UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ 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
UNAUDITED INTERIM FINANCIAL INFORMATION
Net premiums earned	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 188,296	$ 182,025	$ 184,331	$ 183,285	$ 791,366	$ 737,937	$ 728,608
Net investment income	16,962	16,314	14,577	14,232	14,446	14,187	13,238	13,005	62,085	54,876	53,075
Net realized gains (losses)	15,507	18,647	20,849	8,404	5,111	35	(1,359)	624	63,407	4,411	34,645
Equity Securities, FV-NI, Unrealized Gain (Loss)	(64,200)	4,848	(12,611)	(26,772)					(98,735)
Earnings before income taxes	(32,708)	46,349	39,562	14,378	24,972	(862)	34,036	26,443	67,581	84,589	157,082
Net earnings	$ (20,660)	$ 39,372	$ 33,251	$ 12,216	$ 57,258	$ 1,734	$ 26,208	$ 19,828	$ 64,179	$ 105,028	$ 114,920
Basic earnings per share (in dollars per share)	$ (0.46)	$ 0.89	$ 0.75	$ 0.28	$ 1.30	$ 0.04	$ 0.60	$ 0.45	$ 1.45	$ 2.39	$ 2.63
Diluted earnings per share (in dollars per share)	$ (0.46)	$ 0.88	$ 0.74	$ 0.27	$ 1.29	$ 0.04	$ 0.59	$ 0.45	$ 1.43	$ 2.36	$ 2.59