Commitments and Contingencies (Outstanding Performance Letters Of Credit) (Details) $ in Millions	Dec. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	$ 749
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	151
CNA [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	49
AIG Insurance [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	28
Zurich [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	73
All Other [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, amount outstanding	$ 1