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, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 4,917	$ 4,285
Short-term borrowings	175	0
Commercial paper	550	250
Notes	25,425	23,307
Convertible debentures	610	712
Fair Value, Recurring [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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		0
Short-term borrowings	175
Commercial paper	550	250
Notes	25,425	23,307
Convertible debentures	610	712
Fair Value, Recurring [Member] | Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term borrowings	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]
Short-term borrowings	175
Commercial paper	550	250
Notes	22,439	19,677
Convertible debentures	77	108
Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	4,733	4,285
Other Investments [Member] | Fair Value, Recurring [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	4,733	4,285
Other Investments [Member] | Fair Value, Recurring [Member] | Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	0
Other Investments [Member] | 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
Other Investments [Member] | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 4,733	$ 4,285