INVESTMENTS - Contractual maturities of fixed maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 503,716	$ 436,287
Due after one year through five years	4,878,151	4,165,696
Due after five years through ten years	2,478,542	2,344,859
Due after ten years	277,756	258,654
Total fixed maturities with a single maturity date (amortized cost)	8,138,165	7,205,496
Total	12,241,782	11,566,930
Fair value
Due in one year or less	505,602	444,527
Due after one year through five years	4,908,640	4,335,219
Due after five years through ten years	2,488,478	2,489,050
Due after ten years	279,056	273,025
Total fixed maturities with a single maturity date (fair value)	8,181,776	7,541,821
Fair value	$ 12,313,200	$ 12,041,799
% of Total fair value
Due in one year or less	4.10%	3.60%
Due after one year through five years	39.90%	36.00%
Due after five years through ten years	20.20%	20.70%
Due after ten years	2.30%	2.30%
Fixed maturities with a single maturity date (% of total fair value)	66.50%	62.60%
Total	100.00%	100.00%
Agency RMBS
Amortized cost
Fixed maturities without a single maturity date	$ 1,065,973	$ 1,244,727
Total	1,065,973	1,244,727
Fair value
Fixed maturities without a single maturity date (fair value)	1,074,589	1,286,209
Fair value	$ 1,074,589	$ 1,286,209
% of Total fair value
Fixed maturities without a single maturity date (% of total fair value)	8.70%	10.70%
CMBS
Amortized cost
Fixed maturities without a single maturity date	$ 1,223,051	$ 1,268,273
Total	1,223,051	1,268,273
Fair value
Fixed maturities without a single maturity date (fair value)	1,248,191	1,353,587
Fair value	$ 1,248,191	$ 1,353,587
% of Total fair value
Fixed maturities without a single maturity date (% of total fair value)	10.10%	11.20%
Non-agency RMBS
Amortized cost
Fixed maturities without a single maturity date	$ 185,854	$ 136,198
Total	185,854	136,198
Fair value
Fixed maturities without a single maturity date (fair value)	186,164	140,104
Fair value	$ 186,164	$ 140,104
% of Total fair value
Fixed maturities without a single maturity date (% of total fair value)	1.50%	1.20%
ABS
Amortized cost
Fixed maturities without a single maturity date	$ 1,628,739	$ 1,712,236
Total	1,628,739	1,712,236
Fair value
Fixed maturities without a single maturity date (fair value)	1,622,480	1,720,078
Fair value	$ 1,622,480	$ 1,720,078
% of Total fair value
Fixed maturities without a single maturity date (% of total fair value)	13.20%	14.30%