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INVESTMENTS (Fixed Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fixed maturities
Amortized cost	$ 12,201,832		$ 12,263,240
Allowance for expected credit losses	(2,498)	$ (6,257)	0	$ 0	$ 0	$ 0
Gross unrealized gains	463,461		236,817
Gross unrealized losses	(53,554)		(31,852)
Fair value	12,609,241		12,468,205
U.S. government and agency
Fixed maturities
Amortized cost	1,929,224		2,102,849
Allowance for expected credit losses	0		0
Gross unrealized gains	62,872		16,345
Gross unrealized losses	(630)		(6,313)
Fair value	1,991,466		2,112,881
Non-U.S. government
Fixed maturities
Amortized cost	659,744		564,505
Allowance for expected credit losses	0		0
Gross unrealized gains	18,099		14,535
Gross unrealized losses	(4,042)		(2,448)
Fair value	673,801		576,592
Corporate debt
Fixed maturities
Amortized cost	4,607,629		4,797,384
Allowance for expected credit losses	(2,462)		0
Gross unrealized gains	214,924		140,426
Gross unrealized losses	(23,360)		(7,556)
Fair value	4,796,731		4,930,254
Agency RMBS
Fixed maturities
Amortized cost	1,630,014		1,570,823
Allowance for expected credit losses	0		0
Gross unrealized gains	48,465		25,215
Gross unrealized losses	(1,373)		(3,454)
Fair value	1,677,106		1,592,584
CMBS
Fixed maturities
Amortized cost	1,291,291		1,340,156
Allowance for expected credit losses	0		0
Gross unrealized gains	92,012		29,838
Gross unrealized losses	(2,565)		(4,942)
Fair value	1,380,738		1,365,052
Non-agency RMBS
Fixed maturities
Amortized cost	131,937		84,381
Allowance for expected credit losses	(36)		0
Gross unrealized gains	2,733		1,393
Gross unrealized losses	(1,104)		(852)
Fair value	133,530		84,922
ABS
Fixed maturities
Amortized cost	1,679,839		1,599,867
Allowance for expected credit losses	0		0
Gross unrealized gains	12,374		4,706
Gross unrealized losses	(20,038)		(5,880)
Fair value	1,672,175		1,598,693
Municipals
Fixed maturities
Amortized cost	272,154		203,275
Allowance for expected credit losses	0		0
Gross unrealized gains	11,982		4,359
Gross unrealized losses	(442)		(407)
Fair value	$ 283,694		$ 207,227