Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 20,617	$ 19,263	$ 18,163	$ 16,929
Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	6,188	5,979
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	6,324
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,905
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2016	101
Liability for unallocated claim adjustment expenses for accident years presented	67
Total net liability for unpaid claim and claim adjustment expenses	4,073
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	862	856	826	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	699	672	630	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	886	847	779	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	864	801	699	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	751	660	523	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	634	502	356	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	638	453	225	$ 64
Other Professional Liability and Management Liability | 2023 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	594	302	$ 64
Other Professional Liability and Management Liability | 2024 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	315	$ 77
Other Professional Liability and Management Liability | 2025 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 81