INVESTMENTS (Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 331,379	$ 430,390
Due after one year through five years	4,980,949	4,751,064
Due after five years through ten years	2,107,695	1,762,452
Due after ten years	397,393	210,133
Total fixed maturities with a single maturity date	7,817,416	7,154,039
Amortized cost	12,407,704	11,616,312
Fair value
Due in one year or less	330,584	426,142
Due after one year through five years	5,037,089	4,691,263
Due after five years through ten years	2,173,611	1,697,737
Due after ten years	413,812	206,077
Total fixed maturities with a single maturity date	7,955,096	7,021,219
Fair value	$ 12,616,241	$ 11,435,347
% of Total fair value
Due in one year or less	2.60%	3.70%
Due after one year through five years	39.90%	41.00%
Due after five years through ten years	17.20%	14.80%
Due after ten years	3.30%	1.80%
Fixed maturities with a single maturity date	63.00%	61.30%
Total	100.00%	100.00%
Agency RMBS
Amortized cost
Fixed maturities without a single maturity date	$ 1,609,193	$ 1,666,684
Amortized cost	1,609,193	1,666,684
Fair value
Fixed maturities without a single maturity date	1,632,768	1,643,308
Fair value	$ 1,632,768	$ 1,643,308
% of Total fair value
Fixed maturities without a single maturity date	12.90%	14.40%
CMBS
Amortized cost
Fixed maturities without a single maturity date	$ 1,321,029	$ 1,103,507
Amortized cost	1,321,029	1,103,507
Fair value
Fixed maturities without a single maturity date	1,368,890	1,092,530
Fair value	$ 1,368,890	$ 1,092,530
% of Total fair value
Fixed maturities without a single maturity date	10.90%	9.60%
Non-Agency RMBS
Amortized cost
Fixed maturities without a single maturity date	$ 59,308	$ 40,732
Amortized cost	59,308	40,732
Fair value
Fixed maturities without a single maturity date	58,952	40,687
Fair value	$ 58,952	$ 40,687
% of Total fair value
Fixed maturities without a single maturity date	0.50%	0.40%
ABS
Amortized cost
Fixed maturities without a single maturity date	$ 1,600,758	$ 1,651,350
Amortized cost	1,600,758	1,651,350
Fair value
Fixed maturities without a single maturity date	1,600,535	1,637,603
Fair value	$ 1,600,535	$ 1,637,603
% of Total fair value
Fixed maturities without a single maturity date	12.70%	14.30%