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, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,579
IBNR	2,049
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	883	$ 885	$ 888	$ 899	$ 911	$ 944	$ 949	$ 934	$ 908	$ 880
IBNR	$ 17
Cumulative Number of Claims | claim	18,745
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 850	831	833	846	840	878	887	909	$ 923
IBNR	$ 25
Cumulative Number of Claims | claim	18,504
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 846	850	863	866	885	926	894	$ 884
IBNR	$ 36
Cumulative Number of Claims | claim	17,939
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 845	854	835	831	885	898	$ 878
IBNR	$ 57
Cumulative Number of Claims | claim	17,568
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 813	807	832	877	892	$ 888
IBNR	$ 74
Cumulative Number of Claims | claim	17,417
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 907	904	900	900	$ 901
IBNR	$ 120
Cumulative Number of Claims | claim	17,946
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 791	813	845	$ 847
IBNR	$ 220
Cumulative Number of Claims | claim	18,118
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 869	864	$ 850
IBNR	$ 276
Cumulative Number of Claims | claim	19,789
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 845	$ 837
IBNR	$ 447
Cumulative Number of Claims | claim	19,157
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 930
IBNR	$ 777
Cumulative Number of Claims | claim	16,557