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, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	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 ($)	Dec. 31, 2014 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 3,110
IBNR	622
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	201	$ 201	$ 202	$ 201	$ 201	$ 205	$ 205	$ 212	$ 223	$ 234
IBNR	$ 1
Cumulative Number of Claims | claim	33,633
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 184	182	183	181	183	190	190	199	$ 201
IBNR	$ 4
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	$ 197	200	195	190	186	186	186	$ 198
IBNR	$ 4
Cumulative Number of Claims | claim	30,455
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 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	$ 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	$ 325	323	289	266	$ 257
IBNR	$ 10
Cumulative Number of Claims | claim	37,258
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 298	304	303	$ 310
IBNR	$ 21
Cumulative Number of Claims | claim	29,142
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 390	388	$ 397
IBNR	$ 93
Cumulative Number of Claims | claim	32,918
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 465	$ 437
IBNR	$ 137
Cumulative Number of Claims | claim	36,777
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 554
IBNR	$ 347
Cumulative Number of Claims | claim	34,211