Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended	120 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total	$ 33	$ 49	$ 53
Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	28	45	53
Total net development for accident years prior to 2014	2	4	0
Total unallocated claim adjustment expense development	3	0	0
Total	33	49	53
2014 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(1)	1	$ 0	$ (4)	$ 0	$ (7)	$ (11)	$ (11)									$ (33)
2015 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	(1)	2	(2)	(7)	0	(9)	$ (2)									$ (17)
2016 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(3)	5	5	4	0	0	$ (12)									$ (1)
2017 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	7	11	21	2	$ (1)									$ 42
2018 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	9	18	0	$ (2)									$ 26
2019 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	34	23	$ 9									$ 68
2020 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(6)	1	$ (7)									$ (12)
2021 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	$ (9)									$ (7)
2022 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 28									$ 28