INVESTMENTS - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Mar. 01, 2019
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0
Weighted average duration of MBS portfolio	2 years 1 month 6 days
Securities pledged as collateral	$ 1,600,000,000		$ 1,100,000,000
Sale of investment securities available for sale, including interest	198,900,000	$ 602,500,000
Remaining securities	5,900,000
Losses from sale of available-for-sale securities	0	0
Debt securities, available-for-sale, realized gain (less than)		100,000
Unamortized premiums	47,100,000		15,100,000
Unaccreted discounts	3,000,000.0		$ 4,100,000
Owned investment securities	161,900,000
Total unrealized losses on securities	500,000
Held-to-maturity debt securities with an amortized cost basis	113,600,000
Accrued interest	$ 1,000,000.0
Beneficial
Gain (Loss) on Securities [Line Items]
Securities sold		$ 23,700,000		$ 578,800,000