Claim, Claim Adjustment Expense 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, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,767
IBNR	2,326
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	848	$ 850	$ 831	$ 833	$ 846	$ 840	$ 878	$ 887	$ 909	$ 923
IBNR	$ 18
Cumulative Number of Claims | claim	18,506
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 833	846	850	863	866	885	926	894	$ 884
IBNR	$ 30
Cumulative Number of Claims | claim	17,950
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 841	845	854	835	831	885	898	$ 878
IBNR	$ 37
Cumulative Number of Claims | claim	17,577
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 836	813	807	832	877	892	$ 888
IBNR	$ 41
Cumulative Number of Claims | claim	17,436
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 891	907	904	900	900	$ 901
IBNR	$ 84
Cumulative Number of Claims | claim	17,968
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 775	791	813	845	$ 847
IBNR	$ 152
Cumulative Number of Claims | claim	18,159
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 906	869	864	$ 850
IBNR	$ 202
Cumulative Number of Claims | claim	19,926
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 856	845	$ 837
IBNR	$ 283
Cumulative Number of Claims | claim	19,357
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 944	$ 930
IBNR	$ 568
Cumulative Number of Claims | claim	19,095
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,037
IBNR	$ 911
Cumulative Number of Claims | claim	15,487