INVESTMENTS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	$ 1,211,723		$ 1,203,978
Gross unrealized gains	133,308		128,038
Gross unrealized losses	7,251		7,481
Estimated fair value	1,337,780		1,324,535
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	102,752		74,394
Less than twelve months, Unrealized losses	2,221		2,142
Twelve months or longer, Fair value	75,634		152,526
Twelve months or longer, Unrealized losses	5,030		5,339
Total, Fair value	178,386		226,920
Total, Unrealized losses	7,251		7,481
Amortized cost
Due in one year or less	40,128
Due after one year through five years	176,529
Due after five years through ten years	233,367
Due after ten years	495,137
Mortgage-backed securities	139,144
Totals	1,084,305		1,080,006
Estimated fair value
Due in one year or less	40,717
Due after one year through five years	187,627
Due after five years through ten years	244,118
Due after ten years	526,572
Mortgage-backed securities	139,189
Totals	1,138,223		1,127,499
Realized Investment Gains (Losses) [Abstract]
'Other-than-temporary' impairments	(618)	(316)
Net realized investment gains (losses)	783	1,262
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	1,084,305		1,080,006
Gross unrealized gains	60,244		54,148
Gross unrealized losses	6,326		6,655
Estimated fair value	1,138,223		1,127,499
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	86,888		65,655
Less than twelve months, Unrealized losses	1,474		1,633
Twelve months or longer, Fair value	73,747		150,623
Twelve months or longer, Unrealized losses	4,852		5,022
Total, Fair value	160,635		216,278
Total, Unrealized losses	6,326		6,655
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	535	81
Available-for-sale, Gross realized investment losses	0	(92)
US Treasury Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	9,582		9,574
Gross unrealized gains	210		129
Gross unrealized losses	0		0
Estimated fair value	9,792		9,703
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	220,768		215,425
Gross unrealized gains	3,295		2,313
Gross unrealized losses	1,034		2,122
Estimated fair value	223,029		215,616
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	51,725		24,473
Less than twelve months, Unrealized losses	226		94
Twelve months or longer, Fair value	37,523		97,446
Twelve months or longer, Unrealized losses	808		2,028
Total, Fair value	89,248		121,919
Total, Unrealized losses	1,034		2,122
US States and Political Subdivisions Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	294,630		299,258
Gross unrealized gains	27,704		26,840
Gross unrealized losses	41		40
Estimated fair value	322,293		326,058
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,894		0
Less than twelve months, Unrealized losses	41		0
Twelve months or longer, Fair value	0		3,757
Twelve months or longer, Unrealized losses	0		40
Total, Fair value	4,894		3,757
Total, Unrealized losses	41		40
Commercial Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	41,277		42,996
Gross unrealized gains	3,581		3,766
Gross unrealized losses	0		0
Estimated fair value	44,858		46,762
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	221		1,102
Less than twelve months, Unrealized losses	0		0
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	221		1,102
Total, Unrealized losses	0		0
Residential Mortgage Backed Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	97,867		100,296
Gross unrealized gains	1,484		1,402
Gross unrealized losses	5,020		3,745
Estimated fair value	94,331		97,953
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	18,318		21,451
Less than twelve months, Unrealized losses	1,161		1,252
Twelve months or longer, Fair value	23,684		21,163
Twelve months or longer, Unrealized losses	3,859		2,493
Total, Fair value	42,002		42,614
Total, Unrealized losses	5,020		3,745
Collateralized Debt Obligations [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	13,742		14,798
Gross unrealized gains	1,243		1,213
Gross unrealized losses	0		6
Estimated fair value	14,985		16,005
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value			1,889
Less than twelve months, Unrealized losses			6
Twelve months or longer, Fair value			0
Twelve months or longer, Unrealized losses			0
Total, Fair value			1,889
Total, Unrealized losses			6
Corporate Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	406,439		397,659
Gross unrealized gains	22,727		18,485
Gross unrealized losses	231		742
Estimated fair value	428,935		415,402
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	11,730		16,740
Less than twelve months, Unrealized losses	46		281
Twelve months or longer, Fair value	12,540		28,257
Twelve months or longer, Unrealized losses	185		461
Total, Fair value	24,270		44,997
Total, Unrealized losses	231		742
Equity Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	127,418		123,972
