Accounting Policies and Supplemental Disclosures - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Accounting Policies [Abstract]
Cash paid for interest on debt	$ 276	$ 290	$ 902	$ 879
Cash paid for operating leases	1,640	1,029	5,086	3,680
Cash paid for interest on finance leases	157	168	601	650
Cash paid for interest on financing obligations	33	22	113	59
Cash paid for income taxes, net of refunds	801	305	2,209	1,017
Assets acquired under operating leases	3,536	2,408	17,345	9,403
Property and equipment acquired under finance leases	2,067	2,166	11,489	13,262
Property and equipment acquired under build-to-suit arrangements	$ 887	$ 379	$ 2,775	$ 1,304