Debt (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Jan. 11, 2012 USD ($)	Jan. 21, 2011 USD ($)
Debt Instrument [Line Items]
Commercial paper	$ 50,000,000	$ 68,200,000
Balance outstanding	$ 485,605,998	470,442,665
Patronage Capital [Member]
Debt Instrument [Line Items]
Secured debt covenant restriction at period end	5.00%
Indenture [Member] | Minimum [Member] | Power Sales Agreements With Members [Member]
Debt Instrument [Line Items]
Assets excluded from lien, term of contract	3 years
Indenture [Member] | Maximum [Member]
Debt Instrument [Line Items]
Assets excluded from lien, term of contract	5 years
Term Loan Facility [Member]
Debt Instrument [Line Items]
Bonds issuance amount	$ 610,600,000
Commercial Paper [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commercial paper repricing term	1 day
Commercial Paper [Member] | Maximum [Member]
Debt Instrument [Line Items]
Commercial paper repricing term	270 days
Revolving Line Of Credit [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 50,000,000
Line of credit, outstanding balance	$ 0	$ 0
Line of credit, borrowing rate description	The borrowing rate is calculated using the total rate per annum and may be fixed by NRUCFC.
Line of credit, borrowing rate	3.00%	2.90%
Period of time the line of credit needs to be paid down for five consecutive days	12 months
Number of consecutive days debt needs to be paid down to $0 during a twelve month period	5 days
Amended And Unsecured Credit Agreement [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Yield margins for interest multiplier to total interest expense | item	1.10
Amended And Unsecured Credit Agreement [Member] | Unsecured Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 150,000,000
Credit Agreement [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 150,000,000
Line of credit, borrowing rate description	one month LIBOR plus 90.0 basis points, along with a 10.0 basis points facility fee (based on an A/A2/A unsecured debt rating).
Line of credit, commencement date	Jun. 13, 2016
Line of credit, expiration date	Jun. 13, 2021
Facility fee	0.10%
Credit Agreement [Member] | Unsecured Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Variable rate	0.90%
2011 Series A Bonds [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Bonds issuance amount				$ 275,000,000
Bonds commencement payment date	Sep. 15, 2011
Note commencement date	Jan. 21, 2011
2012 Series A Bonds [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Bonds issuance amount			$ 250,000,000
Bonds commencement payment date	Sep. 15, 2012
Note commencement date	Jan. 11, 2012
2016 CoBank Note [Member]
Debt Instrument [Line Items]
Bonds issuance amount	$ 45,600,000
Maturity date	Mar. 15, 2031
Interest rate	2.58%	2.58%
Average Life (Years)	5 years 8 months 12 days
Balance outstanding	$ 40,356,000	$ 43,776,000
2017 Series A Bonds [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Bonds issuance amount	$ 40,000,000
Maturity date	Mar. 15, 2037
Interest rate	3.43%
Average Life (Years)	10 years 2 months 12 days
Balance outstanding	$ 40,000,000	$ 0
CoBank Master Loan Agreement [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Yield margins for interest multiplier to total interest expense | item	1.10
Line of credit facility, first amendment date	Jun. 30, 2016
Second Amended and Restated Master Loan Agreement [Member] | Indenture [Member]
Debt Instrument [Line Items]
Yield margins for interest multiplier to total interest expense | item	1.10
Assignable Margins [Member]
Debt Instrument [Line Items]
Secured debt covenant restriction during period	50.00%
Aggregate Equities And Margins To Total Liabilities And Equities And Margins [Member] | Minimum [Member]
Debt Instrument [Line Items]
Secured debt covenant restriction during period	30.00%