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INVESTMENTS (Fixed Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Fixed maturities
Amortized cost	$ 11,715,885		$ 12,263,240
Allowance for expected credit losses	(6,257)	$ (20,019)	0	$ 0	$ 0	$ 0
Gross unrealized gains	432,693		236,817
Gross unrealized losses	(95,906)		(31,852)
Fair value	12,046,415		12,468,205
U.S. government and agency
Fixed maturities
Amortized cost	1,942,952		2,102,849
Allowance for expected credit losses	0		0
Gross unrealized gains	70,341		16,345
Gross unrealized losses	(26)		(6,313)
Fair value	2,013,267		2,112,881
Non-U.S. government
Fixed maturities
Amortized cost	608,375		564,505
Allowance for expected credit losses	0		0
Gross unrealized gains	9,519		14,535
Gross unrealized losses	(9,797)		(2,448)
Fair value	608,097		576,592
Corporate debt
Fixed maturities
Amortized cost	4,513,997		4,797,384
Allowance for expected credit losses	(4,643)		0
Gross unrealized gains	200,202		140,426
Gross unrealized losses	(47,145)		(7,556)
Fair value	4,662,411		4,930,254
Agency RMBS
Fixed maturities
Amortized cost	1,483,883		1,570,823
Allowance for expected credit losses	0		0
Gross unrealized gains	51,466		25,215
Gross unrealized losses	(276)		(3,454)
Fair value	1,535,073		1,592,584
CMBS
Fixed maturities
Amortized cost	1,302,049		1,340,156
Allowance for expected credit losses	0		0
Gross unrealized gains	79,241		29,838
Gross unrealized losses	(4,026)		(4,942)
Fair value	1,377,264		1,365,052
Non-agency RMBS
Fixed maturities
Amortized cost	118,085		84,381
Allowance for expected credit losses	(20)		0
Gross unrealized gains	2,396		1,393
Gross unrealized losses	(2,242)		(852)
Fair value	118,219		84,922
ABS
Fixed maturities
Amortized cost	1,558,578		1,599,867
Allowance for expected credit losses	(1,594)		0
Gross unrealized gains	9,798		4,706
Gross unrealized losses	(32,218)		(5,880)
Fair value	1,534,564		1,598,693
Municipals
Fixed maturities
Amortized cost	187,966		203,275
Allowance for expected credit losses	0		0
Gross unrealized gains	9,730		4,359
Gross unrealized losses	(176)		(407)
Fair value	$ 197,520		$ 207,227