Fair Value (Carrying And Estimated Fair Values by Level Of Financial Instruments Not Recorded At Fair Value On Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 4,097	$ 4,258
Short-term borrowings	0	700
Commercial paper		400
Notes	24,273	20,470
Convertible debentures	426	904
Fair Value, Recurring [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	4,097	4,258
Short-term borrowings		0
Commercial paper		0
Notes	0	0
Convertible debentures	0	0
Fair Value, Recurring [Member] | Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	0	0
Short-term borrowings		700
Commercial paper		400
Notes	24,273	20,470
Convertible debentures	426	904
Fair Value, Recurring [Member] | Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	0	0
Short-term borrowings		0
Commercial paper		0
Notes	0	0
Convertible debentures	0	0
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	4,097	4,258
Short-term borrowings		700
Commercial paper		400
Notes	21,358	18,840
Convertible debentures	$ 118	$ 145