INVESTMENTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	$ 1,273,132	$ 1,253,166
Fixed maturity securities available-for-sale	1,282,909	1,275,016
Equity securities available-for-sale, Amortized cost		144,274
Equity securities available-for-sale		228,115
Available-for-sale securities, Amortized cost		1,397,440
Available-for-sale securities, Gross unrealized gains		118,284
Available-for-sale securities, Gross unrealized losses		12,593
Available-for-sale Securities		1,503,131
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		247,865
Less than twelve months, Unrealized losses		3,840
Twelve months or longer, Fair value		192,946
Twelve months or longer, Unrealized losses		8,753
Total, Fair value		440,811
Total, Unrealized losses		12,593
Amortized cost
Due in one year or less	32,551
Due after one year through five years	240,690
Due after five years through ten years	320,118
Due after ten years	433,584
Securities not due at a single maturity date	246,189
Fixed maturity securities available-for-sale, Amortized cost	1,273,132	1,253,166
Estimated fair value
Due in one year or less	32,703
Due after one year through five years	241,229
Due after five years through ten years	320,931
Due after ten years	440,277
Securities not due at a single maturity date	247,769
Totals	1,282,909	1,275,016
Realized Investment Gains (Losses) [Abstract]
Fixed maturity secutities available-for-sale, gross realized gains	324
Fixed maturity securities available-for-sale, gross realized losses	18,872
Change in unrealized investment gains	(28,838)
Equity securities:	8,017
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments	(41,252)	6,556	$ 4,074
Investment Income, Net [Abstract]
Interest on fixed maturity securities	42,961	39,992	41,499
Dividends on equity securities	6,078	6,032	6,922
Income on reverse repurchase agreements	339	306	236
Interest on short-term investments	608	273	121
Return on long-term investments	134	1,126	514
Total investment income	50,120	47,729	49,292
Securities litigation income	14	12	111
Investment expenses	(2,497)	(2,262)	(1,913)
Net investment income	47,637	45,479	47,490
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease Abstract
Deferred income tax expense (benefit)	(2,534)	10,205	(5,720)
Net changes in unrealized holding gains (losses) net of tax	(9,539)	18,952	(10,621)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	1,273,132	1,253,166
Fixed maturity securities available-or-sale, Gross unrealized gains	20,713	33,036
Fixed maturity securities available-for-sale, Gross unrealized losses	10,936	11,186
Fixed maturity securities available-for-sale	1,282,909	1,275,016
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	236,333	232,834
Less than twelve months, Unrealized losses	3,329	2,472
Twelve months or longer, Fair value	242,366	190,985
Twelve months or longer, Unrealized losses	7,607	8,714
Total, Fair value	478,699	423,819
Total, Unrealized losses	10,936	11,186
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	1,273,132	1,253,166
Estimated fair value
Totals	1,282,909	1,275,016
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains		545	2,054
Available-for-sale, Gross realized investment losses		(4,849)	(2,829)
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease Abstract
Net changes in unrealized holding gains (losses) before tax	(12,073)	11,676	(20,634)
Deferred income tax expense (benefit)	(2,534)	4,086	(7,222)
Net changes in unrealized holding gains (losses) net of tax	(9,539)	7,590	(13,412)
U.S. treasury
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	8,139	8,115
Fixed maturity securities available-or-sale, Gross unrealized gains	0	0
Fixed maturity securities available-for-sale, Gross unrealized losses	118	37
Fixed maturity securities available-for-sale	8,021	8,078
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0	8,078
Less than twelve months, Unrealized losses	0	37
Twelve months or longer, Fair value	8,021	0
Twelve months or longer, Unrealized losses	118	0
Total, Fair value	8,021	8,078
Total, Unrealized losses	118	37
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	8,139	8,115
Estimated fair value
Totals	8,021	8,078
U.S. government-sponsored agencies
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	303,198	303,932
Fixed maturity securities available-or-sale, Gross unrealized gains	2,799	122
Fixed maturity securities available-for-sale, Gross unrealized losses	1,518	6,105
Fixed maturity securities available-for-sale	304,479	297,949
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	14,620	134,284
Less than twelve months, Unrealized losses	20	1,491
Twelve months or longer, Fair value	92,603	127,604
Twelve months or longer, Unrealized losses	1,498	4,614
Total, Fair value	107,223	261,888
Total, Unrealized losses	1,518	6,105
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	303,198	303,932
Estimated fair value
Totals	304,479	297,949
Obligations of states and political subdivisions
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	273,727	290,038
Fixed maturity securities available-or-sale, Gross unrealized gains	10,375	17,729
Fixed maturity securities available-for-sale, Gross unrealized losses	451	231
Fixed maturity securities available-for-sale	283,651	307,536
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0	0
Less than twelve months, Unrealized losses	0	0
Twelve months or longer, Fair value	14,498	14,416
Twelve months or longer, Unrealized losses	451	231
Total, Fair value	14,498	14,416
Total, Unrealized losses	451	231
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	273,727	290,038
Estimated fair value
Totals	283,651	307,536
Commercial mortgage-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	83,854	84,058
Fixed maturity securities available-or-sale, Gross unrealized gains	1,287	591
Fixed maturity securities available-for-sale, Gross unrealized losses	762	669
Fixed maturity securities available-for-sale	84,379	83,980
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,021	32,155
Less than twelve months, Unrealized losses	21	221
Twelve months or longer, Fair value	24,222	8,530
Twelve months or longer, Unrealized losses	741	448
Total, Fair value	26,243	40,685
Total, Unrealized losses	762	669
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	83,854	84,058
Estimated fair value
Totals	84,379	83,980
Residential mortgage-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	161,055	120,554
Fixed maturity securities available-or-sale, Gross unrealized gains	3,374	2,479
Fixed maturity securities available-for-sale, Gross unrealized losses	2,292	3,234
Fixed maturity securities available-for-sale	162,137	119,799
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	16,852	30,003
Less than twelve months, Unrealized losses	145	394
Twelve months or longer, Fair value	45,597	22,948
