INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,348,947		$ 1,348,947		$ 1,337,004
Gross unrealized gains	112,422		112,422		99,752
Gross unrealized losses	14,590		14,590		23,218
Estimated fair values	1,446,779		1,446,779		1,413,538
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	322,596		322,596		422,453
Less than twelve months, Unrealized losses	9,504		9,504		16,670
Twelve months or longer, Fair value	48,279		48,279		36,223
Twelve months or longer, Unrealized losses	5,086		5,086		6,548
Total, Fair value	370,875		370,875		458,676
Total, Unrealized losses	14,590		14,590		23,218
Amortized cost
Due in one year or less	67,465		67,465
Due after one year through five years	158,450		158,450
Due after five years through ten years	337,009		337,009
Due after ten years	488,531		488,531
Securities not due at a single maturity date	149,344		149,344
Totals	1,200,799		1,200,799		1,189,525
Estimated fair value
Due in one year or less	68,242		68,242
Due after one year through five years	164,964		164,964
Due after five years through ten years	345,245		345,245
Due after ten years	499,403		499,403
Securities not due at a single maturity date	147,274		147,274
Totals	1,225,128		1,225,128		1,199,699
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	(733)	$ (270)	(733)	$ (701)
Net realized investment gains (losses)	3,387	1,634	2,760	549
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,200,799		1,200,799		1,189,525
Gross unrealized gains	37,807		37,807		32,485
Gross unrealized losses	13,478		13,478		22,311
Estimated fair values	1,225,128		1,225,128		1,199,699
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	310,076		310,076		404,167
Less than twelve months, Unrealized losses	8,727		8,727		16,129
Twelve months or longer, Fair value	44,327		44,327		32,250
Twelve months or longer, Unrealized losses	4,751		4,751		6,182
Total, Fair value	354,403		354,403		436,417
Total, Unrealized losses	13,478		13,478		22,311
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	289	1,005	379	1,674
Available-for-sale, Gross realized investment losses	(880)	0	(2,086)	(299)
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	7,853		7,853		7,841
Gross unrealized gains	62		62		0
Gross unrealized losses	0		0		11
Estimated fair values	7,915		7,915		7,830
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					7,830
Less than twelve months, Unrealized losses					11
Twelve months or longer, Fair value					0
Twelve months or longer, Unrealized losses					0
Total, Fair value					7,830
Total, Unrealized losses					11
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	243,258		243,258		249,495
Gross unrealized gains	249		249		311
Gross unrealized losses	6,775		6,775		10,609
Estimated fair values	236,732		236,732		239,197
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	194,957		194,957		202,900
Less than twelve months, Unrealized losses	6,641		6,641		10,609
Twelve months or longer, Fair value	4,867		4,867		0
Twelve months or longer, Unrealized losses	134		134		0
Total, Fair value	199,824		199,824		202,900
Total, Unrealized losses	6,775		6,775		10,609
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	320,598		320,598		319,663
Gross unrealized gains	19,711		19,711		17,034
Gross unrealized losses	348		348		940
Estimated fair values	339,961		339,961		335,757
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	14,415		14,415		43,777
Less than twelve months, Unrealized losses	348		348		940
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	14,415		14,415		43,777
Total, Unrealized losses	348		348		940
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	59,214		59,214		37,964
Gross unrealized gains	683		683		741
Gross unrealized losses	1,065		1,065		1,133
Estimated fair values	58,832		58,832		37,572
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	43,939		43,939		21,695
Less than twelve months, Unrealized losses	1,065		1,065		1,133
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	43,939		43,939		21,695
Total, Unrealized losses	1,065		1,065		1,133
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	87,921		87,921		102,307
Gross unrealized gains	2,481		2,481		1,435
Gross unrealized losses	4,151		4,151		7,308
Estimated fair values	86,251		86,251		96,434
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	13,671		13,671		26,217
Less than twelve months, Unrealized losses	426		426		1,232
Twelve months or longer, Fair value	18,491		18,491		23,625
Twelve months or longer, Unrealized losses	3,725		3,725		6,076
Total, Fair value	32,162		32,162		49,842
Total, Unrealized losses	4,151		4,151		7,308
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	24,696		24,696		26,592
Gross unrealized gains	796		796		732
Gross unrealized losses	472		472		931
Estimated fair values	25,020		25,020		26,393
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	7,336		7,336		19,091
Less than twelve months, Unrealized losses	123		123		931
Twelve months or longer, Fair value	6,516		6,516		0
Twelve months or longer, Unrealized losses	349		349		0
Total, Fair value	13,852		13,852		19,091
Total, Unrealized losses	472		472		931
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	457,259		457,259		445,663
Gross unrealized gains	13,825		13,825		12,232
Gross unrealized losses	667		667		1,379
Estimated fair values	470,417		470,417		456,516
