INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Sep. 30, 2020
Less than 12 Months Fair Value	$ 447,393	$ 481,608
Less than 12 Months Unrealized Loss	(4,221)	(1,814)
12 Months or More Fair Value	145,773	214,322
12 Months or More Unrealized Loss	(5,142)	(4,493)
Total Fair Value	593,166	695,930
Total Unrealized Loss	(9,363)	(6,307)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	19,033	0
Less than 12 Months Unrealized Loss	(349)	0
12 Months or More Fair Value	4,577	5,649
12 Months or More Unrealized Loss	(248)	(115)
Total Fair Value	23,610	5,649
Total Unrealized Loss	(597)	(115)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	138,610	125,889
Less than 12 Months Unrealized Loss	(1,307)	(553)
12 Months or More Fair Value	97,031	54,963
12 Months or More Unrealized Loss	(1,671)	(511)
Total Fair Value	235,641	180,852
Total Unrealized Loss	(2,978)	(1,064)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	81,678	118,623
Less than 12 Months Unrealized Loss	(599)	(457)
12 Months or More Fair Value	0	7,950
12 Months or More Unrealized Loss	0	(36)
Total Fair Value	81,678	126,573
Total Unrealized Loss	(599)	(493)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	42,042	85,248
Less than 12 Months Unrealized Loss	(339)	(297)
12 Months or More Fair Value	1	30,628
12 Months or More Unrealized Loss	0	(196)
Total Fair Value	42,043	115,876
Total Unrealized Loss	(339)	(493)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	145,885	111,658
Less than 12 Months Unrealized Loss	(1,568)	(298)
12 Months or More Fair Value	44,164	104,069
12 Months or More Unrealized Loss	(3,223)	(3,571)
Total Fair Value	190,049	215,727
Total Unrealized Loss	(4,791)	(3,869)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	20,145	40,190
Less than 12 Months Unrealized Loss	(59)	(209)
12 Months or More Fair Value	0	11,063
12 Months or More Unrealized Loss	0	(64)
Total Fair Value	20,145	51,253
Total Unrealized Loss	(59)	(273)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Loss	0	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	0	0
Total Unrealized Loss	0	0
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Loss	0	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	0	0
Total Unrealized Loss	$ 0	$ 0