Fair Value (Carrying and Fair Value By Level of Financial Instruments Not Recorded At Fair Value On Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	$ 4,285	$ 4,258
Convertible debentures	108
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	4,285	4,258
Short-term borrowings		700
Commercial paper	250	400
Notes	19,677	18,840
Convertible debentures	108	145
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	4,285	4,258
Short-term borrowings		700
Commercial paper	250	400
Notes	23,307	20,470
Convertible debentures	712	904
Fair Value, Recurring [Member] | Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	0	0
Short-term borrowings		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, long-term	0	0
Short-term borrowings		700
Commercial paper	250	400
Notes	23,307	20,470
Convertible debentures	712	904
Fair Value, Recurring [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	4,285	4,258
Short-term borrowings		0
Commercial paper	0	0
Notes	0	0
Convertible debentures	$ 0	$ 0