GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Aug. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill resulting from business combinations	$ 58,000		$ 58,000	$ 0
Goodwill		$ 937,771	937,689	937,771	$ 937,689	$ 937,771	$ 937,771	$ 879,727	$ 880,251
Other Depreciation and Amortization					6,500
Intangible assets amortization		$ 2,779	$ 2,432	$ 8,362	$ 6,521
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 3 months 18 days