INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	$ 1,247,717		$ 1,247,717		$ 1,253,166
Fixed maturity securities available-for-sale	1,241,699		1,241,699		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	39,947		39,947
Due after one year through five years	207,717		207,717
Due after five years through ten years	366,053		366,053
Due after ten years	422,003		422,003
Securities not due at a single maturity date	211,997		211,997
Fixed maturity securities available-for-sale, Amortized cost	1,247,717		1,247,717		1,253,166
Estimated fair value
Due in one year or less	40,411		40,411
Due after one year through five years	208,900		208,900
Due after five years through ten years	362,780		362,780
Due after ten years	423,852		423,852
Securities not due at a single maturity date	205,756		205,756
Totals	1,241,699		1,241,699		1,275,016
Realized Investment Gains (Losses) [Abstract]
Fixed maturity securities available-for-sale gross realized gains	0		234
Fixed maturity securities available-for-sale gross realized losses	(5,490)		(5,968)
Equity securities:	1,479		4,195
Change in unrealized investment gains	(447)		(10,301)
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments	(5,860)	$ 3,387	(11,253)	$ 2,760
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	1,247,717		1,247,717		1,253,166
Fixed maturity securities available-for-sale, Gross unrealized gains	17,520		17,520		33,036
Fixed maturity securities available-for-sale, Gross unrealized losses	23,538		23,538		11,186
Fixed maturity securities available-for-sale	1,241,699		1,241,699		1,275,016
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	565,632		565,632		232,834
Less than twelve months, Unrealized losses	14,486		14,486		2,472
Twelve months or longer, Fair value	132,990		132,990		190,985
Twelve months or longer, Unrealized losses	9,052		9,052		8,714
Total, Fair value	698,622		698,622		423,819
Total, Unrealized losses	23,538		23,538		11,186
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	1,247,717		1,247,717		1,253,166
Estimated fair value
Totals	1,241,699		1,241,699		1,275,016
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains		289		379
Available-for-sale, Gross realized investment losses		(880)		(2,086)
U.S. treasury
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	8,127		8,127		8,115
Fixed maturity securities available-for-sale, Gross unrealized gains	0		0		0
Fixed maturity securities available-for-sale, Gross unrealized losses	213		213		37
Fixed maturity securities available-for-sale	7,914		7,914		8,078
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	7,914		7,914		8,078
Less than twelve months, Unrealized losses	213		213		37
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	7,914		7,914		8,078
Total, Unrealized losses	213		213		37
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	8,127		8,127		8,115
Estimated fair value
Totals	7,914		7,914		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	305,833		305,833		303,932
Fixed maturity securities available-for-sale, Gross unrealized gains	519		519		122
Fixed maturity securities available-for-sale, Gross unrealized losses	10,128		10,128		6,105
Fixed maturity securities available-for-sale	296,224		296,224		297,949
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	187,074		187,074		134,284
Less than twelve months, Unrealized losses	5,498		5,498		1,491
Twelve months or longer, Fair value	75,882		75,882		127,604
Twelve months or longer, Unrealized losses	4,630		4,630		4,614
Total, Fair value	262,956		262,956		261,888
Total, Unrealized losses	10,128		10,128		6,105
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	305,833		305,833		303,932
Estimated fair value
Totals	296,224		296,224		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	278,869		278,869		290,038
Fixed maturity securities available-for-sale, Gross unrealized gains	11,448		11,448		17,729
Fixed maturity securities available-for-sale, Gross unrealized losses	587		587		231
Fixed maturity securities available-for-sale	289,730		289,730		307,536
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	12,195		12,195		0
Less than twelve months, Unrealized losses	60		60		0
Twelve months or longer, Fair value	14,002		14,002		14,416
Twelve months or longer, Unrealized losses	527		527		231
Total, Fair value	26,197		26,197		14,416
Total, Unrealized losses	587		587		231
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	278,869		278,869		290,038
Estimated fair value
Totals	289,730		289,730		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	78,977		78,977		84,058
Fixed maturity securities available-for-sale, Gross unrealized gains	222		222		591
Fixed maturity securities available-for-sale, Gross unrealized losses	3,523		3,523		669
Fixed maturity securities available-for-sale	75,676		75,676		83,980
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	60,331		60,331		32,155
Less than twelve months, Unrealized losses	2,750		2,750		221
Twelve months or longer, Fair value	8,193		8,193		8,530
Twelve months or longer, Unrealized losses	773		773		448
Total, Fair value	68,524		68,524		40,685
Total, Unrealized losses	3,523		3,523		669
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	78,977		78,977		84,058
Estimated fair value
Totals	75,676		75,676		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	131,427		131,427		120,554
Fixed maturity securities available-for-sale, Gross unrealized gains	1,772		1,772		2,479
Fixed maturity securities available-for-sale, Gross unrealized losses	4,669		4,669		3,234
Fixed maturity securities available-for-sale	128,530		128,530		119,799
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	97,256		97,256		30,003
Less than twelve months, Unrealized losses	2,616		2,616		394
Twelve months or longer, Fair value	18,781		18,781		22,948
Twelve months or longer, Unrealized losses	2,053		2,053		2,840
Total, Fair value	116,037		116,037		52,951
Total, Unrealized losses	4,669		4,669		3,234
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	131,427		131,427		120,554
Estimated fair value
Totals	128,530		128,530		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	22,325		22,325		23,934
Fixed maturity securities available-for-sale, Gross unrealized gains	351		351		625
Fixed maturity securities available-for-sale, Gross unrealized losses	1,005		1,005		445
Fixed maturity securities available-for-sale	21,671		21,671		24,114
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,976		4,976		0
Less than twelve months, Unrealized losses	122		122		0
Twelve months or longer, Fair value	12,134		12,134		13,440
Twelve months or longer, Unrealized losses	883		883		445
Total, Fair value	17,110		17,110		13,440
Total, Unrealized losses	1,005		1,005		445
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	22,325		22,325		23,934
Estimated fair value
Totals	21,671		21,671		24,114
Corporate
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	422,159		422,159		422,535
Fixed maturity securities available-for-sale, Gross unrealized gains	3,208		3,208		11,490
Fixed maturity securities available-for-sale, Gross unrealized losses	3,413		3,413		465
Fixed maturity securities available-for-sale	421,954		421,954		433,560
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	195,886		195,886		28,314
Less than twelve months, Unrealized losses	3,227		3,227		329
Twelve months or longer, Fair value	3,998		3,998		4,047
Twelve months or longer, Unrealized losses	186		186		136
Total, Fair value	199,884		199,884		32,361
Total, Unrealized losses	3,413		3,413		465
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	422,159		422,159		422,535
Estimated fair value
Totals	421,954		421,954		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]
Net realized investment gains, excluding other-than-temporary impairments		6,026		8,802
Other-than-temporary impairments		(733)		(733)
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]
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments	$ (1,402)	$ (1,315)	$ 587	$ (3,602)