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, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	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 ($)	Dec. 31, 2014 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 9,443
IBNR	2,843
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	838	$ 842	$ 841	$ 845	$ 854	$ 835	$ 831	$ 885	$ 898	$ 878
IBNR	$ 26
Cumulative Number of Claims | claim	17,585
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 858	855	836	813	807	832	877	892	$ 888
IBNR	$ 18
Cumulative Number of Claims | claim	17,454
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 906	888	891	907	904	900	900	$ 901
IBNR	$ 39
Cumulative Number of Claims | claim	17,987
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 746	758	775	791	813	845	$ 847
IBNR	$ 69
Cumulative Number of Claims | claim	18,199
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 941	923	906	869	864	$ 850
IBNR	$ 101
Cumulative Number of Claims | claim	20,038
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 939	876	856	845	$ 837
IBNR	$ 100
Cumulative Number of Claims | claim	19,515
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 945	951	944	$ 930
IBNR	$ 281
Cumulative Number of Claims | claim	19,437
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,009	1,038	$ 1,037
IBNR	$ 532
Cumulative Number of Claims | claim	18,259
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,112	$ 1,120
IBNR	$ 706
Cumulative Number of Claims | claim	18,165
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,149
IBNR	$ 971
Cumulative Number of Claims | claim	16,469