Claim and Claim Adjustment Expense Reserves and Future Policy Benefit 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, 2022 USD ($) claim	Dec. 31, 2021 USD ($)	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 ($)	Dec. 31, 2013 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 9,080
IBNR	2,673
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	829	$ 833	$ 846	$ 850	$ 863	$ 866	$ 885	$ 926	$ 894	$ 884
IBNR	$ 21
Cumulative Number of Claims | claim	17,953
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 842	841	845	854	835	831	885	898	$ 878
IBNR	$ 29
Cumulative Number of Claims | claim	17,583
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 855	836	813	807	832	877	892	$ 888
IBNR	$ 30
Cumulative Number of Claims | claim	17,452
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 888	891	907	904	900	900	$ 901
IBNR	$ 57
Cumulative Number of Claims | claim	17,976
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 758	775	791	813	845	$ 847
IBNR	$ 107
Cumulative Number of Claims | claim	18,181
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 923	906	869	864	$ 850
IBNR	$ 142
Cumulative Number of Claims | claim	19,995
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 876	856	845	$ 837
IBNR	$ 173
Cumulative Number of Claims | claim	19,447
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 951	944	$ 930
IBNR	$ 408
Cumulative Number of Claims | claim	19,333
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,038	$ 1,037
IBNR	$ 706
Cumulative Number of Claims | claim	17,983
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,120
IBNR	$ 1,000
Cumulative Number of Claims | claim	15,327