GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended
	Aug. 30, 2019	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill resulting from business combinations	$ 58,000		$ 58,000
Goodwill		$ 937,771		$ 879,727	$ 937,771	$ 880,251
Other Depreciation and Amortization				2,000
Intangible assets amortization		$ 2,792		$ 2,045
Customer lists
Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 39,400
Intangible assets amortization method		straight-line basis
Estimated weighted average life (in years)		11 years
Core deposits
Finite-Lived Intangible Assets [Line Items]
Intangible assets amortization method		accelerated basis
Estimated weighted average life (in years)		7 years 8 months 12 days