Fair Value (Financial Assets and Liabilities, Not Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 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	$ 735	$ 975
Debt Securities, Held-to-maturity	725	972
Debt and other financing obligations, carrying value	50,509	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	735	975
Debt and other financing obligations	45,272	45,078
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	556	541
Debt and other financing obligations	0	0
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	91	181
Debt and other financing obligations	45,272	45,078
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	88	253
Debt and other financing obligations	0	0
Long-term debt and other financing obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt and other financing obligations, carrying value	$ 40,394	$ 40,278