INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Available-for-sale
Total amortized cost		$ 1,727,075	$ 1,646,411
Available-for-sale Securities, Debt Securities		1,710,612	1,672,239
Amortized Cost
Due in one year or less		37,920
Due after one year through five years		349,532
Due after five years through 10 years		653,536
Due after 10 years		220,714
Mtge/ABS/CMBS	[1]	465,373
Total amortized cost		1,727,075	1,646,411
Fair Value
Due in one year or less		37,787
Due after one year through five years		349,582
Due after five years through 10 years		648,164
Due after 10 years		220,540
Mtge/ABS/CMBS	[1]	454,539
Total fair value		1,710,612	1,672,239
Fair value measured on recurring basis
Available-for-sale
Available-for-sale Securities, Debt Securities		1,710,612	1,672,239
Fair Value
Total fair value		1,710,612	1,672,239
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
INVESTMENTS
Total assets at fair value		0
Debt securities
Available-for-sale
Total amortized cost		1,727,075	1,646,411
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		13,405	34,216
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		29,868	8,388
Available-for-sale Securities, Debt Securities		1,710,612	1,672,239
Amortized Cost
Total amortized cost		1,727,075	1,646,411
Fair Value
Total fair value		1,710,612	1,672,239
U.S. government
Available-for-sale
Total amortized cost		151,446	92,561
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		96	23
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		2,377	895
Available-for-sale Securities, Debt Securities		149,165	91,689
Amortized Cost
Total amortized cost		151,446	92,561
Fair Value
Total fair value		149,165	91,689
U.S. Agency
Available-for-sale
Total amortized cost		30,650	18,541
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		213	347
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		607	110
Available-for-sale Securities, Debt Securities		30,256	18,778
Amortized Cost
Total amortized cost		30,650	18,541
Fair Value
Total fair value		30,256	18,778
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost		8,186	7,501
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax			143
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		291	56
Available-for-sale Securities, Debt Securities		7,895	7,588
Amortized Cost
Total amortized cost		8,186	7,501
Fair Value
Total fair value		7,895	7,588
Mortgage-backed
Available-for-sale
Total amortized cost		364,298	329,129
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		1,685	3,420
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		11,638	4,078
Available-for-sale Securities, Debt Securities		354,345	328,471
Amortized Cost
Total amortized cost		364,298	329,129
Fair Value
Total fair value		354,345	328,471
ABS/CMBS
Available-for-sale
Total amortized cost	[2]	101,075	70,405
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	[2]	152	436
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	[2]	1,033	315
Available-for-sale Securities, Debt Securities	[2]	100,194	70,526
Amortized Cost
Total amortized cost	[2]	101,075	70,405
Fair Value
Total fair value	[2]	100,194	70,526
Corporate Debt
Available-for-sale
Total amortized cost		633,159	508,128
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		3,061	12,575
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		11,534	1,681
Available-for-sale Securities, Debt Securities		624,686	519,022
Amortized Cost
Total amortized cost		633,159	508,128
Fair Value
Total fair value		624,686	519,022
Municipal
Available-for-sale
Total amortized cost		438,261	620,146
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		8,198	17,272
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		2,388	1,253
Available-for-sale Securities, Debt Securities		444,071	636,165
Amortized Cost
Total amortized cost		438,261	620,146
Fair Value
Total fair value		$ 444,071	$ 636,165

[1] Mortgage-backed, asset-backed and commercial mortgage-backed

[2] Non-agency asset-backed and commercial mortgage-backed