UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
UNAUDITED INTERIM FINANCIAL INFORMATION
Net premiums earned	$ 215,626	$ 211,255	$ 207,541	$ 204,689	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 839,111	$ 791,366	$ 737,937
Net investment income	17,775	17,532	16,998	16,565	16,962	16,314	14,577	14,232	68,870	62,085	54,876
Net realized gains	477	3,211	4,764	9,068	15,507	18,647	20,849	8,404	17,520	63,407	4,411
Equity Securities, FV-NI, Unrealized Gain (Loss)	30,876	4,906	8,810	33,498	(64,200)	4,848	(12,611)	(26,772)	78,090	(98,735)
Earnings before income taxes	63,218	38,947	48,828	81,741	(32,708)	46,349	39,562	14,378	232,734	67,581	84,589
Net earnings	$ 53,378	$ 32,324	$ 40,467	$ 65,473	$ (20,660)	$ 39,372	$ 33,251	$ 12,216	$ 191,642	$ 64,179	$ 105,028
Basic earnings per share (in dollars per share)	$ 1.19	$ 0.72	$ 0.91	$ 1.47	$ (0.46)	$ 0.89	$ 0.75	$ 0.28	$ 4.28	$ 1.45	$ 2.39
Diluted earnings per share (in dollars per share)	$ 1.18	$ 0.71	$ 0.89	$ 1.46	$ (0.46)	$ 0.88	$ 0.74	$ 0.27	$ 4.23	$ 1.43	$ 2.36