FEDERAL HOME LOAN BANK AND FEDERAL RESERVE BANK ADVANCES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Borrowing capacity based on pledged loans to the FRB and FHLB	$ 810.0	$ 1,100.0
Maximum short-term debt outstanding during period	77.8
Average short-term debt outstanding during period	$ 23.8
Weighted average interest rate	1.66%
FHLB Advances
Short-term Debt [Line Items]
Mortgage loans pledged as collateral	$ 2,300.0	2,200.0
Letters of credit pledged as collateral	62.6	62.6
FRB Advances
Short-term Debt [Line Items]
Mortgage loans pledged as collateral	$ 422.7	447.4
State of California Treasurer's Office | FHLB Advances
Short-term Debt [Line Items]
Letters of credit pledged as collateral		$ 125.0