INVESTMENTS (Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Due in one year or less (amortized cost)	$ 388,837	$ 430,390
Due after one year through five years (amortized cost)	5,169,170	4,751,064
Due after five years through ten years (amortized cost)	1,921,131	1,762,452
Due after ten years (amortized cost)	388,275	210,133
Total fixed maturities with a single maturity date (amortized cost)	7,867,413	7,154,039
Amortized cost	12,343,774	11,616,312
Due in one year or less (fair value)	388,997	426,142
Due after one year through five years (fair value)	5,237,935	4,691,263
Due after five years through ten years (fair value)	1,967,360	1,697,737
Due after ten years (fair value)	400,732	206,077
Total fixed maturities with a single maturity date (fair value)	7,995,024	7,021,219
Fair Value	$ 12,522,955	$ 11,435,347
Due in one year or less (% of total fair value)	3.10%	3.70%
Due after one year through five years (% of total fair value)	41.80%	41.00%
Due after five years through ten years (% of total fair value)	15.70%	14.80%
Due after ten years (% of total fair value)	3.20%	1.80%
Fixed maturities with a single maturity date (% of total fair value)	63.80%	61.30%
Total	100.00%	100.00%
Agency RMBS
Schedule Of Available For Sale Securities [Line Items]
Fixed maturities without a single maturity date (amortized cost)	$ 1,669,160	$ 1,666,684
Amortized cost	1,669,160	1,666,684
Fixed maturities without a single maturity date (fair value)	1,686,912	1,643,308
Fair Value	$ 1,686,912	$ 1,643,308
Fixed maturities without a single maturity date (% of total fair value)	13.50%	14.40%
CMBS
Schedule Of Available For Sale Securities [Line Items]
Fixed maturities without a single maturity date (amortized cost)	$ 1,150,370	$ 1,103,507
Amortized cost	1,150,370	1,103,507
Fixed maturities without a single maturity date (fair value)	1,184,268	1,092,530
Fair Value	$ 1,184,268	$ 1,092,530
Fixed maturities without a single maturity date (% of total fair value)	9.50%	9.60%
Non-Agency RMBS
Schedule Of Available For Sale Securities [Line Items]
Fixed maturities without a single maturity date (amortized cost)	$ 55,985	$ 40,732
Amortized cost	55,985	40,732
Fixed maturities without a single maturity date (fair value)	55,980	40,687
Fair Value	$ 55,980	$ 40,687
Fixed maturities without a single maturity date (% of total fair value)	0.40%	0.40%
ABS
Schedule Of Available For Sale Securities [Line Items]
Fixed maturities without a single maturity date (amortized cost)	$ 1,600,846	$ 1,651,350
Amortized cost	1,600,846	1,651,350
Fixed maturities without a single maturity date (fair value)	1,600,771	1,637,603
Fair Value	$ 1,600,771	$ 1,637,603
Fixed maturities without a single maturity date (% of total fair value)	12.80%	14.30%