INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Mar. 31, 2022
Less than 12 Months Fair Value	$ 2,802,415	$ 1,970,822
Less than 12 Months Unrealized Loss	(134,020)	(27,427)
12 Months or More Fair Value	435,724	171,838
12 Months or More Unrealized Loss	(37,927)	(4,187)
Total Fair Value	3,238,139	2,142,660
Total Unrealized Loss	(171,947)	(31,614)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	98,897	31,896
Less than 12 Months Unrealized Loss	(2,103)	(354)
12 Months or More Fair Value	11,569	11,877
12 Months or More Unrealized Loss	(681)	(373)
Total Fair Value	110,466	43,773
Total Unrealized Loss	(2,784)	(727)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	625,241	482,118
Less than 12 Months Unrealized Loss	(19,707)	(2,578)
12 Months or More Fair Value	22,710	0
12 Months or More Unrealized Loss	(1,614)	0
Total Fair Value	647,951	482,118
Total Unrealized Loss	(21,321)	(2,578)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	496,869	380,735
Less than 12 Months Unrealized Loss	(38,448)	(7,543)
12 Months or More Fair Value	132,317	55,568
12 Months or More Unrealized Loss	(10,812)	(1,051)
Total Fair Value	629,186	436,303
Total Unrealized Loss	(49,260)	(8,594)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	449,683	369,318
Less than 12 Months Unrealized Loss	(22,616)	(6,110)
12 Months or More Fair Value	52,838	12,485
12 Months or More Unrealized Loss	(5,333)	(387)
Total Fair Value	502,521	381,803
Total Unrealized Loss	(27,949)	(6,497)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	636,688	205,520
Less than 12 Months Unrealized Loss	(16,831)	(2,149)
12 Months or More Fair Value	28,585	13,818
12 Months or More Unrealized Loss	(2,475)	(203)
Total Fair Value	665,273	219,338
Total Unrealized Loss	(19,306)	(2,352)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	388,740	459,098
Less than 12 Months Unrealized Loss	(26,406)	(8,375)
12 Months or More Fair Value	187,705	78,090
12 Months or More Unrealized Loss	(17,012)	(2,173)
Total Fair Value	576,445	537,188
Total Unrealized Loss	(43,418)	(10,548)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	73,673	17,382
Less than 12 Months Unrealized Loss	(5,665)	(128)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	73,673	17,382
Total Unrealized Loss	(5,665)	(128)
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	32,624	24,755
Less than 12 Months Unrealized Loss	(2,244)	(190)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	32,624	24,755
Total Unrealized Loss	$ (2,244)	$ (190)