INVESTMENTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,397,440	$ 1,337,004
Gross unrealized gains	118,284	99,752
Gross unrealized losses	12,593	23,218
Estimated fair values	1,503,131	1,413,538
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	247,865	422,453
Less than twelve months, Unrealized losses	3,840	16,670
Twelve months or longer, Fair value	192,946	36,223
Twelve months or longer, Unrealized losses	8,753	6,548
Total, Fair value	440,811	458,676
Total, Unrealized losses	12,593	23,218
Amortized cost
Due in one year or less	40,543
Due after one year through five years	173,741
Due after five years through ten years	368,782
Due after ten years	463,572
Securities not due at a single maturity date	206,528
Totals	1,253,166	1,189,525
Estimated fair value
Due in one year or less	41,094
Due after one year through five years	177,883
Due after five years through ten years	376,119
Due after ten years	474,251
Securities not due at a single maturity date	205,669
Totals	1,275,016	1,199,699
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	(1,088)	(1,264)	$ (1,481)
Net realized investment gains (losses)	6,556	4,074	6,153
Investment Income, Net [Abstract]
Interest on fixed maturity securities	39,992	41,499	42,261
Dividends on equity securities	6,032	6,922	5,617
Income on reverse repurchase agreements	306	236	117
Interest on short-term investments	273	121	2
Return on long-term investments	1,126	514	(461)
Total investment income	47,729	49,292	47,536
Securities litigation income	12	111	32
Investment expenses	(2,262)	(1,913)	(1,986)
Net investment income (loss)	45,479	47,490	45,582
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax [Abstract]
Deferred income tax expense (benefit)	10,205	(5,720)	(9,689)
Net changes in unrealized holding gains (losses) net of tax	18,952	(10,621)	(17,993)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,253,166	1,189,525
Gross unrealized gains	33,036	32,485
Gross unrealized losses	11,186	22,311
Estimated fair values	1,275,016	1,199,699
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	232,834	404,167
Less than twelve months, Unrealized losses	2,472	16,129
Twelve months or longer, Fair value	190,985	32,250
Twelve months or longer, Unrealized losses	8,714	6,182
Total, Fair value	423,819	436,417
Total, Unrealized losses	11,186	22,311
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	545	2,054	725
Available-for-sale, Gross realized investment losses	(4,849)	(2,829)	(251)
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax [Abstract]
Net changes in unrealized holding gains (losses) before tax	11,676	(20,634)	(16,685)
Deferred income tax expense (benefit)	4,086	(7,222)	(5,840)
Net changes in unrealized holding gains (losses) net of tax	7,590	(13,412)	(10,845)
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	8,115	7,841
Gross unrealized gains	0	0
Gross unrealized losses	37	11
Estimated fair values	8,078	7,830
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	8,078	7,830
Less than twelve months, Unrealized losses	37	11
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	8,078	7,830
Total, Unrealized losses	37	11
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	303,932	249,495
Gross unrealized gains	122	311
Gross unrealized losses	6,105	10,609
Estimated fair values	297,949	239,197
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	134,284	202,900
Less than twelve months, Unrealized losses	1,491	10,609
Twelve months or longer, Fair value	127,604	0
Twelve months or longer, Unrealized losses	4,614	0
Total, Fair value	261,888	202,900
Total, Unrealized losses	6,105	10,609
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	290,038	319,663
Gross unrealized gains	17,729	17,034
Gross unrealized losses	231	940
Estimated fair values	307,536	335,757
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0	43,777
Less than twelve months, Unrealized losses	0	940
Twelve months or longer, Fair value	14,416	0
Twelve months or longer, Unrealized losses	231	0
Total, Fair value	14,416	43,777
Total, Unrealized losses	231	940
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	84,058	37,964
Gross unrealized gains	591	741
Gross unrealized losses	669	1,133
Estimated fair values	83,980	37,572
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	32,155	21,695
Less than twelve months, Unrealized losses	221	1,133
Twelve months or longer, Fair value	8,530	0
Twelve months or longer, Unrealized losses	448	0
Total, Fair value	40,685	21,695
Total, Unrealized losses	669	1,133
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	120,554	102,307
Gross unrealized gains	2,479	1,435
Gross unrealized losses	3,234	7,308
Estimated fair values	119,799	96,434
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	30,003	26,217
Less than twelve months, Unrealized losses	394	1,232
Twelve months or longer, Fair value	22,948	23,625
Twelve months or longer, Unrealized losses	2,840	6,076
Total, Fair value	52,951	49,842
Total, Unrealized losses	3,234	7,308
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	23,934	26,592
Gross unrealized gains	625	732
Gross unrealized losses	445	931
Estimated fair values	24,114	26,393
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0	19,091
Less than twelve months, Unrealized losses	0	931
Twelve months or longer, Fair value	13,440	0
