OTHER COMPREHENSIVE INCOME (LOSS) - Component of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before Tax Amount
Other Comprehensive Income (Loss), before Tax	$ 219,197	$ 69,950	$ (156,244)
Tax (Expense) Benefit
Other Comprehensive Income (Loss), Tax	(4,974)	(3,326)	13,353
Net of Tax Amount
Other Comprehensive Income (Loss), Net of Tax	214,223	66,624	(142,891)
Unrealized gains (losses) on available for sale investments, net of tax
Before Tax Amount
Unrealized investment gains arising during the year	211,151	10,165	(280,512)
Adjustment for reclassification of net realized investment gains and OTTI losses recognized in net income	(33,892)	60,423	145,766
Other Comprehensive Income (Loss), before Tax	177,259	70,588	(134,746)
Tax (Expense) Benefit
Unrealized investment gains arising during the year	(5,732)	(5,093)	14,128
Adjustment for reclassification of net realized investment gains and OTTI losses recognized in net income	758	1,767	(775)
Other Comprehensive Income (Loss), Tax	(4,974)	(3,326)	13,353
Net of Tax Amount
Unrealized investment gains arising during the year	205,419	5,072	(266,384)
Adjustment for reclassification of net realized investment gains and OTTI losses recognized in net income	(33,134)	62,190	144,991
Other Comprehensive Income (Loss), Net of Tax	172,285	67,262	(121,393)
Accumulated Other-than-Temporary Impairment, net of tax
Before Tax Amount
Other Comprehensive Income (Loss), before Tax	0	0	0
Tax (Expense) Benefit
Other Comprehensive Income (Loss), Tax	0	0	0
Net of Tax Amount
Other Comprehensive Income (Loss), Net of Tax	0	0	0
Cumulative foreign currency translation adjustments, net of tax
Before Tax Amount
Other Comprehensive Income (Loss), before Tax	41,938	(638)	(21,498)
Tax (Expense) Benefit
Other Comprehensive Income (Loss), Tax	0	0	0
Net of Tax Amount
Other Comprehensive Income (Loss), Net of Tax	$ 41,938	$ (638)	$ (21,498)