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FAIR VALUE MEASUREMENTS - Level 3 for financial instruments measured at fair value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	$ 59,283	$ 152,468
Transfers into Level 3	9,091	0
Transfers out of Level 3	0	(25,307)
Included in net income	23	22,683
Included in OCI	312	(3,840)
Purchases	28,006	22,500
Sales	0	(54,244)
Settlements/ distributions	(5,622)	(54,977)
Closing balance	91,093	59,283
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	721	4,951
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	9,672	10,299
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in earnings	9,421	(627)
Included in OCI	0	0
Purchases	0	0
Sales	0	0
Settlements/ distributions	(9,971)	0
Closing balance	9,122	9,672
Change in unrealized investment gain/(loss) relating to liabilities held at the reporting date	(550)	(627)
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	8,021	86,679
Transfers into Level 3	9,091	0
Transfers out of Level 3	0	(25,307)
Included in net income	0	4,790
Included in OCI	312	(3,840)
Purchases	969	0
Sales	0	(6,068)
Settlements/ distributions	(3,484)	(48,233)
Closing balance	14,909	8,021
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	0	0
Corporate debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	2,297	49,012
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	0	4,790
Included in OCI	207	(4,168)
Purchases	0	0
Sales	0	(6,068)
Settlements/ distributions	0	(41,269)
Closing balance	2,504	2,297
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	0	0
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	5,235	19,134
Transfers into Level 3	0	0
Transfers out of Level 3	0	(7,077)
Included in net income	0	0
Included in OCI	(11)	142
Purchases	0	0
Sales	0	0
Settlements/ distributions	(3,484)	(6,964)
Closing balance	1,740	5,235
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	0	0
ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	489	18,533
Transfers into Level 3	9,091	0
Transfers out of Level 3	0	(18,230)
Included in net income	0	0
Included in OCI	116	186
Purchases	969	0
Sales	0	0
Settlements/ distributions	0	0
Closing balance	10,665	489
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	0	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	51,262	65,789
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	23	17,893
Included in OCI	0	0
Purchases	27,037	22,500
Sales	0	(48,176)
Settlements/ distributions	(2,138)	(6,744)
Closing balance	76,184	51,262
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	721	4,951
CLO-Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	14,328	21,271
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	(6,017)	(199)
Included in OCI	0	0
Purchases	0	0
Sales	0	0
Settlements/ distributions	(2,138)	(6,744)
Closing balance	6,173	14,328
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	(5,319)	(199)
Other privately held investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	36,934	44,518
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	6,040	18,092
Included in OCI	0	0
Purchases	27,037	22,500
Sales	0	(48,176)
Settlements/ distributions	0	0
Closing balance	70,011	36,934
Change in unrealized investment gain/(loss) relating to assets held at the reporting date	6,040	5,150
Derivative instruments
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening balance	9,672	10,299
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in earnings	9,421	(627)
Included in OCI	0	0
Purchases	0	0
Sales	0	0
Settlements/ distributions	(9,971)	0
Closing balance	9,122	9,672
Change in unrealized investment gain/(loss) relating to liabilities held at the reporting date	$ (550)	$ (627)

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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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