Loans - Available Balance Under Unadvanced Commitments and Maturity (Details) - Unadvanced commitments - USD ($) $ in Thousands	Nov. 30, 2021	May 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available Balance	$ 14,678,351	$ 14,357,039
2022	973,413
2023	5,194,921
2024	2,785,103
2025	2,371,592
2026	1,533,052
Thereafter	1,820,270
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available Balance	9,014,706	8,585,226
2022	705,293
2023	4,351,138
2024	1,184,200
2025	1,516,986
2026	449,196
Thereafter	807,893
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available Balance	5,663,645	$ 5,771,813
2022	268,120
2023	843,783
2024	1,600,903
2025	854,606
2026	1,083,856
Thereafter	$ 1,012,377

Other Commitment, to be Paid, after Year Four

No definition available.

Name:	nru_OtherCommitmentToBePaidAfterYearFour
Namespace Prefix:	nru_
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

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

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in remainder of current fiscal year.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=nru_LineOfCreditLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=nru_LongTermLoansMember
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Data Type:	na
Balance Type:
Period Type: