Guarantees - Additional Information (Details) - USD ($)	Nov. 30, 2021	May 31, 2021
Guarantees
Guarantor obligations	$ 657,981,000	$ 689,080,000
Guarantee obligations unsecured	$ 389,000,000	$ 415,000,000
Percentage of total commitment	59.00%	60.00%
Guarantee liability recorded	$ 10,000,000	$ 10,000,000
Guaranty liabilities	10,000,000	9,000,000
Long-term tax-exempt bonds
Guarantees
Guarantor obligations	123,775,000	145,025,000
Letters of credit
Guarantees
Guarantor obligations	377,738,000	389,735,000
Guarantee obligations secured	121,000,000	104,000,000
Master letter of credit \| Master letter of credit
Guarantees
Maximum additional amount potentially required to be issued	91,000,000
Other guarantees
Guarantees
Guarantor obligations	156,468,000	154,320,000
Guarantor obligations, maximum exposure, undiscounted	157,000,000	154,000,000
Other secured guarantees
Guarantees
Guarantor obligations, maximum exposure, undiscounted	25,000,000	25,000,000
Financial standby letter of credit \| Adjustable and floating rate tax exempt bonds
Guarantees
Guarantor obligations	$ 0	$ 0

Guarantee Liability Recorded

No definition available.

Name:	nru_GuaranteeLiabilityRecorded
Namespace Prefix:	nru_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of guarantee obligations secured.

+ References

No definition available.

+ Details

Name:	nru_GuaranteeObligationsSecured
Namespace Prefix:	nru_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of guarantee obligations with unsecured rights of recovery from the entity's members.

+ References

No definition available.

+ Details

Name:	nru_GuaranteeObligationsUnsecured
Namespace Prefix:	nru_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the unsecured guarantee obligations as a percentage of total guarantee obligations.

+ References

No definition available.

+ Details

Name:	nru_GuaranteeObligationsUnsecuredCommitmentAsPercentageOfTotalCommitment
Namespace Prefix:	nru_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the maximum potential additional amount that may be required to be issued to third parties for the benefit of the entity's members.

+ References

No definition available.

+ Details

Name:	nru_LetterOfCreditFacilityMaximumAdditionalAmountPotentiallyRequiredToBeIssued
Namespace Prefix:	nru_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=124440162&loc=d3e12069-110248

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_GuaranteeObligationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=124440162&loc=d3e12069-110248

+ Details

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

- Definition

This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=123422226&loc=d3e11019-110243

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123422226&loc=d3e11049-110243

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=nru_MasterLetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=nru_AdjustableAndFloatingRateTaxExemptBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: