Short-Term Borrowings - Additional Information (Details) - USD ($)	6 Months Ended
Nov. 30, 2019	May 31, 2019
Short-term Debt [Line Items]
Short-term borrowings	$ 4,789,024,000	$ 3,607,726,000
Short Term Debt as Percentage of Debt Outstanding	19.00%	14.00%
Revolving credit agreements
Short-term Debt [Line Items]
Short-term borrowings	$ 0	$ 0
Maximum borrowing capacity	2,725,000,000	2,975,000,000
Letter of Credit Maximum Amount Available	300,000,000
Three Year Agreement \| Revolving credit agreements
Short-term Debt [Line Items]
Maximum borrowing capacity	1,315,000,000	1,440,000,000
Line Of Credit Facility Terminated	125,000,000
Five Year Agreement \| Revolving credit agreements
Short-term Debt [Line Items]
Maximum borrowing capacity	1,410,000,000	$ 1,535,000,000
Line Of Credit Facility Terminated	$ 125,000,000

Represents the maximum amount of letters of credit that are available under the credit facility.

No definition available.

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

- Definition

Line Of Credit Facility Terminated

+ References

No definition available.

+ Details

Name:	nru_LineOfCreditFacilityTerminated
Namespace Prefix:	nru_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Short Term Debt as Percentage of Debt Outstanding

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: 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(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: 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(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_ShortTermBorrowings
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

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nru_ThreeYearAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nru_FiveYearAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: