 51,331	$ 26,346	$ 176,100	$ 159,366	$ 111,511
Net income per share	$ 1.41	$ 0.47	$ 0.92	$ 0.64	$ 0.94	$ 0.66	$ 1.01	$ 0.52	$ 3.44	$ 3.12	$ 2.18
Net income per share - assuming dilution	$ 1.41	$ 0.46	$ 0.92	$ 0.63	$ 0.94	$ 0.66	$ 1.00	$ 0.51	$ 3.43	$ 3.11	$ 2.16