INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,336,322		$ 1,336,322		$ 1,274,393
Gross unrealized gains	131,567		131,567		107,244
Gross unrealized losses	11,457		11,457		14,369
Estimated fair values	1,456,432		1,456,432		1,367,268
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	153,226		153,226		243,068
Less than twelve months, Unrealized losses	3,619		3,619		7,792
Twelve months or longer, Fair value	36,355		36,355		73,043
Twelve months or longer, Unrealized losses	7,838		7,838		6,577
Total, Fair value	189,581		189,581		316,111
Total, Unrealized losses	11,457		11,457		14,369
Amortized cost
Due in one year or less	33,776		33,776
Due after one year through five years	165,885		165,885
Due after five years through ten years	324,442		324,442
Due after ten years	510,334		510,334
Securities not due at a single maturity date	150,033		150,033
Totals	1,184,470		1,184,470		1,130,217
Estimated fair value
Due in one year or less	34,148		34,148
Due after one year through five years	174,885		174,885
Due after five years through ten years	344,608		344,608
Due after ten years	538,909		538,909
Securities not due at a single maturity date	144,600		144,600
Totals	1,237,150		1,237,150		1,161,025
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	(275)	$ (628)	(976)	$ (1,293)
Net realized investment gains (losses)	(1,192)	7,498	(643)	11,555
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,184,470		1,184,470		1,130,217
Gross unrealized gains	62,530		62,530		42,885
Gross unrealized losses	9,850		9,850		12,077
Estimated fair values	1,237,150		1,237,150		1,161,025
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	134,163		134,163		214,736
Less than twelve months, Unrealized losses	2,094		2,094		5,661
Twelve months or longer, Fair value	34,437		34,437		71,072
Twelve months or longer, Unrealized losses	7,756		7,756		6,416
Total, Fair value	168,600		168,600		285,808
Total, Unrealized losses	9,850		9,850		12,077
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	261	61	1,935	642
Available-for-sale, Gross realized investment losses	(218)	0	(517)	0
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	7,835		7,835		12,566
Gross unrealized gains	372		372		23
Gross unrealized losses	0		0		0
Estimated fair values	8,207		8,207		12,589
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	236,698		236,698		202,486
Gross unrealized gains	1,408		1,408		1,817
Gross unrealized losses	612		612		1,637
Estimated fair values	237,494		237,494		202,666
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	56,223		56,223		78,800
Less than twelve months, Unrealized losses	612		612		1,228
Twelve months or longer, Fair value	0		0		34,079
Twelve months or longer, Unrealized losses	0		0		409
Total, Fair value	56,223		56,223		112,879
Total, Unrealized losses	612		612		1,637
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	320,330		320,330		319,940
Gross unrealized gains	29,601		29,601		24,419
Gross unrealized losses	19		19		0
Estimated fair values	349,912		349,912		344,359
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,836		2,836
Less than twelve months, Unrealized losses	19		19
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	2,836		2,836
Total, Unrealized losses	19		19
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	40,800		40,800		44,433
Gross unrealized gains	1,208		1,208		1,692
Gross unrealized losses	126		126		17
Estimated fair values	41,882		41,882		46,108
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	20,698		20,698		6,807
Less than twelve months, Unrealized losses	126		126		17
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	20,698		20,698		6,807
Total, Unrealized losses	126		126		17
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	106,044		106,044		94,279
Gross unrealized gains	2,245		2,245		1,059
Gross unrealized losses	8,780		8,780		6,795
Estimated fair values	99,509		99,509		88,543
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	24,298		24,298		22,028
Less than twelve months, Unrealized losses	1,103		1,103		1,694
Twelve months or longer, Fair value	25,791		25,791		22,781
Twelve months or longer, Unrealized losses	7,677		7,677		5,101
Total, Fair value	50,089		50,089		44,809
Total, Unrealized losses	8,780		8,780		6,795
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	27,239		27,239		17,000
Gross unrealized gains	1,112		1,112		883
Gross unrealized losses	100		100		39
Estimated fair values	28,251		28,251		17,844
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	7,526		7,526		6,013
Less than twelve months, Unrealized losses	100		100		39
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	7,526		7,526		6,013
Total, Unrealized losses	100		100		39
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	445,524		445,524		439,513
Gross unrealized gains	26,584		26,584		12,992
Gross unrealized losses	213		213		3,589
Estimated fair values	471,895		471,895		448,916
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	22,582		22,582		101,088
Less than twelve months, Unrealized losses	134		134		2,683
Twelve months or longer, Fair value	8,646		8,646		14,212
Twelve months or longer, Unrealized losses	79		79		906
