EARNINGS PER SHARE - Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings per common share:
Net income	$ 22,198	$ 21,454	$ 21,713	$ 21,682	$ 21,363	$ 20,570	$ 20,142	$ 18,336	$ 87,047	$ 80,411	$ 57,330
Weighted-average common shares outstanding									25,588,404	25,288,533	25,181,208
Basic earnings per common share	$ 0.86	$ 0.84	$ 0.85	$ 0.85	$ 0.84	$ 0.81	$ 0.80	$ 0.73	$ 3.40	$ 3.18	$ 2.28
Diluted earnings per common share:
Net income	$ 22,198	$ 21,454	$ 21,713	$ 21,682	$ 21,363	$ 20,570	$ 20,142	$ 18,336	$ 87,047	$ 80,411	$ 57,330
Weighted-average common shares outstanding for basic earnings per common share									25,588,404	25,288,533	25,181,208
Add: Dilutive effect of assumed exercise of warrant									0	225,831	219,273
Add: Dilutive effect of assumed exercises of stock options and awards									170,489	213,467	262,900
Average shares and dilutive potential common shares									25,758,893	25,727,831	25,663,381
Diluted earnings per common share	$ 0.86	$ 0.83	$ 0.85	$ 0.84	$ 0.83	$ 0.80	$ 0.79	$ 0.71	$ 3.38	$ 3.13	$ 2.23