INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Jun. 30, 2021
Less than 12 Months Fair Value	$ 1,281,858	$ 426,607
Less than 12 Months Unrealized Loss	(14,442)	(2,402)
12 Months or More Fair Value	82,951	117,126
12 Months or More Unrealized Loss	(224)	(2,989)
Total Fair Value	1,364,809	543,733
Total Unrealized Loss	(14,666)	(5,391)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	31,913	4,992
Less than 12 Months Unrealized Loss	(337)	(8)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	31,913	4,992
Total Unrealized Loss	(337)	(8)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	175,728	84,749
Less than 12 Months Unrealized Loss	(528)	(435)
12 Months or More Fair Value	25,449	68,967
12 Months or More Unrealized Loss	(110)	(391)
Total Fair Value	201,177	153,716
Total Unrealized Loss	(638)	(826)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	265,630	60,008
Less than 12 Months Unrealized Loss	(3,581)	(291)
12 Months or More Fair Value	14,443	0
12 Months or More Unrealized Loss	(6)	0
Total Fair Value	280,073	60,008
Total Unrealized Loss	(3,587)	(291)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	273,715	49,161
Less than 12 Months Unrealized Loss	(2,727)	(445)
12 Months or More Fair Value	4,220	1
12 Months or More Unrealized Loss	(48)	0
Total Fair Value	277,935	49,162
Total Unrealized Loss	(2,775)	(445)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	69,725	169,825
Less than 12 Months Unrealized Loss	(507)	(986)
12 Months or More Fair Value	38,839	48,158
12 Months or More Unrealized Loss	(60)	(2,598)
Total Fair Value	108,564	217,983
Total Unrealized Loss	(567)	(3,584)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	456,487	57,872
Less than 12 Months Unrealized Loss	(6,758)	(237)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	456,487	57,872
Total Unrealized Loss	(6,758)	(237)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	8,660	0
Less than 12 Months Unrealized Loss	(4)	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	8,660	0
Total Unrealized Loss	(4)	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