Long-Term Debt - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Total borrowings at face value	$ 35,061	$ 35,235
Unamortized premium, net	707	748
Debt issuance costs	(147)	(156)
Net book value of debt	35,621	35,827
Long-term finance leases	332	316
Current finance leases	50	42
Total debt and finance leases	36,003	36,185
Less current maturities of financing leases	(50)	(42)
Less current maturities of long-term debt	(601)	(398)
Long-term debt, net	$ 35,352	$ 35,745