GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
	Aug. 30, 2019	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill resulting from business combinations	$ 58,000		$ 58,000		$ 0
Goodwill		$ 937,771		$ 879,727	937,771	$ 879,727	$ 937,771	$ 937,771	$ 879,727	$ 880,251
Other Depreciation and Amortization						4,100
Intangible assets amortization		$ 2,791		$ 2,044	$ 5,583	$ 4,089
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 6 months