INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	$ 1,255,775		$ 1,273,132
Fixed maturity securities available-for-sale	1,291,860		1,282,909
Amortized cost
Due in one year or less	23,719
Due after one year through five years	265,818
Due after five years through ten years	314,904
Due after ten years	402,224
Securities not due at a single maturity date	249,110
Fixed maturity securities available-for-sale, Amortized cost	1,255,775		1,273,132
Estimated fair value
Due in one year or less	23,824
Due after one year through five years	270,454
Due after five years through ten years	325,430
Due after ten years	415,475
Securities not due at a single maturity date	256,677
Totals	1,291,860		1,282,909
Realized Investment Gains (Losses) [Abstract]
Fixed maturity securities available-for-sale gross realized gains	0	$ 234
Fixed maturity securities available-for-sale gross realized losses	(272)	(478)
Net realized investment gains	4,097	2,716
Change in unrealized investment gains	19,829	(9,854)
Net realized investment gains/losses and change in unrealized gains on equity investments	22,643	(5,393)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	1,255,775		1,273,132
Fixed maturity securities available-for-sale, Gross unrealized gains	39,086		20,713
Fixed maturity securities available-for-sale, Gross unrealized losses	3,001		10,936
Fixed maturity securities available-for-sale	1,291,860		1,282,909
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	12,367		236,333
Less than twelve months, Unrealized losses	443		3,329
Twelve months or longer, Fair value	168,136		242,366
Twelve months or longer, Unrealized losses	2,558		7,607
Total, Fair value	180,503		478,699
Total, Unrealized losses	3,001		10,936
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	1,255,775		1,273,132
Estimated fair value
Totals	1,291,860		1,282,909
U.S. treasury
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	8,146		8,139
Fixed maturity securities available-for-sale, Gross unrealized gains	0		0
Fixed maturity securities available-for-sale, Gross unrealized losses	33		118
Fixed maturity securities available-for-sale	8,113		8,021
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	8,113		8,021
Twelve months or longer, Unrealized losses	33		118
Total, Fair value	8,113		8,021
Total, Unrealized losses	33		118
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	8,146		8,139
Estimated fair value
Totals	8,113		8,021
U.S. government-sponsored agencies
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	302,811		303,198
Fixed maturity securities available-for-sale, Gross unrealized gains	6,489		2,799
Fixed maturity securities available-for-sale, Gross unrealized losses	180		1,518
Fixed maturity securities available-for-sale	309,120		304,479
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		14,620
Less than twelve months, Unrealized losses	0		20
Twelve months or longer, Fair value	47,961		92,603
Twelve months or longer, Unrealized losses	180		1,498
Total, Fair value	47,961		107,223
Total, Unrealized losses	180		1,518
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	302,811		303,198
Estimated fair value
Totals	309,120		304,479
Obligations of states and political subdivisions
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	263,064		273,727
Fixed maturity securities available-for-sale, Gross unrealized gains	13,852		10,375
Fixed maturity securities available-for-sale, Gross unrealized losses	59		451
Fixed maturity securities available-for-sale	276,857		283,651
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	12,372		14,498
Twelve months or longer, Unrealized losses	59		451
Total, Fair value	12,372		14,498
Total, Unrealized losses	59		451
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	263,064		273,727
Estimated fair value
Totals	276,857		283,651
Commercial mortgage-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	83,882		83,854
Fixed maturity securities available-for-sale, Gross unrealized gains	2,869		1,287
Fixed maturity securities available-for-sale, Gross unrealized losses	87		762
Fixed maturity securities available-for-sale	86,664		84,379
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		2,021
Less than twelve months, Unrealized losses	0		21
Twelve months or longer, Fair value	11,140		24,222
Twelve months or longer, Unrealized losses	87		741
Total, Fair value	11,140		26,243
Total, Unrealized losses	87		762
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	83,882		83,854
Estimated fair value
Totals	86,664		84,379
Residential mortgage-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	163,955		161,055
Fixed maturity securities available-for-sale, Gross unrealized gains	6,233		3,374
Fixed maturity securities available-for-sale, Gross unrealized losses	1,423		2,292
Fixed maturity securities available-for-sale	168,765		162,137
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,466		16,852
Less than twelve months, Unrealized losses	94		145
Twelve months or longer, Fair value	40,869		45,597
Twelve months or longer, Unrealized losses	1,329		2,147
Total, Fair value	45,335		62,449
Total, Unrealized losses	1,423		2,292
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	163,955		161,055
Estimated fair value
Totals	168,765		162,137
Other asset-backed
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	17,892		21,596
Fixed maturity securities available-for-sale, Gross unrealized gains	301		273
Fixed maturity securities available-for-sale, Gross unrealized losses	474		1,035
Fixed maturity securities available-for-sale	17,719		20,834
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		4,810
Less than twelve months, Unrealized losses	0		147
Twelve months or longer, Fair value	11,167		11,691
Twelve months or longer, Unrealized losses	474		888
Total, Fair value	11,167		16,501
Total, Unrealized losses	474		1,035
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	17,892		21,596
Estimated fair value
Totals	17,719		20,834
Corporate
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	416,025		421,563
Fixed maturity securities available-for-sale, Gross unrealized gains	9,342		2,605
Fixed maturity securities available-for-sale, Gross unrealized losses	745		4,760
Fixed maturity securities available-for-sale	424,622		419,408
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	7,901		198,030
Less than twelve months, Unrealized losses	349		2,996
Twelve months or longer, Fair value	36,514		45,734
Twelve months or longer, Unrealized losses	396		1,764
Total, Fair value	44,415		243,764
Total, Unrealized losses	745		4,760
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	416,025		421,563
Estimated fair value
Totals	424,622		$ 419,408
Other Long-term Investments [Member]
Realized Investment Gains (Losses) [Abstract]
Net realized investment gains/losses and change in unrealized gains on equity investments	$ (1,011)	$ 1,989