INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,338,690		$ 1,337,004
Gross unrealized gains	108,998		99,752
Gross unrealized losses	19,193		23,218
Estimated fair values	1,428,495		1,413,538
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	369,384		422,453
Less than twelve months, Unrealized losses	13,949		16,670
Twelve months or longer, Fair value	31,022		36,223
Twelve months or longer, Unrealized losses	5,244		6,548
Total, Fair value	400,406		458,676
Total, Unrealized losses	19,193		23,218
Amortized cost
Due in one year or less	54,950
Due after one year through five years	165,447
Due after five years through ten years	326,400
Due after ten years	506,694
Securities not due at a single maturity date	129,587
Totals	1,183,078		1,189,525
Estimated fair value
Due in one year or less	55,913
Due after one year through five years	172,042
Due after five years through ten years	331,623
Due after ten years	513,339
Securities not due at a single maturity date	125,376
Totals	1,198,293		1,199,699
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	0	$ (431)
Net realized investment gains (losses)	(627)	(1,085)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,183,078		1,189,525
Gross unrealized gains	33,230		32,485
Gross unrealized losses	18,015		22,311
Estimated fair values	1,198,293		1,199,699
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	353,029		404,167
Less than twelve months, Unrealized losses	13,284		16,129
Twelve months or longer, Fair value	26,193		32,250
Twelve months or longer, Unrealized losses	4,731		6,182
Total, Fair value	379,222		436,417
Total, Unrealized losses	18,015		22,311
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	90	669
Available-for-sale, Gross realized investment losses	(1,206)	(299)
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	7,848		7,841
Gross unrealized gains	15		0
Gross unrealized losses	0		11
Estimated fair values	7,863		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	238,001		249,495
Gross unrealized gains	213		311
Gross unrealized losses	8,543		10,609
Estimated fair values	229,671		239,197
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	201,763		202,900
Less than twelve months, Unrealized losses	8,543		10,609
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	201,763		202,900
Total, Unrealized losses	8,543		10,609
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	334,908		319,663
Gross unrealized gains	17,442		17,034
Gross unrealized losses	642		940
Estimated fair values	351,708		335,757
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	32,765		43,777
Less than twelve months, Unrealized losses	642		940
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	32,765		43,777
Total, Unrealized losses	642		940
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	36,048		37,964
Gross unrealized gains	663		741
Gross unrealized losses	1,092		1,133
Estimated fair values	35,619		37,572
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	20,388		21,695
Less than twelve months, Unrealized losses	1,092		1,133
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	20,388		21,695
Total, Unrealized losses	1,092		1,133
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	90,865		102,307
Gross unrealized gains	2,045		1,435
Gross unrealized losses	5,811		7,308
Estimated fair values	87,099		96,434
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	17,962		26,217
Less than twelve months, Unrealized losses	1,150		1,232
Twelve months or longer, Fair value	17,525		23,625
Twelve months or longer, Unrealized losses	4,661		6,076
Total, Fair value	35,487		49,842
Total, Unrealized losses	5,811		7,308
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	25,545		26,592
Gross unrealized gains	682		732
Gross unrealized losses	610		931
Estimated fair values	25,617		26,393
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	18,875		19,091
Less than twelve months, Unrealized losses	610		931
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	18,875		19,091
Total, Unrealized losses	610		931
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	449,863		445,663
Gross unrealized gains	12,170		12,232
Gross unrealized losses	1,317		1,379
Estimated fair values	460,716		456,516
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	61,276		82,657
Less than twelve months, Unrealized losses	1,247		1,273
Twelve months or longer, Fair value	8,668		8,625
Twelve months or longer, Unrealized losses	70		106
Total, Fair value	69,944		91,282
Total, Unrealized losses	1,317		1,379
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	155,612		147,479
Gross unrealized gains	75,768		67,267
Gross unrealized losses	1,178		907
Estimated fair values	230,202		213,839
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	16,355		18,286
Less than twelve months, Unrealized losses	665		541
Twelve months or longer, Fair value	4,829		3,973
Twelve months or longer, Unrealized losses	513		366
Total, Fair value	21,184		22,259
Total, Unrealized losses	1,178		907
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	2,875	2,082
Available-for-sale, Gross realized investment losses	(99)	(1,192)
'Other-than-temporary' impairments	0	(431)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	26,886		22,922
Gross unrealized gains	12,976		12,410
Gross unrealized losses	207		210
Estimated fair values	39,655		35,122
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,305		1,462
Less than twelve months, Unrealized losses	56		12
Twelve months or longer, Fair value	956		908
Twelve months or longer, Unrealized losses	151		198
Total, Fair value	5,261		2,370
Total, Unrealized losses	207		210
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	20,816		19,832
Gross unrealized gains	14,773		10,739
Gross unrealized losses	14		29
Estimated fair values	35,575		30,542
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	836		1,947
Less than twelve months, Unrealized losses	14		29
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	836		1,947
Total, Unrealized losses	14		29
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	17,139		16,092
Gross unrealized gains	10,385		8,700
Gross unrealized losses	2		85
Estimated fair values	27,522		24,707
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	151		3,585
Less than twelve months, Unrealized losses	2		85
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	151		3,585
Total, Unrealized losses	2		85
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,382		13,438
Gross unrealized gains	6,532		5,787
Gross unrealized losses	48		125
Estimated fair values	19,866		19,100
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,705		2,427
Less than twelve months, Unrealized losses	46		125
Twelve months or longer, Fair value	93		0
Twelve months or longer, Unrealized losses	2		0
Total, Fair value	2,798		2,427
Total, Unrealized losses	48		125
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,430		14,812
Gross unrealized gains	8,105		7,672
Gross unrealized losses	168		163
Estimated fair values	20,367		22,321
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,477		1,637
Less than twelve months, Unrealized losses	168		163
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	2,477		1,637
Total, Unrealized losses	168		163
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	14,276		14,276
Gross unrealized gains	3,986		4,873
Gross unrealized losses	640		78
Estimated fair values	17,622		19,071
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,720		1,621
Less than twelve months, Unrealized losses	345		33
Twelve months or longer, Fair value	1,846		1,188
Twelve months or longer, Unrealized losses	295		45
Total, Fair value	5,566		2,809
Total, Unrealized losses	640		78
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,059		13,005
Gross unrealized gains	12,145		11,258
Gross unrealized losses	33		18
Estimated fair values	25,171		24,245
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,384		779
Less than twelve months, Unrealized losses	33		18
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	1,384		779
Total, Unrealized losses	33		18
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,594		13,071
Gross unrealized gains	5,720		5,345
Gross unrealized losses	1		32
Estimated fair values	18,313		18,384
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	777		1,472
Less than twelve months, Unrealized losses	1		32
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	777		1,472
Total, Unrealized losses	1		32
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	25,030		20,031
Gross unrealized gains	1,146		483
Gross unrealized losses	65		167
Estimated fair values	26,111		20,347
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		3,356
Less than twelve months, Unrealized losses	0		44
Twelve months or longer, Fair value	1,934		1,877
Twelve months or longer, Unrealized losses	65		123
Total, Fair value	1,934		5,233
Total, Unrealized losses	65		$ 167
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Net realized investment gains (losses)	$ (2,287)	$ (1,914)