INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	$ 1,280,928		$ 1,280,928		$ 1,273,132
Fixed maturity securities available-for-sale	1,340,066		1,340,066		1,282,909
Amortized cost
Due in one year or less	15,670		15,670
Due after one year through five years	304,720		304,720
Due after five years through ten years	312,299		312,299
Due after ten years	373,887		373,887
Securities not due at a single maturity date	274,352		274,352
Fixed maturity securities available-for-sale, Amortized cost	1,280,928		1,280,928		1,273,132
Estimated fair value
Due in one year or less	15,732		15,732
Due after one year through five years	314,153		314,153
Due after five years through ten years	328,334		328,334
Due after ten years	392,008		392,008
Securities not due at a single maturity date	289,839		289,839
Totals	1,340,066		1,340,066		1,282,909
Realized Investment Gains (Losses) [Abstract]
Fixed maturity securities available-for-sale gross realized gains	51	$ 0	51	$ 234
Fixed maturity securities available-for-sale gross realized losses	(1)	(5,490)	(273)	(5,968)
Net realized investment gains	9,652	1,479	13,749	4,195
Change in unrealized investment gains	(4,674)	(447)	15,155	(10,301)
Net realized investment gains/losses and change in unrealized gains on equity investments	4,258	(5,860)	26,901	(11,253)
Debt Securities [Member]
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	1,280,928		1,280,928		1,273,132
Fixed maturity securities available-for-sale, Gross unrealized gains	60,905		60,905		20,713
Fixed maturity securities available-for-sale, Gross unrealized losses	1,767		1,767		10,936
Fixed maturity securities available-for-sale	1,340,066		1,340,066		1,282,909
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	8,625		8,625		236,333
Less than twelve months, Unrealized losses	389		389		3,329
Twelve months or longer, Fair value	42,297		42,297		242,366
Twelve months or longer, Unrealized losses	1,378		1,378		7,607
Total, Fair value	50,922		50,922		478,699
Total, Unrealized losses	1,767		1,767		10,936
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	1,280,928		1,280,928		1,273,132
Estimated fair value
Totals	1,340,066		1,340,066		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	8,152		8,152		8,139
Fixed maturity securities available-for-sale, Gross unrealized gains	118		118		0
Fixed maturity securities available-for-sale, Gross unrealized losses	0		0		118
Fixed maturity securities available-for-sale	8,270		8,270		8,021
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					0
Less than twelve months, Unrealized losses					0
Twelve months or longer, Fair value					8,021
Twelve months or longer, Unrealized losses					118
Total, Fair value					8,021
Total, Unrealized losses					118
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	8,152		8,152		8,139
Estimated fair value
Totals	8,270		8,270		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	311,194		311,194		303,198
Fixed maturity securities available-for-sale, Gross unrealized gains	9,659		9,659		2,799
Fixed maturity securities available-for-sale, Gross unrealized losses	26		26		1,518
Fixed maturity securities available-for-sale	320,827		320,827		304,479
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	4,987		4,987		14,620
Less than twelve months, Unrealized losses	14		14		20
Twelve months or longer, Fair value	4,404		4,404		92,603
Twelve months or longer, Unrealized losses	12		12		1,498
Total, Fair value	9,391		9,391		107,223
Total, Unrealized losses	26		26		1,518
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	311,194		311,194		303,198
Estimated fair value
Totals	320,827		320,827		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	252,931		252,931		273,727
Fixed maturity securities available-for-sale, Gross unrealized gains	16,771		16,771		10,375
Fixed maturity securities available-for-sale, Gross unrealized losses	0		0		451
Fixed maturity securities available-for-sale	269,702		269,702		283,651
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					0
Less than twelve months, Unrealized losses					0
Twelve months or longer, Fair value					14,498
Twelve months or longer, Unrealized losses					451
Total, Fair value					14,498
Total, Unrealized losses					451
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	252,931		252,931		273,727
Estimated fair value
Totals	269,702		269,702		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	88,573		88,573		83,854
Fixed maturity securities available-for-sale, Gross unrealized gains	5,731		5,731		1,287
Fixed maturity securities available-for-sale, Gross unrealized losses	0		0		762
Fixed maturity securities available-for-sale	94,304		94,304		84,379
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value					2,021
Less than twelve months, Unrealized losses					21
Twelve months or longer, Fair value					24,222
Twelve months or longer, Unrealized losses					741
Total, Fair value					26,243
Total, Unrealized losses					762
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	88,573		88,573		83,854
Estimated fair value
Totals	94,304		94,304		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	184,680		184,680		161,055
Fixed maturity securities available-for-sale, Gross unrealized gains	10,895		10,895		3,374
Fixed maturity securities available-for-sale, Gross unrealized losses	1,131		1,131		2,292
Fixed maturity securities available-for-sale	194,444		194,444		162,137
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	3,638		3,638		16,852
Less than twelve months, Unrealized losses	375		375		145
Twelve months or longer, Fair value	14,201		14,201		45,597
Twelve months or longer, Unrealized losses	756		756		2,147
Total, Fair value	17,839		17,839		62,449
Total, Unrealized losses	1,131		1,131		2,292
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	184,680		184,680		161,055
Estimated fair value
Totals	194,444		194,444		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	17,315		17,315		21,596
Fixed maturity securities available-for-sale, Gross unrealized gains	492		492		273
Fixed maturity securities available-for-sale, Gross unrealized losses	163		163		1,035
Fixed maturity securities available-for-sale	17,644		17,644		20,834
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0		4,810
Less than twelve months, Unrealized losses	0		0		147
Twelve months or longer, Fair value	7,154		7,154		11,691
Twelve months or longer, Unrealized losses	163		163		888
Total, Fair value	7,154		7,154		16,501
Total, Unrealized losses	163		163		1,035
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	17,315		17,315		21,596
Estimated fair value
Totals	17,644		17,644		20,834
Corporate
Amortized cost and estimated fair value of available-for-sale securities [Abstract]
Fixed maturity securities available-for-sale, Amortized cost	418,083		418,083		421,563
Fixed maturity securities available-for-sale, Gross unrealized gains	17,239		17,239		2,605
Fixed maturity securities available-for-sale, Gross unrealized losses	447		447		4,760
Fixed maturity securities available-for-sale	434,875		434,875		419,408
Available for sale securities continuous unrealized loss position [Abstract]
Less than twelve months, Fair value	0		0		198,030
Less than twelve months, Unrealized losses	0		0		2,996
Twelve months or longer, Fair value	16,538		16,538		45,734
Twelve months or longer, Unrealized losses	447		447		1,764
Total, Fair value	16,538		16,538		243,764
Total, Unrealized losses	447		447		4,760
Amortized cost
Fixed maturity securities available-for-sale, Amortized cost	418,083		418,083		421,563
Estimated fair value
Totals	434,875		434,875		$ 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	$ (770)	$ (1,402)	$ (1,781)	$ 587