Summary Selected Quarterly Financial Data (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of selected unaudited consolidated quarterly financial data
Total revenues	$ 75,066	$ 68,511	$ 79,093	$ 70,260	$ 74,650	$ 69,774	$ 73,450	$ 65,742	$ 292,930	$ 283,616	$ 250,726
Income from operations	7,393	7,177	11,461	4,628	13,434	9,749	9,616	4,198
Net income	$ 4,251	$ 10,373	$ 7,769	$ 3,181	$ 8,897	$ 5,732	$ 5,470	$ 2,911	$ 25,574	$ 23,010	$ 20,507
Basic income per common share	$ 0.19	$ 0.46	$ 0.34	$ 0.14	$ 0.39	$ 0.25	$ 0.24	$ 0.13	$ 1.14	$ 1.02	$ 0.91
Diluted income per common share	$ 0.19	$ 0.45	$ 0.34	$ 0.14	$ 0.38	$ 0.25	$ 0.24	$ 0.13	$ 1.12	$ 0.99	$ 0.89