SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator:
Net income	$ 12,997	$ 13,593	$ 12,458	$ 9,063	$ 13,613	$ 13,001	$ 11,629	$ 8,584	$ 48,111	$ 46,827	$ 42,681
Denominator:
Denominator for basic earnings per share	26,569	26,542	26,501	26,499	26,459	26,423	26,363	26,346	26,528	26,398	26,246
Dilutive effect of employee stock awards									191	218	266
Denominator for diluted earnings per share	26,738	26,736	26,691	26,671	26,632	26,622	26,616	26,593	26,719	26,616	26,512
Earnings per share:
Basic	$ 0.49	$ 0.51	$ 0.47	$ 0.34	$ 0.51	$ 0.49	$ 0.44	$ 0.33	$ 1.81	$ 1.77	$ 1.63
Diluted	$ 0.49	$ 0.51	$ 0.47	$ 0.34	$ 0.51	$ 0.49	$ 0.44	$ 0.32	$ 1.80	$ 1.76	$ 1.61