Quarterly Financial Results (Unaudited) - Summarized 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	$ 47,339	$ 46,181	$ 47,969	$ 48,138	$ 51,269	$ 49,791	$ 49,094	$ 50,119	$ 189,628	$ 200,273	$ 209,752
Operating income	6,550	6,657	5,621	6,273	4,618	8,073	7,368	8,092	25,100	28,151	45,494
Net income	$ 72,721	$ 4,220	$ 3,376	$ 3,917	$ 6,912	$ 4,652	$ 4,291	$ 4,890	$ 84,235	$ 20,745	$ 27,530
Basic net income per common share (usd per share)	$ 3.54	$ 0.20	$ 0.16	$ 0.18	$ 0.32	$ 0.21	$ 0.20	$ 0.23	$ 3.99	$ 0.96	$ 1.27
Diluted net income per common share (usd per share)	$ 3.53	$ 0.20	$ 0.16	$ 0.18	$ 0.31	$ 0.21	$ 0.19	$ 0.22	$ 3.98	$ 0.94	$ 1.25
Release from valuation allowance	$ 68,400