INVESTMENTS - Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Gross investment income									$ 465,806	$ 427,994	$ 383,544
Investment expenses									(27,299)	(27,189)	(30,209)
Net investment income	$ 113,128	$ 114,421	$ 109,960	$ 100,999	$ 100,908	$ 95,169	$ 106,063	$ 98,664	438,507	400,805	353,335
Fixed maturities
Net Investment Income [Line Items]
Gross investment income									356,273	312,662	305,459
Other investments
Net Investment Income [Line Items]
Gross investment income									48,959	76,858	42,514
Equity securities
Net Investment Income [Line Items]
Gross investment income									10,077	14,919	16,306
Mortgage loans
Net Investment Income [Line Items]
Gross investment income									13,566	10,780	7,996
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									27,566	10,057	9,209
Short-term investments
Net Investment Income [Line Items]
Gross investment income									$ 9,365	$ 2,718	$ 2,060