Accounting Policies and Supplemental Disclosures - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounting Policies [Abstract]
Cash paid for interest on debt	$ 276	$ 285	$ 731	$ 715	$ 933	$ 869
Cash paid for operating leases	1,812	1,159	5,029	3,275	6,230	4,215
Cash paid for interest on finance leases	121	155	407	484	535	650
Cash paid for interest on financing obligations	48	28	116	71	147	90
Cash paid for income taxes, net of refunds	750	502	3,354	1,293	3,774	1,481
Assets acquired under operating leases	10,447	6,115	19,561	11,870	23,908	14,346
Property and equipment acquired under finance leases	1,744	3,571	5,453	8,892	8,149	13,075
Property and equipment acquired under build-to-suit arrangements	$ 1,721	$ 366	$ 3,702	$ 1,228	$ 4,742	$ 1,480