Long-Term Debt - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Total borrowings at face value	$ 30,760	$ 35,235
Unamortized premium, net	688	748
Debt issuance costs	(135)	(156)
Net book value of debt	31,313	35,827
Long-term finance leases	330	316
Current finance leases	52	42
Total debt and finance leases	31,695	36,185
Less current maturities of financing leases	(52)	(42)
Less current maturities of long-term debt	(728)	(398)
Long-term debt, net	$ 30,915	$ 35,745