Leases (Supplemental Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases:
Operating lease right-of-use assets, net	$ 680	$ 504
Other current liabilities	96	62
Operating lease liabilities	633	450
Total operating lease liabilities	729	512
Finance Leases:
Property and equipment, at cost	29,405	28,332
Accumulated depreciation	(13,663)	(12,995)
Property and equipment, net	15,742	15,337
Current portion of long-term debt	10	9
Long-term debt	163	271
Total finance lease liabilities	173	280
Leaseholds and Leasehold Improvements [Member]
Finance Leases:
Property and equipment, at cost	184	294
Accumulated depreciation	(27)	(52)
Property and equipment, net	$ 157	$ 242