Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease right-of-use assets	$ 1,928	$ 2,285
Accounts payable and other current liabilities	391	455
Operating lease liabilities	1,286	1,632
Total operating lease liabilities	1,677	2,087
Property and equipment	804	794
Accumulated depreciation	(386)	(349)
Property and equipment, net	418	445
Long-term debt due within one year	186	48
Long-term debt	139	324
Total finance lease liabilities	$ 325	$ 372