UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
UNAUDITED INTERIM FINANCIAL INFORMATION
Net premiums earned	$ 179,371	$ 179,448	$ 172,339	$ 169,003	$ 179,892	$ 177,747	$ 168,604	$ 161,132	$ 700,161	$ 687,375	$ 630,802
Net investment income	13,754	13,964	13,431	13,495	13,844	14,200	13,982	13,582	54,644	55,608	52,763
Net realized gains (losses)	14,207	7,534	4,802	13,286	9,542	5,708	10,431	6,501	39,829	32,182	22,036
Earnings before income taxes	45,582	51,937	54,650	44,513	46,479	48,594	52,423	41,991	196,682	189,487	175,666
Net earnings	$ 33,853	$ 35,908	$ 37,185	$ 30,598	$ 37,497	$ 33,254	$ 35,725	$ 28,969	$ 137,544	$ 135,445	$ 126,255
Basic earnings per share (in dollars per share)	$ 0.78	$ 0.83	$ 0.86	$ 0.71	$ 0.87	$ 0.77	$ 0.83	$ 0.67	$ 3.18	$ 3.15	$ 2.95
Diluted earnings per share (in dollars per share)	$ 0.76	$ 0.81	$ 0.84	$ 0.70	$ 0.85	$ 0.76	$ 0.82	$ 0.66	$ 3.12	$ 3.09	$ 2.90