INCOME TAXES - RECONCILIATION OF EFFECTIVE TAX RATES (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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)	(14.10%)	(16.30%)	(14.70%)
Historic and low income tax credits (as a percent)	(9.40%)	(8.20%)	(7.00%)
Insurance captive	(3.30%)	(3.70%)	(1.80%)
Non-deductible interest expense related to carrying tax-exempt investments (as a percent)	0.40%	0.40%	0.40%
Non-deductible merger expenses		0.90%
Other (as a percent)	0.70%	0.10%	0.40%
Effective tax rates (as a percent)	8.30%	7.20%	11.30%