Debt - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Commercial paper	$ 25,287	$ 27,313
Debt	$ 98,228	$ 98,353
Weighted average contractual interest rates	2.38%	2.04%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 58,412	$ 57,402
Weighted average contractual interest rates	2.42%	2.18%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 14,529	$ 13,638
Weighted average contractual interest rates	2.36%	1.95%
Commercial paper [Member]
Debt Instrument [Line Items]
Weighted average contractual interest rates	2.32%	1.80%

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_CommercialPaper
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant