Claim and Claim Adjustment Expense Reserves 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, 2022	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
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 19,908	$ 19,205	$ 18,701	$ 17,885
Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,563	5,265
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	5,721
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,359
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2013	99
Liability for unallocated claim adjustment expenses for accident years presented	56
Total net liability for unpaid claim and claim adjustment expenses	3,514
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	791	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	802	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	794	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	784	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	596	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	706	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	567	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	400	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	217	$ 58
Other Professional Liability and Management Liability | 2022 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 64