SEGMENT INFORMATION (Segments) (Details) - USD ($) $ 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
Segment Reporting Information [Line Items]
Net premiums earned									$ 1,023,401	$ 930,890	$ 828,330
Investment income, net of investment expenses									106,822	100,781	104,609
Net realized investment gains									6,103	2,846	7,270
Other income									621	401	1,685
Total revenues	$ 301,995	$ 289,494	$ 279,773	$ 265,685	$ 275,956	$ 264,560	$ 255,918	$ 238,484	1,136,947	1,034,918	941,894
Benefit, losses and expenses									1,078,280	913,467	865,431
Income before income taxes	13,854	15,175	864	28,774	43,263	26,824	19,533	31,831	58,667	121,451	76,463
Income tax expense									8,763	32,325	17,326
Net income	11,995	$ 12,368	$ 3,114	$ 22,427	30,895	$ 19,534	$ 15,018	$ 23,679	49,904	89,126	59,137
Total assets	4,054,758				3,890,376				4,054,758	3,890,376	3,856,689
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net premiums earned									936,131	851,695	766,939
Total revenues									996,362	899,271	816,263
Income before income taxes									57,497	115,370	66,025
Income tax expense									8,379	30,050	13,649
Net income									49,118	85,320	52,376
Total assets	2,449,140				2,280,674				2,449,140	2,280,674	2,127,623
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									87,270	79,984	61,921
Total revenues									140,585	135,647	125,631
Income before income taxes									1,170	6,081	10,438
Income tax expense									384	2,275	3,677
Net income									786	3,806	6,761
Total assets	1,605,618				1,609,702				1,605,618	1,609,702	1,729,066
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net premiums earned									936,131	851,695	766,939
Investment income, net of investment expenses									55,441	46,606	44,219
Net realized investment gains									4,947	1,124	4,177
Other income									0	(107)	910
Total revenues									996,519	899,318	816,245
Benefit, losses and expenses									938,864	784,691	750,768
Total net income before eliminations									57,655	114,627	65,477
Total assets	2,662,272				2,490,138				2,662,272	2,490,138	2,360,764
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									87,270	79,984	61,921
Investment income, net of investment expenses									51,538	54,222	60,373
Net realized investment gains									1,156	1,722	3,093
Other income									621	508	774
Total revenues									140,585	136,436	126,161
Benefit, losses and expenses									139,671	129,771	115,361
Total net income before eliminations									914	6,665	10,800
Intersegment eliminations | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total revenues									(157)	(47)	18
Intersegment eliminations									(158)	743	548
Total assets	$ (213,132)				$ (209,464)				(213,132)	(209,464)	(233,141)
Intersegment eliminations | Life Insurance
Segment Reporting Information [Line Items]
Total revenues									0	(789)	(530)
Intersegment eliminations									$ 256	$ (584)	$ (362)