Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Other Professional Liability and Management Liability - Specialty $ in Millions	Dec. 31, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 9,796
IBNR	2,688
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	865	$ 858	$ 855	$ 836	$ 813	$ 807	$ 832	$ 877	$ 892	$ 888
IBNR	$ 21
Cumulative Number of Claims | claim	17,457
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 912	906	888	891	907	904	900	900	$ 901
IBNR	$ 37
Cumulative Number of Claims | claim	17,989
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 752	746	758	775	791	813	845	$ 847
IBNR	$ 51
Cumulative Number of Claims | claim	18,215
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 987	941	923	906	869	864	$ 850
IBNR	$ 57
Cumulative Number of Claims | claim	20,071
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 970	939	876	856	845	$ 837
IBNR	$ 96
Cumulative Number of Claims | claim	19,548
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 945	945	951	944	$ 930
IBNR	$ 168
Cumulative Number of Claims | claim	19,509
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 965	1,009	1,038	$ 1,037
IBNR	$ 311
Cumulative Number of Claims | claim	18,377
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,084	1,112	$ 1,120
IBNR	$ 465
Cumulative Number of Claims | claim	18,376
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,166	$ 1,149
IBNR	$ 564
Cumulative Number of Claims | claim	19,587
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,150
IBNR	$ 918
Cumulative Number of Claims | claim	17,921