Fair Value Measurements - Assets and Liabilities Not Carried at Fair Value on Recurring Basis on Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Financial liabilities
Financial liabilities	$ 28,041	$ 26,632
Financial liabilities	98,497	98,233
Unsecured notes and loans payable [Member]
Financial liabilities
Financial liabilities	56,742	57,282
Secured notes and loans payable [Member]
Financial liabilities
Financial liabilities	13,714	14,319
Fair value [Member] \| Commercial paper [Member]
Financial liabilities
Financial liabilities	28,041	26,632
Fair value [Member] \| Unsecured notes and loans payable [Member]
Financial liabilities
Financial liabilities	57,433	58,223
Fair value [Member] \| Secured notes and loans payable [Member]
Financial liabilities
Financial liabilities	13,708	14,322
Fair value [Member] \| Level 1 [Member] \| Commercial paper [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] \| Level 1 [Member] \| Unsecured notes and loans payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] \| Level 1 [Member] \| Secured notes and loans payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] \| Level 2 [Member] \| Commercial paper [Member]
Financial liabilities
Financial liabilities	28,041	26,632
Fair value [Member] \| Level 2 [Member] \| Unsecured notes and loans payable [Member]
Financial liabilities
Financial liabilities	55,078	55,838
Fair value [Member] \| Level 2 [Member] \| Secured notes and loans payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] \| Level 3 [Member] \| Commercial paper [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] \| Level 3 [Member] \| Unsecured notes and loans payable [Member]
Financial liabilities
Financial liabilities	2,355	2,385
Fair value [Member] \| Level 3 [Member] \| Secured notes and loans payable [Member]
Financial liabilities
Financial liabilities	13,708	14,322
Retail Loan [Member] \| Carrying value [Member]
Financial assets
Finance receivables, net	51,945	50,682
Retail Loan [Member] \| Fair value [Member]
Financial assets
Finance receivables, net	51,823	50,733
Retail Loan [Member] \| Fair value [Member] \| Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Retail Loan [Member] \| Fair value [Member] \| Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Retail Loan [Member] \| Fair value [Member] \| Level 3 [Member]
Financial assets
Finance receivables, net	51,823	50,733
Wholesale [Member] \| Carrying value [Member]
Financial assets
Finance receivables, net	9,980	10,819
Wholesale [Member] \| Fair value [Member]
Financial assets
Finance receivables, net	10,040	10,881
Wholesale [Member] \| Fair value [Member] \| Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Wholesale [Member] \| Fair value [Member] \| Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Wholesale [Member] \| Fair value [Member] \| Level 3 [Member]
Financial assets
Finance receivables, net	10,040	10,881
Real estate [Member] \| Carrying value [Member]
Financial assets
Finance receivables, net	4,547	4,602
Real estate [Member] \| Fair value [Member]
Financial assets
Finance receivables, net	4,471	4,459
Real estate [Member] \| Fair value [Member] \| Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Real estate [Member] \| Fair value [Member] \| Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Real estate [Member] \| Fair value [Member] \| Level 3 [Member]
Financial assets
Finance receivables, net	4,471	4,459
Working capital [Member] \| Carrying value [Member]
Financial assets
Finance receivables, net	2,269	2,218
Working capital [Member] \| Fair value [Member]
Financial assets
Finance receivables, net	2,301	2,222
Working capital [Member] \| Fair value [Member] \| Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Working capital [Member] \| Fair value [Member] \| Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Working capital [Member] \| Fair value [Member] \| Level 3 [Member]
Financial assets
Finance receivables, net	$ 2,301	$ 2,222

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://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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- Definition

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+ References

No definition available.

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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