Commitments and Contingencies - Commitments and Guarantees (Details) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023
Commitments:
Credit facilities commitments with dealers	$ 3,005	$ 3,153
Commitments under operating lease agreements	101	106
Total commitments	3,106	3,259
Total commitments and guarantees	3,206	3,359
Performance Guarantee [Member] \| Putnam and Gibson Counties [Member]
Commitments:
Guarantees of affiliate pollution control and solid waste disposal bonds	$ 100	$ 100

Commitments:

No definition available.

Name:	ck0000834071_CommitmentAbstract
Namespace Prefix:	ck0000834071_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of commitments the company entered into.

+ References

No definition available.

+ Details

Name:	ck0000834071_Commitments
Namespace Prefix:	ck0000834071_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate amount of commitments and guarantor obligations.

+ References

No definition available.

+ Details

Name:	ck0000834071_CommitmentsAndGuarantees
Namespace Prefix:	ck0000834071_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of credit facilities commitments with vehicle dealers.

+ References

No definition available.

+ Details

Name:	ck0000834071_CreditFacilitiesCommitmentsWithVehicleDealers
Namespace Prefix:	ck0000834071_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482425/460-10-50-4

+ Details

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

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-6

+ Details

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

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_PerformanceGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=ck0000834071_PutnamAndGibsonCountiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: