INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Mar. 31, 2021
Less than 12 Months Fair Value	$ 1,199,433	$ 426,607
Less than 12 Months Unrealized Loss	(25,886)	(2,402)
12 Months or More Fair Value	115,645	117,126
12 Months or More Unrealized Loss	(2,249)	(2,989)
Total Fair Value	1,315,078	543,733
Total Unrealized Loss	(28,135)	(5,391)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	34,096	4,992
Less than 12 Months Unrealized Loss	(654)	(8)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	34,096	4,992
Total Unrealized Loss	(654)	(8)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	160,329	84,749
Less than 12 Months Unrealized Loss	(970)	(435)
12 Months or More Fair Value	36,344	68,967
12 Months or More Unrealized Loss	(220)	(391)
Total Fair Value	196,673	153,716
Total Unrealized Loss	(1,190)	(826)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	318,750	60,008
Less than 12 Months Unrealized Loss	(8,527)	(291)
12 Months or More Fair Value	22,104	0
12 Months or More Unrealized Loss	(14)	0
Total Fair Value	340,854	60,008
Total Unrealized Loss	(8,541)	(291)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	142,823	49,161
Less than 12 Months Unrealized Loss	(2,815)	(445)
12 Months or More Fair Value	1	1
12 Months or More Unrealized Loss	0	0
Total Fair Value	142,824	49,162
Total Unrealized Loss	(2,815)	(445)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	60,681	169,825
Less than 12 Months Unrealized Loss	(1,665)	(986)
12 Months or More Fair Value	57,196	48,158
12 Months or More Unrealized Loss	(2,015)	(2,598)
Total Fair Value	117,877	217,983
Total Unrealized Loss	(3,680)	(3,584)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	482,754	57,872
Less than 12 Months Unrealized Loss	(11,255)	(237)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	482,754	57,872
Total Unrealized Loss	(11,255)	(237)
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