Loans - Troubled Debt Restructured Loans (Details) - USD ($) $ in Thousands	Nov. 30, 2019	May 31, 2019	Nov. 30, 2018
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total recorded investment in loans	$ 26,426,879	$ 25,905,664	$ 25,282,953
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage	100.00%	100.00%
Performing TDR loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans outstanding	$ 11,097	$ 11,853
Performing TDR Loans As Percentage of Total Loans	0.04%	0.05%
Total TDR loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total recorded investment in loans	$ 11,097	$ 11,853
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage	0.04%	0.05%
CFC
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total recorded investment in loans	$ 25,368,276	$ 24,817,676
CFC \| Performing TDR loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans outstanding	$ 5,755	$ 6,261
Performing TDR Loans As Percentage of Total Loans	0.02%	0.03%
RTFC
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total recorded investment in loans	$ 356,324	$ 345,100
RTFC \| Performing TDR loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans outstanding	$ 5,342	$ 5,592
Performing TDR Loans As Percentage of Total Loans	0.02%	0.02%

Represents the percentage of total loan that is represented by a particular financial receivable category.

No definition available.

Name:	nru_LoansAndLeasesReceivableCommercialNetOfDeferredIncomePercentage
Namespace Prefix:	nru_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Performing TDR Loans As Percentage of Total Loans

+ References

No definition available.

+ Details

Name:	nru_PerformingTDRLoansAsPercentageofTotalLoans
Namespace Prefix:	nru_
Data Type:	num:percentItemType
Balance Type:	na
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_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nru_PerformingTroubledDebtRestructuringLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nru_TroubledDebtRestructuringFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=nru_RuralTelephoneFinanceCooperativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: