INCOME TAXES - Reconciliation Effective Income Tax Amount (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal income statutory rate									21.00%	35.00%	35.00%
Tax at federal income tax rate									$ 31,813	$ 43,499	$ 36,836
Change resulting from:
State income tax, net of federal benefit									1,965	(680)	695
Tax-exempt interest									(3,095)	(4,390)	(3,916)
Increase in cash value of bank owned life insurance									(382)	(556)	(607)
Excess tax benefit from stock compensation									(602)	(939)	0
Nondeductible merger expenses									1,002	0	0
Other									(238)	175	138
Remeasurement of deferred tax assets and deferred tax liabilities at reduced federal corporate tax rate									0	13,625	0
Provision for income taxes	$ 7,017	$ 13,317	$ 2,423	$ 7,706	$ 22,063	$ 8,142	$ 10,315	$ 10,214	$ 30,463	$ 50,734	$ 33,146