Selected Quarterly Data (Unaudited) - Summary of Selected Quarterly Data (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 79,850	$ 82,324	$ 80,085	$ 80,295	$ 86,115	$ 92,375	$ 99,656	$ 103,831	$ 322,554	$ 381,977	$ 450,598
Operating Income	12,783	16,538	8,743	12,750	16,506	23,112	16,208	24,542	50,814	80,378	130,720
Net Income Attributable to Common Stockholders	$ 12,426	$ 15,625	$ 8,088	$ 12,363	$ 6,636	$ (649)	$ 9,777	$ 19,342	$ 48,502	$ 35,106	$ 97,700
Earnings (loss) per share—Basic (in $ per share)	$ 1.94	$ 2.04	$ 0.99	$ 1.48	$ 0.78	$ (0.07)	$ 1.10	$ 2.16	$ 6.34	$ 3.99	$ 10.75
Earnings (loss) per share—Diluted (in $ per share)	$ 1.87	$ 1.99	$ 0.97	$ 1.45	$ 0.76	$ (0.07)	$ 1.08	$ 2.11	$ 6.20	$ 3.92	$ 10.51