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
Quarterly results of consolidated operations
Total revenues	$ 79,615	$ 77,415	$ 70,135	$ 69,957	$ 68,506	$ 68,053	$ 68,192	$ 73,788	$ 297,122	$ 278,539	$ 239,310
Earnings (loss) before taxes	(737)	7,428	1,751	5,507	(8,062)	6,897	3,533	8,362	13,949	10,730	27,869
Net earnings (loss)	644	6,078	1,985	4,401	(4,080)	5,728	4,082	8,394	11,839	10,842	30,508
Comprehensive income (loss)	$ (5,502)	$ 11,824	$ 7,438	$ 7,291	$ (4,186)	$ 15,489	$ 10,495	$ (4,668)	$ 22,198	$ 20,516	$ 42,742
Net earnings (loss) per share:
Basic	$ (0.06)	$ 0.61	$ 0.21	$ 0.40	$ (0.67)	$ 0.56	$ 0.39	$ 0.76	$ 1.18	$ 1.04	$ 2.96
Diluted	$ (0.06)	$ 0.60	$ 0.21	$ 0.39	$ (0.67)	$ 0.54	$ 0.38	$ 0.73	$ 1.14	$ 1.01	$ 2.87
Maximum [Member]
Common stock price ranges:
Share Price	$ 19.05	$ 19.61	$ 19.39	$ 22.33	$ 22.00	$ 20.10	$ 21.52	$ 21.95	$ 19.05	$ 22.00
Minimum [Member]
Common stock price ranges:
Share Price	$ 15.75	$ 16.71	$ 17.08	$ 18.04	$ 17.38	$ 16.97	$ 17.28	$ 18.82	$ 15.75	$ 17.38