Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - General Liability - 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
General Liability | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,929
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,741
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	119
Liability for unallocated claim adjustment expenses for accident years presented	51
Total net liability for unpaid claim and claim adjustment expenses	2,911
General Liability | 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	602	597	579	559	$ 510	$ 454	$ 374	$ 247	$ 132	$ 28
General Liability | 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	582	572	551	510	450	352	240	128	$ 31
General Liability | 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	607	569	547	481	376	247	119	$ 31
General Liability | 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	530	501	446	357	230	110	$ 19
General Liability | 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	524	481	407	279	163	$ 32
General Liability | 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	471	399	250	118	$ 23
General Liability | 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	307	228	107	$ 33
General Liability | 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	181	98	$ 25
General Liability | 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	99	$ 23
General Liability | 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	$ 26