INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Jun. 30, 2020
Less than 12 Months Fair Value	$ 477,818	$ 481,608
Less than 12 Months Unrealized Loss	(9,748)	(1,814)
12 Months or More Fair Value	102,456	214,322
12 Months or More Unrealized Loss	(6,556)	(4,493)
Total Fair Value	580,274	695,930
Total Unrealized Loss	(16,304)	(6,307)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	58,500	0
Less than 12 Months Unrealized Loss	(2,126)	0
12 Months or More Fair Value	4,303	5,649
12 Months or More Unrealized Loss	(517)	(115)
Total Fair Value	62,803	5,649
Total Unrealized Loss	(2,643)	(115)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	190,047	125,889
Less than 12 Months Unrealized Loss	(4,702)	(553)
12 Months or More Fair Value	65,165	54,963
12 Months or More Unrealized Loss	(2,245)	(511)
Total Fair Value	255,212	180,852
Total Unrealized Loss	(6,947)	(1,064)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	37,979	118,623
Less than 12 Months Unrealized Loss	(66)	(457)
12 Months or More Fair Value	0	7,950
12 Months or More Unrealized Loss	0	(36)
Total Fair Value	37,979	126,573
Total Unrealized Loss	(66)	(493)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	43,648	85,248
Less than 12 Months Unrealized Loss	(139)	(297)
12 Months or More Fair Value	1	30,628
12 Months or More Unrealized Loss	0	(196)
Total Fair Value	43,649	115,876
Total Unrealized Loss	(139)	(493)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	131,442	111,658
Less than 12 Months Unrealized Loss	(2,627)	(298)
12 Months or More Fair Value	32,987	104,069
12 Months or More Unrealized Loss	(3,794)	(3,571)
Total Fair Value	164,429	215,727
Total Unrealized Loss	(6,421)	(3,869)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	16,202	40,190
Less than 12 Months Unrealized Loss	(88)	(209)
12 Months or More Fair Value	0	11,063
12 Months or More Unrealized Loss	0	(64)
Total Fair Value	16,202	51,253
Total Unrealized Loss	(88)	(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