Gross unrealized gains	73,064		73,890
Gross unrealized losses	925		826
Estimated fair value	199,557		197,036
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	15,864		8,739
Less than twelve months, Unrealized losses	747		509
Twelve months or longer, Fair value	1,887		1,903
Twelve months or longer, Unrealized losses	178		317
Total, Fair value	17,751		10,642
Total, Unrealized losses	925		826
Realized Investment Gains (Losses) [Abstract]
Available-for-sale, Gross realized investment gains	2,594	2,033
Available-for-sale, Gross realized investment losses	(329)	(76)
'Other-than-temporary' impairments	(618)	(316)
Common stocks Financial services [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	23,724		22,586
Gross unrealized gains	10,719		11,835
Gross unrealized losses	166		42
Estimated fair value	34,277		34,379
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	5,130		1,162
Less than twelve months, Unrealized losses	158		9
Twelve months or longer, Fair value	57		187
Twelve months or longer, Unrealized losses	8		33
Total, Fair value	5,187		1,349
Total, Unrealized losses	166		42
Common stocks Information technology [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	17,470		15,755
Gross unrealized gains	9,964		11,110
Gross unrealized losses	54		0
Estimated fair value	27,380		26,865
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,867
Less than twelve months, Unrealized losses	54
Twelve months or longer, Fair value	0
Twelve months or longer, Unrealized losses	0
Total, Fair value	1,867
Total, Unrealized losses	54
Common stocks Healthcare [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	15,536		14,673
Gross unrealized gains	13,665		12,179
Gross unrealized losses	2		0
Estimated fair value	29,199		26,852
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	161
Less than twelve months, Unrealized losses	2
Twelve months or longer, Fair value	0
Twelve months or longer, Unrealized losses	0
Total, Fair value	161
Total, Unrealized losses	2
Common stocks Consumer staples [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	8,827		10,584
Gross unrealized gains	6,106		6,112
Gross unrealized losses	10		2
Estimated fair value	14,923		16,694
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	266		1,051
Less than twelve months, Unrealized losses	10		2
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	266		1,051
Total, Unrealized losses	10		2
Common stocks Consumer discretionary [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,706		11,304
Gross unrealized gains	11,868		11,420
Gross unrealized losses	15		33
Estimated fair value	24,559		22,691
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,324		822
Less than twelve months, Unrealized losses	15		33
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	1,324		822
Total, Unrealized losses	15		33
Common stocks Energy [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	16,375		15,837
Gross unrealized gains	7,558		7,458
Gross unrealized losses	351		432
Estimated fair value	23,582		22,863
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,460		4,298
Less than twelve months, Unrealized losses	351		432
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	3,460		4,298
Total, Unrealized losses	351		432
Common stocks Industrials [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	9,555		9,658
Gross unrealized gains	8,695		8,596
Gross unrealized losses	89		33
Estimated fair value	18,161		18,221
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	2,189		1,406
Less than twelve months, Unrealized losses	89		33
Twelve months or longer, Fair value	0		0
Twelve months or longer, Unrealized losses	0		0
Total, Fair value	2,189		1,406
Total, Unrealized losses	89		33
Common stocks Other [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	11,143		11,493
Gross unrealized gains	3,669		4,563
Gross unrealized losses	68		0
Estimated fair value	14,744		16,056
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	1,467
Less than twelve months, Unrealized losses	68
Twelve months or longer, Fair value	0
Twelve months or longer, Unrealized losses	0
Total, Fair value	1,467
Total, Unrealized losses	68
Nonredeemable Preferred Stock [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Amortized cost	12,082		12,082
Gross unrealized gains	820		617
Gross unrealized losses	170		284
Estimated fair value	12,732		12,415
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0
Less than twelve months, Unrealized losses	0		0
Twelve months or longer, Fair value	1,830		1,716
Twelve months or longer, Unrealized losses	170		284
Total, Fair value	1,830		1,716
Total, Unrealized losses	170		284
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Other long-term investments, Gross realized investment losses	$ (1,399)	$ (368)