LEASES - Narrative (Details) $ in Thousands		3 Months Ended		6 Months Ended
	Apr. 01, 2020 USD ($) Operating-Leases Locations	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Description		The Company leases real estate properties for its network of bank branches, financial centers and corporate offices.
Lessee, Operating Lease, Option to Extend		Most lease agreements include one or more options to renew, with renewal terms that can extend the original lease term from one to twenty years or more.
Operating Lease, Right-of-Use Asset		$ 70,200		$ 70,200
Operating Lease Liability		81,215		81,215
Operating Lease, Expense		3,400	$ 2,800	6,100	$ 5,700
ROU asset obtained in exchange for lease obligations		300
Cash paid for amounts included in the measurement of lease liabilities		$ 3,300	$ 2,200	$ 6,000	$ 4,300
Weighted average remaining lease term		9 years 7 months 6 days		9 years 7 months 6 days
Weighted average discount rate		3.07%		3.07%
Revere acquisition [Member]
Lessee, Lease, Description [Line Items]
Acquisition related unfavorable fair value marks	$ 1,100
Tenant allowance	200
Expense related to lease termination	2,300
Operating Lease Liability	8,700
ROU asset obtained in exchange for lease obligations	$ 7,400
Number of operating leases | Operating-Leases	15
Number of locations operating leases | Locations	12
Lease operation commence in 2021 [Member]
Lessee, Lease, Description [Line Items]
Acquisition related unfavorable fair value marks		$ 500		$ 500
Tenant allowance		200		200
Operating Lease, Right-of-Use Asset		1,400		1,400
Operating Lease Liability		$ 2,100		$ 2,100
Minimum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Renewal Term		1 year		1 year
Maximum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Renewal Term		20 years		20 years