INCOME TAXES - RECONCILIATION OF EFFECTIVE TAX RATES (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of effective tax rates and federal statutory rates applied to financial statement income
U. S. federal income tax rate (as a percent)	34.00%	34.00%	34.00%
Changes from the statutory rate
Tax-exempt interest income (as a percent)	(9.00%)	(14.10%)	(16.30%)
Historic and low income tax credits (as a percent)	(4.00%)	(9.40%)	(8.20%)
Insurance captive	(1.90%)	(3.30%)	(3.70%)
Non-deductible interest expense related to carrying tax-exempt investments (as a percent)	0.30%	0.40%	0.40%
Non-deductible merger expenses			0.90%
2017 Tax Cuts and Jobs Act	3.10%
Other (as a percent)	0.30%	0.70%	0.10%
Effective tax rates (as a percent)	22.80%	8.30%	7.20%