INVESTMENTS - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Mar. 01, 2019
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0
Weighted average duration of MBS portfolio	2 years 2 months 12 days
Securities pledged as collateral	$ 1,300,000,000		$ 1,100,000,000
Sale of investment securities available for sale, including interest	30,600,000
Remaining securities	700,000
Debt securities, available-for-sale, realized gain (less than)		$ 100,000
Losses from sale of available-for-sale securities	0	0
Securities sold		583,900,000
Sale of investment securities available-for-sale	30,641,000	583,852,000
Unamortized premiums	16,400,000		15,100,000
Unaccreted discounts	3,800,000		$ 4,100,000
Owned investment securities	9,300,000
Total unrealized losses on securities	100,000
Held-to-maturity debt securities with an amortized cost basis	134,100,000
Accrued interest	$ 1,200,000
Beneficial
Gain (Loss) on Securities [Line Items]
Securities sold		$ 5,100,000		$ 578,800,000