INCOME TAXES - Reconciliation Effective Income Tax Amount (Details) - USD ($) $ 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
Income Tax Disclosure [Abstract]
Federal income statutory rate									21.00%	21.00%	35.00%
Tax at federal income tax rate									$ 44,433	$ 31,813	$ 43,499
Change resulting from:
State income tax, net of federal benefit									7,389	1,965	(680)
Tax-exempt interest									(2,911)	(3,095)	(4,390)
Increase in cash value of bank owned life insurance									(581)	(382)	(556)
Excess tax benefit from stock compensation									(108)	(602)	(939)
Nondeductible merger expenses									799	1,002	0
Other									1,122	(238)	175
Remeasurement of deferred tax assets and deferred tax liabilities at reduced federal corporate tax rate									0	0	13,625
Provision for income taxes	$ 20,959	$ 5,692	$ 12,064	$ 11,428	$ 7,017	$ 13,317	$ 2,423	$ 7,706	$ 50,143	$ 30,463	$ 50,734