INVESTMENTS - Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net investment income
Gross investment income									$ 57,883	$ 59,482	$ 60,726
Less investment expenses									(4,808)	(4,838)	(5,118)
Net Investment Income	$ 13,153	$ 13,504	$ 13,048	$ 13,370	$ 13,754	$ 13,964	$ 13,431	$ 13,495	53,075	54,644	55,608
Debt securities
Net investment income
Gross investment income									46,834	48,064	48,757
Equity securities
Net investment income
Gross investment income									10,929	11,407	11,962
Cash and Short-term Investments
Net investment income
Gross investment income									$ 120	$ 11	$ 7