Quarterly Financial Results (Unaudited) - Summarized Quarterly Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 51,269	$ 49,791	$ 49,094	$ 50,119	$ 54,685	$ 49,669	$ 52,268	$ 53,130	$ 200,273	$ 209,752	$ 219,696
Operating income	4,618	8,073	7,368	8,092	12,315	9,534	11,905	11,740	28,151	45,494	45,728
Net income	$ 6,912	$ 4,652	$ 4,291	$ 4,890	$ 8,015	$ 5,762	$ 6,828	$ 6,925	$ 20,745	$ 27,530	$ 26,984
Basic net income per common share	$ 0.32	$ 0.21	$ 0.20	$ 0.23	$ 0.37	$ 0.27	$ 0.32	$ 0.32	$ 0.96	$ 1.27	$ 1.23
Diluted net income per common share	$ 0.31	$ 0.21	$ 0.19	$ 0.22	$ 0.36	$ 0.26	$ 0.31	$ 0.32	$ 0.94	$ 1.25	$ 1.20