Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - 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
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 141,874	$ 108,348	$ 102,342	$ 146,296	$ 104,567	$ 91,913	$ 92,682	$ 170,081	$ 498,860	$ 459,244	$ 498,834
Operating Income	21,819	10,156	15,742	36,380	16,171	10,909	13,170	37,508	84,096	77,758	62,279
Net Income	$ 11,863	$ 4,416	$ 8,029	$ 20,367	$ 8,619	$ 5,119	$ 6,294	$ 21,109	$ 44,675	$ 41,140	$ 36,092
Earnings per share:
Basic (in usd per share)	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 0.56	$ 0.34	$ 0.41	$ 1.45	$ 2.87	$ 2.73	$ 2.48
Diluted (in usd per share)	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 0.56	$ 0.33	$ 0.41	$ 1.44	$ 2.86	$ 2.72	$ 2.47