Twelve months or longer, Unrealized losses	445	0
Total, Fair value	13,440	19,091
Total, Unrealized losses	445	931
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	422,535	445,663
Gross unrealized gains	11,490	12,232
Gross unrealized losses	465	1,379
Estimated fair values	433,560	456,516
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	28,314	82,657
Less than twelve months, Unrealized losses	329	1,273
Twelve months or longer, Fair value	4,047	8,625
Twelve months or longer, Unrealized losses	136	106
Total, Fair value	32,361	91,282
Total, Unrealized losses	465	1,379
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	144,274	147,479
Gross unrealized gains	85,248	67,267
Gross unrealized losses	1,407	907
Estimated fair values	228,115	213,839
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	15,031	18,286
Less than twelve months, Unrealized losses	1,368	541
Twelve months or longer, Fair value	1,961	3,973
Twelve months or longer, Unrealized losses	39	366
Total, Fair value	16,992	22,259
Total, Unrealized losses	1,407	907
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	19,792	15,078	12,741
Available-for-sale, Gross realized investment losses	(1,592)	(2,675)	(4,110)
'Other-than-temporary' impairments	(1,088)	(1,055)	(1,481)
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax [Abstract]
Net changes in unrealized holding gains (losses) before tax	17,481	4,293	(10,997)
Deferred income tax expense (benefit)	6,119	1,502	(3,849)
Net changes in unrealized holding gains (losses) net of tax	11,362	2,791	(7,148)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	30,103	22,922
Gross unrealized gains	13,594	12,410
Gross unrealized losses	175	210
Estimated fair values	43,522	35,122
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,391	1,462
Less than twelve months, Unrealized losses	175	12
Twelve months or longer, Fair value	0	908
Twelve months or longer, Unrealized losses	0	198
Total, Fair value	4,391	2,370
Total, Unrealized losses	175	210
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	18,308	19,832
Gross unrealized gains	17,504	10,739
Gross unrealized losses	2	29
Estimated fair values	35,810	30,542
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	344	1,947
Less than twelve months, Unrealized losses	2	29
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	344	1,947
Total, Unrealized losses	2	29
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	18,877	16,092
Gross unrealized gains	11,876	8,700
Gross unrealized losses	158	85
Estimated fair values	30,595	24,707
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,532	3,585
Less than twelve months, Unrealized losses	158	85
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	2,532	3,585
Total, Unrealized losses	158	85
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	9,275	13,438
Gross unrealized gains	4,917	5,787
Gross unrealized losses	65	125
Estimated fair values	14,127	19,100
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	575	2,427
Less than twelve months, Unrealized losses	65	125
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	575	2,427
Total, Unrealized losses	65	125
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	10,935	14,812
Gross unrealized gains	9,640	7,672
Gross unrealized losses	37	163
Estimated fair values	20,538	22,321
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	992	1,637
Less than twelve months, Unrealized losses	37	163
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	992	1,637
Total, Unrealized losses	37	163
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,441	14,276
Gross unrealized gains	5,381	4,873
Gross unrealized losses	917	78
Estimated fair values	16,905	19,071
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,181	1,621
Less than twelve months, Unrealized losses	917	33
Twelve months or longer, Fair value	0	1,188
Twelve months or longer, Unrealized losses	0	45
Total, Fair value	3,181	2,809
Total, Unrealized losses	917	78
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,746	13,005
Gross unrealized gains	15,757	11,258
Gross unrealized losses	14	18
Estimated fair values	28,489	24,245
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,016	779
Less than twelve months, Unrealized losses	14	18
Twelve months or longer, Fair value	0	0
Twelve months or longer, Unrealized losses	0	0
Total, Fair value	3,016	779
Total, Unrealized losses	14	18
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	11,058	13,071
Gross unrealized gains	5,363	5,345
Gross unrealized losses	0	32
Estimated fair values	16,421	18,384
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		1,472
Less than twelve months, Unrealized losses		32
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		1,472
Total, Unrealized losses		32
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	20,531	20,031
Gross unrealized gains	1,216	483
Gross unrealized losses	39	167
Estimated fair values	21,708	20,347
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0	3,356
Less than twelve months, Unrealized losses	0	44
Twelve months or longer, Fair value	1,961	1,877
Twelve months or longer, Unrealized losses	39	123
Total, Fair value	1,961	5,233
Total, Unrealized losses	39	167
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments		(209)
Net realized investment gains (losses)	$ (6,252)	$ (6,499)	$ (1,471)