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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 54,685	$ 49,669	$ 52,268	$ 53,130	$ 51,884	$ 55,116	$ 55,961	$ 56,735	$ 209,752	$ 219,696	$ 233,693
Operating income	12,315	9,534	11,905	11,740	4,614	13,047	13,810	14,257	45,494	45,728	51,829
Net income	$ 8,015	$ 5,762	$ 6,828	$ 6,925	$ 2,027	$ 8,048	$ 8,447	$ 8,462	$ 27,530	$ 26,984	$ 83,786
Basic net income per common share	$ 0.37	$ 0.27	$ 0.32	$ 0.32	$ 0.09	$ 0.37	$ 0.38	$ 0.38	$ 1.27	$ 1.23	$ 3.79
Diluted net income per common share	$ 0.36	$ 0.26	$ 0.31	$ 0.32	$ 0.09	$ 0.36	$ 0.37	$ 0.37	$ 1.25	$ 1.20	$ 3.72