UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 168,398		$ 162,378		$ 160,336		$ 153,871		$ 147,479		$ 165,104		$ 158,808		$ 152,335		$ 644,983	$ 623,726	$ 594,467
Income before income tax expense	29,196		5,047		8,087		20,877		13,923		15,921		11,875		29,937		63,207	71,656	40,907
Income tax expense	7,904		918		1,959		6,223		4,028		4,732		3,127		9,607		17,004	21,494	10,915
Net income	$ 21,292		$ 4,129		$ 6,128		$ 14,654		$ 9,895		$ 11,189		$ 8,748		$ 20,330		$ 46,203	$ 50,162	$ 29,992
Net income per common share - basic and diluted1	$ 1.01	[1]	$ 0.20	[1]	$ 0.29	[1]	$ 0.70	[1]	$ 0.48	[1]	$ 0.54	[1]	$ 0.42	[1]	$ 1.00	[1]	$ 2.20	$ 2.43	$ 1.48

[1] Since the weighted-average number of shares outstanding for the quarters are calculated independently of the weighted-average number of shares outstanding for the year, quarterly net income per share may not total to annual net income per share.