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, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,466
IBNR	1,957
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	821	$ 821	$ 827	$ 824	$ 837	$ 847	$ 848	$ 848	$ 828	$ 828
IBNR	$ 9
Cumulative Number of Claims | claim	17,891
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 885	888	899	911	944	949	934	908	$ 880
IBNR	$ 21
Cumulative Number of Claims | claim	18,738
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 831	833	846	840	878	887	909	$ 923
IBNR	$ 18
Cumulative Number of Claims | claim	18,499
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 850	863	866	885	926	894	$ 884
IBNR	$ 45
Cumulative Number of Claims | claim	17,928
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 854	835	831	885	898	$ 878
IBNR	$ 74
Cumulative Number of Claims | claim	17,553
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 807	832	877	892	$ 888
IBNR	$ 120
Cumulative Number of Claims | claim	17,390
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 904	900	900	$ 901
IBNR	$ 188
Cumulative Number of Claims | claim	17,890
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 813	845	$ 847
IBNR	$ 308
Cumulative Number of Claims | claim	18,015
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 864	$ 850
IBNR	$ 460
Cumulative Number of Claims | claim	19,468
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 837
IBNR	$ 714
Cumulative Number of Claims | claim	16,722