Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ in Millions	Dec. 31, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 2,582
IBNR	433
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	295	$ 296	$ 297	$ 299	$ 299	$ 307	$ 303	$ 299	$ 289	$ 275
IBNR	$ 1
Cumulative Number of Claims | claim	46,288
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 241	241	241	245	245	249	265	265	$ 246
IBNR	$ 2
Cumulative Number of Claims | claim	39,430
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 202	201	201	205	205	212	223	$ 234
IBNR	$ 2
Cumulative Number of Claims | claim	33,628
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 183	181	183	190	190	199	$ 201
IBNR	$ 3
Cumulative Number of Claims | claim	30,427
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 195	190	186	186	186	$ 198
IBNR	$ 3
Cumulative Number of Claims | claim	30,449
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 232	221	200	198	$ 199
IBNR	$ 5
Cumulative Number of Claims | claim	30,940
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 245	227	227	$ 229
IBNR	$ 8
Cumulative Number of Claims | claim	34,292
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 289	266	$ 257
IBNR	$ 31
Cumulative Number of Claims | claim	37,142
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 303	$ 310
IBNR	$ 107
Cumulative Number of Claims | claim	28,837
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 397
IBNR	$ 271
Cumulative Number of Claims | claim	27,182