LOANS - Corporate Debt Amortized Costs Basis by Risk Rating and Vintage Year (Details) - Corporate Loans $ in Thousands	Mar. 31, 2021 USD ($)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	$ 2,074,475
2021	47,167
2020	516,980
2019	357,342
2018	718,577
2017	353,541
2016	46,394
2015	34,474
Accrued interest receivable	8,800
Deferred loan fees on unfunded loans	1,100
Performing
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	1,618,878
2021	44,236
2020	478,783
2019	309,602
2018	415,939
2017	289,450
2016	46,394
2015	34,474
Closely Monitored
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	374,004
2021	2,931
2020	26,358
2019	21,567
2018	259,057
2017	64,091
2016	0
2015	0
Substandard
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	81,593
2021	0
2020	11,839
2019	26,173
2018	43,581
2017	0
2016	0
2015	0
Doubtful
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	0
2021	0
2020	0
2019	0
2018	0
2017	0
2016	0
2015	0
Loss
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	0
2021	0
2020	0
2019	0
2018	0
2017	0
2016	0
2015	$ 0