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Fair Value (Financial Assets and Liabilities, Not Measured at Fair Value on a Recurring Basis) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Held-to-maturity, Fair Value $ 647 $ 975
Debt Securities, Held-to-maturity 638 972
Debt and other financing obligations, carrying value 42,280 39,474
Fair Value, Nonrecurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Held-to-maturity, Fair Value 647 975
Fair Value, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Held-to-maturity, Fair Value 466 541
Fair Value, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Held-to-maturity, Fair Value 108 181
Fair Value, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Held-to-maturity, Fair Value 73 253
Long-term debt and other financing obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and other financing obligations, carrying value 42,171 40,278
Long-term debt and other financing obligations [Member] | Fair Value, Nonrecurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and other financing obligations 51,254 45,078
Long-term debt and other financing obligations [Member] | Fair Value, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and other financing obligations 0 0
Long-term debt and other financing obligations [Member] | Fair Value, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and other financing obligations 51,254 45,078
Long-term debt and other financing obligations [Member] | Fair Value, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and other financing obligations $ 0 $ 0