Selected Quarterly Data (Unaudited) - Summary of Selected Quarterly Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 86,115	$ 92,375	$ 99,656	$ 103,831	$ 112,137	$ 117,841	$ 112,749	$ 107,871	$ 381,977	$ 450,598	$ 389,215
Operating Income	16,506	23,122	16,208	24,542	36,665	38,927	22,502	32,626	80,378	130,720	113,504
Net Income (Loss) Attributable to Common Stockholders	$ 6,636	$ (649)	$ 9,777	$ 19,342	$ 18,879	$ 37,340	$ 19,543	$ 21,938	$ 35,106	$ 97,700	$ 75,190
Earnings (loss) per share—Basic (in $ per share)	$ 0.78	$ (0.07)	$ 1.10	$ 2.16	$ 2.09	$ 4.10	$ 2.14	$ 2.41	$ 3.99	$ 10.75	$ 9.18
Earnings (loss) per share—Diluted (in $ per share)	$ 0.76	$ (0.07)	$ 1.08	$ 2.11	$ 2.05	$ 4.02	$ 2.10	$ 2.34	$ 3.92	$ 10.51	$ 8.92
Net tax benefit of federal corporate income tax return						$ 15,500