Total, Fair value	31,228		31,228		115,300
Total, Unrealized losses	213		213		3,589
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	151,852		151,852		144,176
Gross unrealized gains	69,037		69,037		64,359
Gross unrealized losses	1,607		1,607		2,292
Estimated fair values	219,282		219,282		206,243
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	19,063		19,063		28,332
Less than twelve months, Unrealized losses	1,525		1,525		2,131
Twelve months or longer, Fair value	1,918		1,918		1,971
Twelve months or longer, Unrealized losses	82		82		161
Total, Fair value	20,981		20,981		30,303
Total, Unrealized losses	1,607		1,607		2,292
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	1,001	3,345	5,849	11,681
Available-for-sale, Gross realized investment losses	(50)	(2,525)	(1,683)	(3,278)
'Other-than-temporary' impairments	(275)	(628)	(976)	(1,293)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	24,115		24,115		24,557
Gross unrealized gains	8,748		8,748		9,731
Gross unrealized losses	444		444		333
Estimated fair values	32,419		32,419		33,955
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,802		4,802		6,387
Less than twelve months, Unrealized losses	444		444		333
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	4,802		4,802		6,387
Total, Unrealized losses	444		444		333
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	19,482		19,482		19,427
Gross unrealized gains	11,457		11,457		8,807
Gross unrealized losses	2		2		132
Estimated fair values	30,937		30,937		28,102
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	52		52		1,316
Less than twelve months, Unrealized losses	2		2		132
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	52		52		1,316
Total, Unrealized losses	2		2		132
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	16,522		16,522		15,599
Gross unrealized gains	11,040		11,040		10,359
Gross unrealized losses	43		43		64
Estimated fair values	27,519		27,519		25,894
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	873		873		3,199
Less than twelve months, Unrealized losses	43		43		64
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	873		873		3,199
Total, Unrealized losses	43		43		64
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	14,059		14,059		11,136
Gross unrealized gains	7,066		7,066		7,090
Gross unrealized losses	59		59		26
Estimated fair values	21,066		21,066		18,200
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,079		1,079		1,244
Less than twelve months, Unrealized losses	59		59		26
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	1,079		1,079		1,244
Total, Unrealized losses	59		59		26
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	15,399		15,399		10,270
Gross unrealized gains	8,983		8,983		8,658
Gross unrealized losses	336		336		5
Estimated fair values	24,046		24,046		18,923
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,813		3,813		176
Less than twelve months, Unrealized losses	336		336		5
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	3,813		3,813		176
Total, Unrealized losses	336		336		5
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	14,021		14,021		16,384
Gross unrealized gains	5,167		5,167		5,972
Gross unrealized losses	359		359		1,288
Estimated fair values	18,829		18,829		21,068
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,899		4,899		8,233
Less than twelve months, Unrealized losses	359		359		1,272
Twelve months or longer, Fair value	0		0		116
Twelve months or longer, Unrealized losses	0		0		16
Total, Fair value	4,899		4,899		8,349
Total, Unrealized losses	359		359		1,288
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,390		13,390		11,525
Gross unrealized gains	10,375		10,375		8,902
Gross unrealized losses	71		71		11
Estimated fair values	23,694		23,694		20,416
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,097		1,097		1,263
Less than twelve months, Unrealized losses	71		71		11
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	1,097		1,097		1,263
Total, Unrealized losses	71		71		11
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	16,833		16,833		17,246
Gross unrealized gains	4,700		4,700		3,672
Gross unrealized losses	211		211		235
Estimated fair values	21,322		21,322		20,683
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,448		2,448		4,064
Less than twelve months, Unrealized losses	211		211		235
Twelve months or longer, Fair value	0		0		0
Twelve months or longer, Unrealized losses	0		0		0
Total, Fair value	2,448		2,448		4,064
Total, Unrealized losses	211		211		235
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	18,031		18,031		18,032
Gross unrealized gains	1,501		1,501		1,168
Gross unrealized losses	82		82		198
Estimated fair values	19,450		19,450		19,002
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0		2,450
Less than twelve months, Unrealized losses	0		0		53
Twelve months or longer, Fair value	1,918		1,918		1,855
Twelve months or longer, Unrealized losses	82		82		145
Total, Fair value	1,918		1,918		4,305
Total, Unrealized losses	82		82		$ 198
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Other long-term investments, net	$ (1,911)	$ 7,245	$ (5,251)	$ 3,803