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	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 4,181	$ 4,258
Short-term borrowings	1,075	700
Senior revolving credit facility	300
Commercial paper	1,305	400
Notes	20,343	20,470
Convertible debentures	680	904
Fair Value, Recurring [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	4,181	4,258
Short-term borrowings	0	0
Senior revolving credit facility	0
Commercial paper	0	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	1,075	700
Senior revolving credit facility	300
Commercial paper	1,305	400
Notes	20,343	20,470
Convertible debentures	680	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	0
Senior revolving credit facility	0
Commercial paper	0	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	4,181	4,258
Short-term borrowings	1,075	700
Senior revolving credit facility	300
Commercial paper	1,305	400
Notes	18,867	18,840
Convertible debentures	$ 136	$ 145