INCOME TAXES - Reconciliation Effective Income Tax Amount (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at federal income tax rate									$ 43,499	$ 36,836	$ 19,860
Change resulting from:
Tax-exempt interest									(4,390)	(3,916)	(2,490)
Increase in cash value of bank owned life insurance									(556)	(607)	(484)
Excess tax benefit from stock compensation									(939)	0	0
State income tax, net of federal benefit									(680)	695	667
Remeasurement of deferred tax assets and deferred tax liabilities at reduced federal corporate tax rate									13,625	0	0
Other									175	138	(1,656)
Provision for income taxes	$ 22,063	$ 8,142	$ 10,315	$ 10,214	$ 6,987	$ 10,364	$ 9,671	$ 6,124	$ 50,734	$ 33,146	$ 15,897