INVESTMENTS - Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Gross investment income									$ 427,994	$ 383,544	$ 337,034
Investment expenses									(27,189)	(30,209)	(31,698)
Net investment income	$ 100,908	$ 95,169	$ 106,063	$ 98,664	$ 95,517	$ 116,923	$ 91,730	$ 49,164	400,805	353,335	305,336
Fixed maturities
Net Investment Income [Line Items]
Gross investment income									312,662	305,459	294,725
Other investments
Net Investment Income [Line Items]
Gross investment income									76,858	42,514	20,148
Equity securities
Net Investment Income [Line Items]
Gross investment income									14,919	16,306	11,289
Mortgage loans
Net Investment Income [Line Items]
Gross investment income									10,780	7,996	1,861
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									10,057	9,209	8,572
Short-term investments
Net Investment Income [Line Items]
Gross investment income									$ 2,718	$ 2,060	$ 439