SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 10,840	$ 14,020	$ 12,769	$ 13,985	$ 13,643	$ 16,235	$ 13,061	$ 12,894	$ 51,614	$ 55,833	$ 41,979
Less: Income allocated to participating securities									(479)	(492)	(385)
Net income available to common stockholders									$ 51,135	$ 55,341	$ 41,594
Denominator:
Weighted average shares - basic (in shares)	66,270	66,267	66,248	66,046	65,981	65,978	65,953	65,739	66,209	65,913	65,555
Performance-based restricted stock awards (in shares)									57	53	85
Weighted average shares - diluted (in shares)	66,334	66,267	66,248	66,046	66,041	66,005	65,967	65,739	66,266	65,966	65,640
Basic net income per share:
Net income (in usd per share)	$ 0.16	$ 0.21	$ 0.19	$ 0.21	$ 0.20	$ 0.24	$ 0.20	$ 0.19	$ 0.77	$ 0.84	$ 0.63
Diluted net income per share:
Net income (in usd per share)	$ 0.16	$ 0.21	$ 0.19	$ 0.21	$ 0.20	$ 0.24	$ 0.20	$ 0.19	$ 0.77	$ 0.84	$ 0.63