Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ in Millions	Dec. 31, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 3,745
IBNR	820
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	184	$ 184	$ 182	$ 183	$ 181	$ 183	$ 190	$ 190	$ 199	$ 201
IBNR	$ 2
Cumulative Number of Claims | claim	30,430
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 195	197	200	195	190	186	186	186	$ 198
IBNR	$ 1
Cumulative Number of Claims | claim	30,457
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 241	241	239	232	221	200	198	$ 199
IBNR	$ 4
Cumulative Number of Claims | claim	30,947
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 260	255	254	245	227	227	$ 229
IBNR	$ 2
Cumulative Number of Claims | claim	34,333
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 327	325	323	289	266	$ 257
IBNR	$ 7
Cumulative Number of Claims | claim	37,280
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 303	298	304	303	$ 310
IBNR	$ 14
Cumulative Number of Claims | claim	29,182
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 393	390	388	$ 397
IBNR	$ 51
Cumulative Number of Claims | claim	33,028
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 496	465	$ 437
IBNR	$ 90
Cumulative Number of Claims | claim	37,230
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 620	$ 554
IBNR	$ 202
Cumulative Number of Claims | claim	42,345
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 726
IBNR	$ 447
Cumulative Number of Claims | claim	40,718