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Fair Value of Financial Instruments (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets measured at fair value $ 303,060 $ 304,561
Liabilities measured at fair value 6,845 7,091
Fair Value, Nonrecurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets measured at fair value 0 0
Liabilities measured at fair value $ 0 $ 0