Quarterly Financial Data - Schedule of Quarterly Financial Information (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
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 180,403	$ 126,936	$ 125,084	$ 185,160	$ 141,874	$ 108,348	$ 102,342	$ 146,296	$ 617,583	$ 498,860	$ 459,244
Operating Income	23,263	14,239	13,666	34,676	21,819	10,156	15,742	36,380	85,843	84,096	77,758
Net Income	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 11,863	$ 4,416	$ 8,029	$ 20,367	$ 58,124	$ 44,675	$ 41,140
Earnings per share:
Basic (in usd per share)	$ 1.60	$ 0.42	$ 0.37	$ 1.17	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 3.56	$ 2.87	$ 2.73
Diluted (in usd per share)	$ 1.59	$ 0.42	$ 0.37	$ 1.17	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 3.55	$ 2.86	$ 2.72