Summary Selected Quarterly Financial Data - Summary of Selected Consolidated Quarterly Financial 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]
Total revenues	$ 80,681	$ 75,363	$ 82,947	$ 73,071	$ 77,521	$ 69,815	$ 78,953	$ 71,361	$ 312,062	$ 297,650	$ 293,031
Income from operations	6,188	4,925	7,080	20	4,464	4,301	9,788	3,280	18,213	21,833	30,760
Net income	$ 3,599	$ 3,043	$ 4,289	$ (66)	$ 2,161	$ 2,509	$ 6,185	$ 2,169	$ 10,865	$ 13,024	$ 26,296
Basic income per common share	$ 0.20	$ 0.17	$ 0.23		$ 0.11	$ 0.12	$ 0.29	$ 0.10	$ 0.58	$ 0.62	$ 1.17
Diluted income per common share	$ 0.19	$ 0.16	$ 0.23		$ 0.11	$ 0.12	$ 0.28	$ 0.10	$ 0.57	$ 0.61	$ 1.15