Long-Term Debt - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Total borrowings at face value	$ 35,061	$ 35,235
Unamortized premium, net	727	748
Debt issuance costs	(152)	(156)
Net book value of debt	35,636	35,827
Long-term finance leases	330	316
Current finance leases	59	42
Total debt and finance leases	36,025	36,185
Less current maturities of financing leases	(59)	(42)
Less current maturities of long-term debt	(500)	(398)
Long-term debt, net	$ 35,466	$ 35,745