DERIVATIVES AND HEDGING ACTIVITIES - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) agreement
Derivative [Line Items]
Cumulative amount of last-of-layer fair value hedging basis adjustments	$ 14,800,000		$ 14,800,000		$ (405,000)
Carrying amount of the last-of-layer hedged assets	1,000,000,000.0		1,000,000,000.0		1,000,000,000.0
Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Closed portfolio loans used in hedging relationship	2,300,000,000		2,300,000,000		2,500,000,000
Cumulative amount of last-of-layer fair value hedging basis adjustments	14,784,000		14,784,000		$ (405,000)
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Number of agreements entered into | agreement					2
Term of contract					2 years
Notional amounts	1,000,000,000		1,000,000,000		$ 1,000,000,000
Net effect on interest income	(3,500,000)	$ 113,000	(3,891,000)	$ 113,000
Carrying amount of the last-of-layer hedged assets	1,014,784,000		1,014,784,000		999,595,000
Cash collateral	$ 15,700,000		$ 15,700,000		$ 2,800,000