Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - 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
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 201,190	$ 140,279	$ 136,664	$ 239,356	$ 180,403	$ 126,936	$ 125,084	$ 185,160	$ 717,489	$ 617,583	$ 498,860
Operating Income	28,930	12,036	13,248	40,406	23,628	14,632	14,061	35,099	94,620	87,420	85,983
Net Income	$ 17,801	$ 5,538	$ 6,387	$ 26,855	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 56,580	$ 58,124	$ 44,675
Earnings per share:
Basic (in usd per share)	$ 1.09	$ 0.34	$ 0.39	$ 1.64	$ 1.60	$ 0.42	$ 0.37	$ 1.17	$ 3.46	$ 3.56	$ 2.87
Diluted (in usd per share)	$ 1.08	$ 0.34	$ 0.39	$ 1.64	$ 1.59	$ 0.42	$ 0.37	$ 1.17	$ 3.45	$ 3.55	$ 2.86