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, 2025 USD ($) claim	Dec. 31, 2024 USD ($)	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 ($)	Dec. 31, 2016 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,515
IBNR	1,085
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	194	$ 195	$ 197	$ 200	$ 195	$ 190	$ 186	$ 186	$ 186	$ 198
IBNR	$ 0
Cumulative Number of Claims | claim	30,459
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 237	241	241	239	232	221	200	198	$ 199
IBNR	$ 0
Cumulative Number of Claims | claim	30,948
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 259	260	255	254	245	227	227	$ 229
IBNR	$ 1
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	$ 323	327	325	323	289	266	$ 257
IBNR	$ 0
Cumulative Number of Claims | claim	37,281
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 299	303	298	304	303	$ 310
IBNR	$ 7
Cumulative Number of Claims | claim	29,192
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 377	393	390	388	$ 397
IBNR	$ 16
Cumulative Number of Claims | claim	33,063
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 506	496	465	$ 437
IBNR	$ 55
Cumulative Number of Claims | claim	37,327
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 635	620	$ 554
IBNR	$ 120
Cumulative Number of Claims | claim	42,921
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 799	$ 726
IBNR	$ 270
Cumulative Number of Claims | claim	49,670
2025
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 886
IBNR	$ 616
Cumulative Number of Claims | claim	41,075