Investments - Fixed Maturity Securities and Short Term Investments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Percent of Total	100.00%		100.00%
Amortized Cost	$ 6,565		$ 6,746
Allowance for Credit Losses	(66)	$ (69)	(77)	$ (86)	$ (79)	$ (65)
Gross Unrealized Gains	96		55
Gross Unrealized Losses	(311)		(417)
Estimated Fair Value	$ 6,284		$ 6,307
Government agency obligations as a percentage of total mortgage backed securities	67.00%		42.00%
Obligations of state and political subdivisions
Schedule of Investments [Line Items]
Percent of Total	33.00%		41.00%
Amortized Cost	$ 2,171		$ 2,733
Allowance for Credit Losses	(14)		(13)
Gross Unrealized Gains	33		33
Gross Unrealized Losses	(71)		(92)
Estimated Fair Value	$ 2,119		$ 2,661
U.S. government and agencies
Schedule of Investments [Line Items]
Percent of Total	1.00%		1.00%
Amortized Cost	$ 75		$ 65
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(5)		(6)
Estimated Fair Value	$ 71		$ 60
Corporate securities
Schedule of Investments [Line Items]
Percent of Total	38.00%		34.00%
Amortized Cost	$ 2,503		$ 2,327
Allowance for Credit Losses	(6)		(6)
Gross Unrealized Gains	43		17
Gross Unrealized Losses	(136)		(197)
Estimated Fair Value	$ 2,404		$ 2,141
RMBS
Schedule of Investments [Line Items]
Percent of Total	10.00%		6.00%
Amortized Cost	$ 613		$ 428
Allowance for Credit Losses	(20)		(21)
Gross Unrealized Gains	11		3
Gross Unrealized Losses	(60)		(68)
Estimated Fair Value	$ 544		$ 342
CMBS
Schedule of Investments [Line Items]
Percent of Total	3.00%		2.00%
Amortized Cost	$ 180		$ 157
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3		0
Gross Unrealized Losses	(3)		(6)
Estimated Fair Value	$ 180		$ 151
CLOs
Schedule of Investments [Line Items]
Percent of Total	6.00%		7.00%
Amortized Cost	$ 384		$ 456
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(2)		(7)
Estimated Fair Value	$ 383		$ 450
Others
Schedule of Investments [Line Items]
Percent of Total	8.00%		7.00%
Amortized Cost	$ 556		$ 465
Allowance for Credit Losses	(26)		(37)
Gross Unrealized Gains	3		0
Gross Unrealized Losses	(25)		(26)
Estimated Fair Value	$ 508		$ 402
Non-U.S. government securities
Schedule of Investments [Line Items]
Percent of Total	1.00%		2.00%
Amortized Cost	$ 83		$ 115
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		0
Gross Unrealized Losses	(9)		(15)
Estimated Fair Value	$ 75		$ 100