Selected Quarterly Data (Unaudited) - Summary of Selected Quarterly 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	$ 128,024	$ 123,675	$ 94,132	$ 79,776	$ 79,850	$ 82,324	$ 80,085	$ 80,295	$ 425,607	$ 322,554	$ 381,977
Operating Income (Loss)	28,015	16,789	3,184	10,047	12,783	16,538	8,743	12,750	58,035	50,814	80,378
Net Income (Loss) Attributable to Common Stockholders	$ 3,414	$ 16,708	$ (2,389)	$ 10,943	$ 12,426	$ 15,625	$ 8,088	$ 12,363	$ 28,676	$ 48,502	$ 35,106
Earnings (loss) per share—Basic (in $ per share)	$ 0.48	$ 2.32	$ (0.34)	$ 1.67	$ 1.94	$ 2.04	$ 0.99	$ 1.48	$ 4.09	$ 6.34	$ 3.99
Earnings (loss) per share—Diluted (in $ per share)	$ 0.46	$ 2.21	$ (0.34)	$ 1.62	$ 1.87	$ 1.99	$ 0.97	$ 1.45	$ 3.96	$ 6.20	$ 3.92