Accounting Policies and Supplemental Disclosures - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounting Policies [Abstract]
Cash paid for interest on debt	$ 285	$ 287	$ 715	$ 720	$ 869	$ 842
Cash paid for operating leases	1,159	872	3,275	2,420	4,215	2,420
Cash paid for interest on finance leases	155	167	484	481	650	585
Cash paid for interest on financing obligations	28	14	71	20	90	72
Cash paid for income taxes, net of refunds	502	241	1,293	692	1,481	863
Assets acquired under operating leases	6,115	2,299	11,870	5,393	14,346	5,393
Property and equipment acquired under finance leases	3,571	3,606	8,892	9,541	13,075	13,222
Property and equipment acquired under build-to-suit arrangements	$ 366	$ 390	$ 1,228	$ 1,109	$ 1,480	$ 2,252