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, 2018 USD ($) claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,559
IBNR	1,948
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	892	$ 904	$ 906	$ 897	$ 903	$ 893	$ 903	$ 908	$ 875	$ 831
IBNR	$ 18
Cumulative Number of Claims | claims	17,374
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 821	827	824	837	847	848	848	828	$ 828
IBNR	$ 19
Cumulative Number of Claims | claims	17,888
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 888	899	911	944	949	934	908	$ 880
IBNR	$ 39
Cumulative Number of Claims | claims	18,728
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 833	846	840	878	887	909	$ 923
IBNR	$ 41
Cumulative Number of Claims | claims	18,491
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 863	866	885	926	894	$ 884
IBNR	$ 65
Cumulative Number of Claims | claims	17,918
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 835	831	885	898	$ 878
IBNR	$ 88
Cumulative Number of Claims | claims	17,515
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 832	877	892	$ 888
IBNR	$ 194
Cumulative Number of Claims | claims	17,333
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 900	900	$ 901
IBNR	$ 279
Cumulative Number of Claims | claims	17,787
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 845	$ 847
IBNR	$ 479
Cumulative Number of Claims | claims	17,780
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 850
IBNR	$ 726
Cumulative Number of Claims | claims	16,564