UNAUDITED INTERIM FINANCIAL INFORMATION (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended								12 Months Ended
	Jan. 15, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
UNAUDITED INTERIM FINANCIAL INFORMATION
Common stock split conversion ratio	2
Net premiums earned		$ 168,396	$ 163,702	$ 154,553	$ 144,151	$ 147,764	$ 149,943	$ 141,584	$ 137,280	$ 630,802	$ 576,571	$ 538,452
Net investment income		13,432	13,598	12,847	12,886	14,491	14,221	14,826	15,293	52,763	58,831	63,681
Net realized investment gains (losses)		3,611	10,999	3,742	3,684	9,139	5,481	(664)	11,416	22,036	25,372	17,036
Earnings before income taxes		40,812	55,309	43,576	35,969	28,575	36,480	36,568	41,109	175,666	142,732	183,586
Net earnings		$ 33,859	$ 37,647	$ 29,902	$ 24,847	$ 25,097	$ 25,463	$ 24,748	$ 28,038	$ 126,255	$ 103,346	$ 126,598
Basic earnings per share (in dollars per share)		$ 0.79	$ 0.88	$ 0.70	$ 0.58	$ 0.59	$ 0.60	$ 0.58	$ 0.66	$ 2.95	$ 2.44	$ 3.00
Diluted earnings per share (in dollars per share)		$ 0.77	$ 0.86	$ 0.69	$ 0.57	$ 0.58	$ 0.59	$ 0.58	$ 0.65	$ 2.90	$ 2.39	$ 2.95