Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 19,205	$ 18,701	$ 17,885	$ 17,965
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	8,065	7,475
Commercial Auto | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	1,928
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	654
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	5
Liability for unallocated claim adjustment expenses for accident years presented	14
Total net liability for unpaid claim and claim adjustment expenses	673
Commercial Auto | Commercial | 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	292	291	291	290	$ 285	$ 282	$ 259	$ 220	$ 160	$ 78
Commercial Auto | Commercial | 2013
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	239	239	238	234	225	200	168	135	$ 74
Commercial Auto | Commercial | 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	199	198	196	187	166	137	102	$ 64
Commercial Auto | Commercial | 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	178	175	172	153	130	96	$ 52
Commercial Auto | Commercial | 2016
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	185	175	154	126	93	$ 52
Commercial Auto | Commercial | 2017
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	203	178	150	107	$ 58
Commercial Auto | Commercial | 2018
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	212	175	128	$ 66
Commercial Auto | Commercial | 2019
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	203	147	$ 77
Commercial Auto | Commercial | 2020
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	134	$ 71
Commercial Auto | Commercial | 2021
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 83