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	35,758		35,758		82,657
Less than twelve months, Unrealized losses	124		124		1,273
Twelve months or longer, Fair value	14,453		14,453		8,625
Twelve months or longer, Unrealized losses	543		543		106
Total, Fair value	50,211		50,211		91,282
Total, Unrealized losses	667		667		1,379
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	148,148		148,148		147,479
Gross unrealized gains	74,615		74,615		67,267
Gross unrealized losses	1,112		1,112		907
Estimated fair values	221,651		221,651		213,839
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	12,520		12,520		18,286
Less than twelve months, Unrealized losses	777		777		541
Twelve months or longer, Fair value	3,952		3,952		3,973
Twelve months or longer, Unrealized losses	335		335		366
Total, Fair value	16,472		16,472		22,259
Total, Unrealized losses	1,112		1,112		907
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	6,398	2,766	9,273	4,848
Available-for-sale, Gross realized investment losses	(372)	(441)	(471)	(1,633)
'Other-than-temporary' impairments	(733)	(270)	(733)	(701)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	25,852		25,852		22,922
Gross unrealized gains	12,415		12,415		12,410
Gross unrealized losses	146		146		210
Estimated fair values	38,121		38,121		35,122
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,407		1,407		1,462
Less than twelve months, Unrealized losses	20		20		12
Twelve months or longer, Fair value	945		945		908
Twelve months or longer, Unrealized losses	126		126		198
Total, Fair value	2,352		2,352		2,370
Total, Unrealized losses	146		146		210
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	21,830		21,830		19,832
Gross unrealized gains	14,141		14,141		10,739
Gross unrealized losses	109		109		29
Estimated fair values	35,862		35,862		30,542
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,313		2,313		1,947
Less than twelve months, Unrealized losses	109		109		29
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	2,313		2,313		1,947
Total, Unrealized losses	109		109		29
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	16,820		16,820		16,092
Gross unrealized gains	11,071		11,071		8,700
Gross unrealized losses	0		0		85
Estimated fair values	27,891		27,891		24,707
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					3,585
Less than twelve months, Unrealized losses					85
Twelve months or longer, Fair value					0
Twelve months or longer, Unrealized losses					0
Total, Fair value					3,585
Total, Unrealized losses					85
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	10,894		10,894		13,438
Gross unrealized gains	5,843		5,843		5,787
Gross unrealized losses	44		44		125
Estimated fair values	16,693		16,693		19,100
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,293		1,293		2,427
Less than twelve months, Unrealized losses	44		44		125
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	1,293		1,293		2,427
Total, Unrealized losses	44		44		125
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	11,848		11,848		14,812
Gross unrealized gains	8,310		8,310		7,672
Gross unrealized losses	89		89		163
Estimated fair values	20,069		20,069		22,321
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,280		1,280		1,637
Less than twelve months, Unrealized losses	89		89		163
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	1,280		1,280		1,637
Total, Unrealized losses	89		89		163
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,404		12,404		14,276
Gross unrealized gains	3,708		3,708		4,873
Gross unrealized losses	663		663		78
Estimated fair values	15,449		15,449		19,071
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,635		3,635		1,621
Less than twelve months, Unrealized losses	474		474		33
Twelve months or longer, Fair value	1,027		1,027		1,188
Twelve months or longer, Unrealized losses	189		189		45
Total, Fair value	4,662		4,662		2,809
Total, Unrealized losses	663		663		78
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,267		12,267		13,005
Gross unrealized gains	12,255		12,255		11,258
Gross unrealized losses	13		13		18
Estimated fair values	24,509		24,509		24,245
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	340		340		779
Less than twelve months, Unrealized losses	13		13		18
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	340		340		779
Total, Unrealized losses	13		13		18
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,203		13,203		13,071
Gross unrealized gains	5,126		5,126		5,345
Gross unrealized losses	28		28		32
Estimated fair values	18,301		18,301		18,384
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,252		2,252		1,472
Less than twelve months, Unrealized losses	28		28		32
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	2,252		2,252		1,472
Total, Unrealized losses	28		28		32
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	23,030		23,030		20,031
Gross unrealized gains	1,746		1,746		483
Gross unrealized losses	20		20		167
Estimated fair values	24,756		24,756		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	1,980		1,980		1,877
Twelve months or longer, Unrealized losses	20		20		123
Total, Fair value	1,980		1,980		5,233
Total, Unrealized losses	20		20		$ 167
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Net realized investment gains (losses)	$ (1,315)	$ (1,426)	$ (3,602)	$ (3,340)