INCOME TAX- Differences between effective tax rates and the federal tax rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Differences between effective tax rates and the federal statutory rate applied to income before income taxes
Federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Federal statutory income tax	$ 21,831	$ 17,661	$ 16,021
Tax exempt interest	(5,258)	(4,799)	(4,676)
Effect of state income taxes	103	75	169
Non-deductible expenses	615	436	193
Tax exempt income on life insurance	(773)	(530)	(537)
Tax credits	(720)	(397)	(501)
Change in valuation allowance	(26)	(453)	(190)
Captive insurance premiums	(289)	(335)	(361)
Tax rate change - Tax Cuts & Jobs Act	2,505
Stock based compensation	(364)
Low income housing proportional amortization	497	419	363
Other	(175)	60	(248)
Total income tax expense	$ 12,936	$ 12,137	$ 10,233