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, 2017 USD ($) claims	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,398
IBNR	1,897
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	818	$ 827	$ 845	$ 850	$ 880	$ 915	$ 924	$ 954	$ 933	$ 916
IBNR	$ 26
Cumulative Number of Claims | claims	16,331
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 901	903	895	900	891	898	903	873	$ 829
IBNR	$ 32
Cumulative Number of Claims | claims	17,274
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 826	823	836	846	848	850	827	$ 825
IBNR	$ 31
Cumulative Number of Claims | claims	17,805
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 898	910	944	948	933	904	$ 876
IBNR	$ 71
Cumulative Number of Claims | claims	18,643
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 832	833	870	876	894	$ 907
IBNR	$ 73
Cumulative Number of Claims | claims	18,262
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 824	840	879	841	$ 844
IBNR	$ 83
Cumulative Number of Claims | claims	17,362
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 798	854	859	$ 841
IBNR	$ 158
Cumulative Number of Claims | claims	16,984
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 832	851	$ 847
IBNR	$ 296
Cumulative Number of Claims | claims	16,603
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 859	$ 859
IBNR	$ 426
Cumulative Number of Claims | claims	17,004
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 810
IBNR	$ 701
Cumulative Number of Claims | claims	15,206