INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Sep. 30, 2021
Less than 12 Months Fair Value	$ 1,418,947	$ 426,607
Less than 12 Months Unrealized Loss	(20,254)	(2,402)
12 Months or More Fair Value	87,340	117,126
12 Months or More Unrealized Loss	(1,025)	(2,989)
Total Fair Value	1,506,287	543,733
Total Unrealized Loss	(21,279)	(5,391)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	43,960	4,992
Less than 12 Months Unrealized Loss	(540)	(8)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	43,960	4,992
Total Unrealized Loss	(540)	(8)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	111,942	84,749
Less than 12 Months Unrealized Loss	(770)	(435)
12 Months or More Fair Value	16,462	68,967
12 Months or More Unrealized Loss	(38)	(391)
Total Fair Value	128,404	153,716
Total Unrealized Loss	(808)	(826)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	391,674	60,008
Less than 12 Months Unrealized Loss	(8,725)	(291)
12 Months or More Fair Value	19,537	0
12 Months or More Unrealized Loss	(154)	0
Total Fair Value	411,211	60,008
Total Unrealized Loss	(8,879)	(291)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	309,109	49,161
Less than 12 Months Unrealized Loss	(2,975)	(445)
12 Months or More Fair Value	18,819	1
12 Months or More Unrealized Loss	(466)	0
Total Fair Value	327,928	49,162
Total Unrealized Loss	(3,441)	(445)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	121,782	169,825
Less than 12 Months Unrealized Loss	(1,140)	(986)
12 Months or More Fair Value	18,671	48,158
12 Months or More Unrealized Loss	(96)	(2,598)
Total Fair Value	140,453	217,983
Total Unrealized Loss	(1,236)	(3,584)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	423,116	57,872
Less than 12 Months Unrealized Loss	(6,029)	(237)
12 Months or More Fair Value	13,851	0
12 Months or More Unrealized Loss	(271)	0
Total Fair Value	436,967	57,872
Total Unrealized Loss	(6,300)	(237)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	17,364	0
Less than 12 Months Unrealized Loss	(75)	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	17,364	0
Total Unrealized Loss	(75)	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