Twelve months or longer, Unrealized losses	2,147	2,840
Total, Fair value	62,449	52,951
Total, Unrealized losses	2,292	3,234
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	161,055	120,554
Estimated fair value
Totals	162,137	119,799
Other asset-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	21,596	23,934
Fixed maturity securities available-or-sale, Gross unrealized gains	273	625
Fixed maturity securities available-for-sale, Gross unrealized losses	1,035	445
Fixed maturity securities available-for-sale	20,834	24,114
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,810	0
Less than twelve months, Unrealized losses	147	0
Twelve months or longer, Fair value	11,691	13,440
Twelve months or longer, Unrealized losses	888	445
Total, Fair value	16,501	13,440
Total, Unrealized losses	1,035	445
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	21,596	23,934
Estimated fair value
Totals	20,834	24,114
Corporate
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	421,563	422,535
Fixed maturity securities available-or-sale, Gross unrealized gains	2,605	11,490
Fixed maturity securities available-for-sale, Gross unrealized losses	4,760	465
Fixed maturity securities available-for-sale	419,408	433,560
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	198,030	28,314
Less than twelve months, Unrealized losses	2,996	329
Twelve months or longer, Fair value	45,734	4,047
Twelve months or longer, Unrealized losses	1,764	136
Total, Fair value	243,764	32,361
Total, Unrealized losses	4,760	465
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	421,563	422,535
Estimated fair value
Totals	419,408	433,560
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		144,274
Equity securities available-for-sale, Gross unrealized gains		85,248
Equity securities available-for-sale, Gross unrealized loss		1,407
Equity securities available-for-sale		228,115
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		15,031
Less than twelve months, Unrealized losses		1,368
Twelve months or longer, Fair value		1,961
Twelve months or longer, Unrealized losses		39
Total, Fair value		16,992
Total, Unrealized losses		1,407
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments		(1,088)	(1,055)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		18,200	12,403
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease Abstract
Net changes in unrealized holding gains (losses) before tax		17,481	4,293
Deferred income tax expense (benefit)		6,119	1,502
Net changes in unrealized holding gains (losses) net of tax		11,362	2,791
Financial services
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		30,103
Equity securities available-for-sale, Gross unrealized gains		13,594
Equity securities available-for-sale, Gross unrealized loss		175
Equity securities available-for-sale		43,522
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		4,391
Less than twelve months, Unrealized losses		175
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		4,391
Total, Unrealized losses		175
Information technology
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		18,308
Equity securities available-for-sale, Gross unrealized gains		17,504
Equity securities available-for-sale, Gross unrealized loss		2
Equity securities available-for-sale		35,810
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		344
Less than twelve months, Unrealized losses		2
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		344
Total, Unrealized losses		2
Healthcare
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		18,877
Equity securities available-for-sale, Gross unrealized gains		11,876
Equity securities available-for-sale, Gross unrealized loss		158
Equity securities available-for-sale		30,595
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		2,532
Less than twelve months, Unrealized losses		158
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		2,532
Total, Unrealized losses		158
Consumer staples
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		9,275
Equity securities available-for-sale, Gross unrealized gains		4,917
Equity securities available-for-sale, Gross unrealized loss		65
Equity securities available-for-sale		14,127
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		575
Less than twelve months, Unrealized losses		65
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		575
Total, Unrealized losses		65
Consumer discretionary
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		10,935
Equity securities available-for-sale, Gross unrealized gains		9,640
Equity securities available-for-sale, Gross unrealized loss		37
Equity securities available-for-sale		20,538
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		992
Less than twelve months, Unrealized losses		37
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		992
Total, Unrealized losses		37
Energy
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		12,441
Equity securities available-for-sale, Gross unrealized gains		5,381
Equity securities available-for-sale, Gross unrealized loss		917
Equity securities available-for-sale		16,905
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		3,181
Less than twelve months, Unrealized losses		917
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		3,181
Total, Unrealized losses		917
Industrials
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		12,746
Equity securities available-for-sale, Gross unrealized gains		15,757
Equity securities available-for-sale, Gross unrealized loss		14
Equity securities available-for-sale		28,489
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		3,016
Less than twelve months, Unrealized losses		14
Twelve months or longer, Fair value		0
Twelve months or longer, Unrealized losses		0
Total, Fair value		3,016
Total, Unrealized losses		14
Other
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		11,058
Equity securities available-for-sale, Gross unrealized gains		5,363
Equity securities available-for-sale, Gross unrealized loss		0
Equity securities available-for-sale		16,421
Non-redeemable preferred stocks
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Equity securities available-for-sale, Amortized cost		20,531
Equity securities available-for-sale, Gross unrealized gains		1,216
Equity securities available-for-sale, Gross unrealized loss		39
Equity securities available-for-sale		21,708
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value		0
Less than twelve months, Unrealized losses		0
Twelve months or longer, Fair value		1,961
Twelve months or longer, Unrealized losses		39
Total, Fair value		1,961
Total, Unrealized losses		39
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments		(209)
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments	$ (1,883)	$ (6,252)	$ (6,499)