INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,384,200		$ 1,384,200		$ 1,337,004
Gross unrealized gains	119,612		119,612		99,752
Gross unrealized losses	13,753		13,753		23,218
Estimated fair values	1,490,059		1,490,059		1,413,538
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	175,958		175,958		422,453
Less than twelve months, Unrealized losses	3,501		3,501		16,670
Twelve months or longer, Fair value	204,024		204,024		36,223
Twelve months or longer, Unrealized losses	10,252		10,252		6,548
Total, Fair value	379,982		379,982		458,676
Total, Unrealized losses	13,753		13,753		23,218
Amortized cost
Due in one year or less	51,964		51,964
Due after one year through five years	167,769		167,769
Due after five years through ten years	378,169		378,169
Due after ten years	462,916		462,916
Securities not due at a single maturity date	172,954		172,954
Totals	1,233,772		1,233,772		1,189,525
Estimated fair value
Due in one year or less	52,464		52,464
Due after one year through five years	173,934		173,934
Due after five years through ten years	387,220		387,220
Due after ten years	473,065		473,065
Securities not due at a single maturity date	171,657		171,657
Totals	1,258,340		1,258,340		1,199,699
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	(355)	$ (275)	(1,088)	$ (976)
Net realized investment gains (losses)	(594)	(1,192)	2,166	(643)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,233,772		1,233,772		1,189,525
Gross unrealized gains	37,181		37,181		32,485
Gross unrealized losses	12,613		12,613		22,311
Estimated fair values	1,258,340		1,258,340		1,199,699
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	164,692		164,692		404,167
Less than twelve months, Unrealized losses	2,367		2,367		16,129
Twelve months or longer, Fair value	201,927		201,927		32,250
Twelve months or longer, Unrealized losses	10,246		10,246		6,182
Total, Fair value	366,619		366,619		436,417
Total, Unrealized losses	12,613		12,613		22,311
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	29	261	408	1,935
Available-for-sale, Gross realized investment losses	(230)	(218)	(2,316)	(517)
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	8,109		8,109		7,841
Gross unrealized gains	57		57		0
Gross unrealized losses	2		2		11
Estimated fair values	8,164		8,164		7,830
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	248		248		7,830
Less than twelve months, Unrealized losses	2		2		11
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	248		248		7,830
Total, Unrealized losses	2		2		11
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	262,050		262,050		249,495
Gross unrealized gains	237		237		311
Gross unrealized losses	6,668		6,668		10,609
Estimated fair values	255,619		255,619		239,197
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	104,235		104,235		202,900
Less than twelve months, Unrealized losses	1,695		1,695		10,609
Twelve months or longer, Fair value	125,982		125,982		0
Twelve months or longer, Unrealized losses	4,973		4,973		0
Total, Fair value	230,217		230,217		202,900
Total, Unrealized losses	6,668		6,668		10,609
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	318,016		318,016		319,663
Gross unrealized gains	18,954		18,954		17,034
Gross unrealized losses	332		332		940
Estimated fair values	336,638		336,638		335,757
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0		43,777
Less than twelve months, Unrealized losses	0		0		940
Twelve months or longer, Fair value	14,372		14,372		0
Twelve months or longer, Unrealized losses	332		332		0
Total, Fair value	14,372		14,372		43,777
Total, Unrealized losses	332		332		940
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	66,138		66,138		37,964
Gross unrealized gains	583		583		741
Gross unrealized losses	911		911		1,133
Estimated fair values	65,810		65,810		37,572
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	26,070		26,070		21,695
Less than twelve months, Unrealized losses	133		133		1,133
Twelve months or longer, Fair value	15,653		15,653		0
Twelve months or longer, Unrealized losses	778		778		0
Total, Fair value	41,723		41,723		21,695
Total, Unrealized losses	911		911		1,133
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	104,620		104,620		102,307
Gross unrealized gains	2,593		2,593		1,435
Gross unrealized losses	3,547		3,547		7,308
Estimated fair values	103,666		103,666		96,434
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	19,872		19,872		26,217
Less than twelve months, Unrealized losses	431		431		1,232
Twelve months or longer, Fair value	21,155		21,155		23,625
Twelve months or longer, Unrealized losses	3,116		3,116		6,076
Total, Fair value	41,027		41,027		49,842
Total, Unrealized losses	3,547		3,547		7,308
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	24,501		24,501		26,592
Gross unrealized gains	756		756		732
Gross unrealized losses	290		290		931
Estimated fair values	24,967		24,967		26,393
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	5,211		5,211		19,091
Less than twelve months, Unrealized losses	39		39		931
Twelve months or longer, Fair value	8,809		8,809		0
Twelve months or longer, Unrealized losses	251		251		0
Total, Fair value	14,020		14,020		19,091
Total, Unrealized losses	290		290		931
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	450,338		450,338		445,663
Gross unrealized gains	14,001		14,001		12,232
Gross unrealized losses	863		863		1,379
Estimated fair values	463,476		463,476		456,516
