Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Specialty - Other Professional Liability and Management 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 Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 19,205	$ 18,701	$ 17,885	$ 17,965
Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,265	4,898
Other Professional Liability and Management Liability | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	5,735
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,032
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	74
Liability for unallocated claim adjustment expenses for accident years presented	53
Total net liability for unpaid claim and claim adjustment expenses	3,159
Other Professional Liability and Management Liability | 2012 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	816	812	792	755	$ 711	$ 651	$ 573	$ 400	$ 248	$ 56
Other Professional Liability and Management Liability | 2013 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	787	779	771	754	702	618	447	249	$ 54
Other Professional Liability and Management Liability | 2014 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	787	743	707	647	515	392	223	$ 51
Other Professional Liability and Management Liability | 2015 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	725	677	612	542	404	234	$ 60
Other Professional Liability and Management Liability | 2016 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	736	701	625	466	248	$ 64
Other Professional Liability and Management Liability | 2017 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	557	498	394	222	$ 57
Other Professional Liability and Management Liability | 2018 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	599	473	282	$ 54
Other Professional Liability and Management Liability | 2019 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	422	263	$ 64
Other Professional Liability and Management Liability | 2020 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	248	$ 67
Other Professional Liability and Management Liability | 2021 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 58