FEDERAL HOME LOAN BANK AND FEDERAL RESERVE BANK ADVANCES - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Short-term Debt [Line Items]
FHLB advances, maximum percentage of assets	40.00%		40.00%
Borrowing capacity based on pledged loans to the FRB and FHLB	$ 877,400,000		$ 877,400,000		$ 1,100,000,000
Loans available to be pledged as collateral	2,000,000,000.0		2,000,000,000.0		2,400,000,000
Maximum short-term debt outstanding during period		$ 28,100,000	77,800,000	$ 209,800,000
Average short-term debt outstanding during period	0	$ 1,800,000	$ 9,200,000	$ 65,200,000
Weighted average interest rate		2.44%	1.45%	2.57%
FHLB Advances
Short-term Debt [Line Items]
Mortgage loans pledged as collateral	2,300,000,000		$ 2,300,000,000		2,200,000,000
Letters of credit pledged as collateral	62,600,000		62,600,000		62,600,000
FRB Advances
Short-term Debt [Line Items]
Mortgage loans pledged as collateral	443,300,000		443,300,000		447,400,000
Borrowings outstanding	$ 0		$ 0		$ 0