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	9,056		9,056		82,657
Less than twelve months, Unrealized losses	67		67		1,273
Twelve months or longer, Fair value	15,956		15,956		8,625
Twelve months or longer, Unrealized losses	796		796		106
Total, Fair value	25,012		25,012		91,282
Total, Unrealized losses	863		863		1,379
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	150,428		150,428		147,479
Gross unrealized gains	82,431		82,431		67,267
Gross unrealized losses	1,140		1,140		907
Estimated fair values	231,719		231,719		213,839
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	11,266		11,266		18,286
Less than twelve months, Unrealized losses	1,134		1,134		541
Twelve months or longer, Fair value	2,097		2,097		3,973
Twelve months or longer, Unrealized losses	6		6		366
Total, Fair value	13,363		13,363		22,259
Total, Unrealized losses	1,140		1,140		907
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	1,396	1,001	10,669	5,849
Available-for-sale, Gross realized investment losses	(435)	(50)	(906)	(1,683)
'Other-than-temporary' impairments	(355)	(275)	(1,088)	(976)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	28,130		28,130		22,922
Gross unrealized gains	12,679		12,679		12,410
Gross unrealized losses	71		71		210
Estimated fair values	40,738		40,738		35,122
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,964		2,964		1,462
Less than twelve months, Unrealized losses	71		71		12
Twelve months or longer, Fair value	0		0		908
Twelve months or longer, Unrealized losses	0		0		198
Total, Fair value	2,964		2,964		2,370
Total, Unrealized losses	71		71		210
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	19,772		19,772		19,832
Gross unrealized gains	16,912		16,912		10,739
Gross unrealized losses	0		0		29
Estimated fair values	36,684		36,684		30,542
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					1,947
Less than twelve months, Unrealized losses					29
Twelve months or longer, Fair value					0
Twelve months or longer, Unrealized losses					0
Total, Fair value					1,947
Total, Unrealized losses					29
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	18,417		18,417		16,092
Gross unrealized gains	12,457		12,457		8,700
Gross unrealized losses	144		144		85
Estimated fair values	30,730		30,730		24,707
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,594		1,594		3,585
Less than twelve months, Unrealized losses	144		144		85
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	1,594		1,594		3,585
Total, Unrealized losses	144		144		85
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	10,258		10,258		13,438
Gross unrealized gains	4,794		4,794		5,787
Gross unrealized losses	8		8		125
Estimated fair values	15,044		15,044		19,100
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	159		159		2,427
Less than twelve months, Unrealized losses	8		8		125
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	159		159		2,427
Total, Unrealized losses	8		8		125
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,224		12,224		14,812
Gross unrealized gains	9,464		9,464		7,672
Gross unrealized losses	83		83		163
Estimated fair values	21,605		21,605		22,321
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,023		1,023		1,637
Less than twelve months, Unrealized losses	79		79		163
Twelve months or longer, Fair value	99		99		0
Twelve months or longer, Unrealized losses	4		4		0
Total, Fair value	1,122		1,122		1,637
Total, Unrealized losses	83		83		163
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,273		13,273		14,276
Gross unrealized gains	4,692		4,692		4,873
Gross unrealized losses	769		769		78
Estimated fair values	17,196		17,196		19,071
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,323		4,323		1,621
Less than twelve months, Unrealized losses	769		769		33
Twelve months or longer, Fair value	0		0		1,188
Twelve months or longer, Unrealized losses	0		0		45
Total, Fair value	4,323		4,323		2,809
Total, Unrealized losses	769		769		78
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,209		13,209		13,005
Gross unrealized gains	15,138		15,138		11,258
Gross unrealized losses	32		32		18
Estimated fair values	28,315		28,315		24,245
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	309		309		779
Less than twelve months, Unrealized losses	32		32		18
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	309		309		779
Total, Unrealized losses	32		32		18
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,114		12,114		13,071
Gross unrealized gains	4,704		4,704		5,345
Gross unrealized losses	31		31		32
Estimated fair values	16,787		16,787		18,384
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	894		894		1,472
Less than twelve months, Unrealized losses	31		31		32
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	894		894		1,472
Total, Unrealized losses	31		31		32
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	23,031		23,031		20,031
Gross unrealized gains	1,591		1,591		483
Gross unrealized losses	2		2		167
Estimated fair values	24,620		24,620		20,347
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0		3,356
Less than twelve months, Unrealized losses	0		0		44
Twelve months or longer, Fair value	1,998		1,998		1,877
Twelve months or longer, Unrealized losses	2		2		123
Total, Fair value	1,998		1,998		5,233
Total, Unrealized losses	2		2		$ 167
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Net realized investment gains (losses)	$ (999)	$ (1,911)	$ (4,601)	$ (5,251)