INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Dec. 31, 2021
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Jun. 30, 2022
Less than 12 Months Fair Value	$ 3,127,669	$ 1,970,822
Less than 12 Months Unrealized Loss	(222,238)	(27,427)
12 Months or More Fair Value	490,322	171,838
12 Months or More Unrealized Loss	(70,326)	(4,187)
Total Fair Value	3,617,991	2,142,660
Total Unrealized Loss	(292,564)	(31,614)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	116,117	31,896
Less than 12 Months Unrealized Loss	(3,883)	(354)
12 Months or More Fair Value	11,744	11,877
12 Months or More Unrealized Loss	(507)	(373)
Total Fair Value	127,861	43,773
Total Unrealized Loss	(4,390)	(727)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	714,112	482,118
Less than 12 Months Unrealized Loss	(39,664)	(2,578)
12 Months or More Fair Value	25,917	0
12 Months or More Unrealized Loss	(2,388)	0
Total Fair Value	740,029	482,118
Total Unrealized Loss	(42,052)	(2,578)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	626,316	380,735
Less than 12 Months Unrealized Loss	(67,853)	(7,543)
12 Months or More Fair Value	126,912	55,568
12 Months or More Unrealized Loss	(20,513)	(1,051)
Total Fair Value	753,228	436,303
Total Unrealized Loss	(88,366)	(8,594)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	429,951	369,318
Less than 12 Months Unrealized Loss	(29,304)	(6,110)
12 Months or More Fair Value	73,602	12,485
12 Months or More Unrealized Loss	(11,228)	(387)
Total Fair Value	503,553	381,803
Total Unrealized Loss	(40,532)	(6,497)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	737,115	205,520
Less than 12 Months Unrealized Loss	(30,907)	(2,149)
12 Months or More Fair Value	27,765	13,818
12 Months or More Unrealized Loss	(3,361)	(203)
Total Fair Value	764,880	219,338
Total Unrealized Loss	(34,268)	(2,352)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	400,570	459,098
Less than 12 Months Unrealized Loss	(37,708)	(8,375)
12 Months or More Fair Value	224,382	78,090
12 Months or More Unrealized Loss	(32,329)	(2,173)
Total Fair Value	624,952	537,188
Total Unrealized Loss	(70,037)	(10,548)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	70,108	17,382
Less than 12 Months Unrealized Loss	(9,576)	(128)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	70,108	17,382
Total Unrealized Loss	(9,576)	(128)
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	33,380	24,755
Less than 12 Months Unrealized Loss	(3,343)	(190)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	33,380	24,755
Total Unrealized Loss	$ (3,343)	$ (190)