EARNINGS PER SHARE - Computation of EPS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
EARNINGS PER SHARE.
Net income	$ 12,927	$ 12,073
Dividends paid on and earnings allocated to participating securities	(277)	(253)
Income attributable to common stock	$ 12,650	$ 11,820
Weighted average common shares outstanding, including participating securities	19,926	19,834
Weighted average participating securities (in shares)	(426)	(421)
Weighted average common shares outstanding	19,500	19,413
Basic earnings per common share (in dollars per share)	$ 0.65	$ 0.61
Income attributable to common stock	$ 12,650	$ 11,820
Weighted average common shares outstanding	19,500	19,413
Incremental shares from assumed conversions of options and restricted stock units	26	25
Weighted average common and equivalent shares outstanding (in shares)	19,526	19,438
Diluted earnings per common share (in dollars per share)	$ 0.65	$ 0.61