FEDERAL HOME LOAN BANK AND FEDERAL RESERVE BANK ADVANCES - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	$ 780,300,000		$ 780,300,000		$ 1,100,000,000
Loans available to be pledged as collateral	2,600,000,000		2,600,000,000		2,400,000,000
Maximum short-term debt outstanding during period	34,000,000.0	$ 78,000,000.0	77,800,000	$ 209,700,000
Average short-term debt outstanding during period	$ 4,100,000	$ 52,300,000	$ 13,900,000	$ 97,400,000
Weighted average interest rate	0.22%	2.57%	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	459,600,000		459,600,000		447,400,000
Borrowings outstanding	$ 0		$ 0		0
State of California Treasurer's Office | FHLB Advances
Short-term Debt [Line Items]
Letters of credit pledged as collateral					$ 125,000,000.0