UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ 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
UNAUDITED INTERIM FINANCIAL INFORMATION
Net premiums earned	$ 187,869	$ 183,595	$ 180,226	$ 176,918	$ 179,371	$ 179,448	$ 172,339	$ 169,003	$ 728,608	$ 700,161	$ 687,375
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
Net realized gains (losses)	11,283	9,252	2,710	11,400	14,207	7,534	4,802	13,286	34,645	39,829	32,182
Earnings before income taxes	38,006	31,142	42,341	45,593	45,582	51,937	54,650	44,513	157,082	196,682	189,487
Net earnings	$ 32,187	$ 22,263	$ 29,077	$ 31,393	$ 33,853	$ 35,908	$ 37,185	$ 30,598	$ 114,920	$ 137,544	$ 135,445
Basic earnings per share (in dollars per share)	$ 0.73	$ 0.51	$ 0.67	$ 0.72	$ 0.78	$ 0.83	$ 0.86	$ 0.71	$ 2.63	$ 3.18	$ 3.15
Diluted earnings per share (in dollars per share)	$ 0.72	$ 0.50	$ 0.65	$ 0.71	$ 0.76	$ 0.81	$ 0.84	$ 0.70	$ 2.59	$ 3.12	$ 3.09