Debt (Narrative) (Details)		12 Months Ended
	Jun. 29, 2012	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 11, 2012 USD ($)	Jan. 21, 2011 USD ($)	Nov. 17, 2010 USD ($)
Debt Instrument [Line Items]
Commercial paper		$ 68,200,000	$ 20,000,000
Long-term Obligations		470,442,665	473,024,497
Repayments of long-term debt		$ 48,181,832	23,889,777	$ 24,682,812
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		$ 570,600,000
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		2.90%	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
Line of credit, expiration date		Oct. 12, 2017
Amended And Unsecured Credit Agreement [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity		$ 100,000,000
Yield margins for interest multiplier to total interest expense | item		1.10
Line of credit, borrowing rate description		one month London Interbank Offered Rate (LIBOR) plus 107.5 basis points, along with a 17.5 basis points facility fee (based on an A- unsecured debt rating).
Line of credit, expiration date		Nov. 17, 2016
Line of credit facility, first amendment date		May 04, 2012
Line of credit facility, second amendment date		Jun. 29, 2012
Facility fee	0.175%
Amended And Unsecured Credit Agreement [Member] | Unsecured Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Variable rate	1.075%
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
Amended And Unsecured Credit Agreement [Member] | Unsecured Credit Facility [Member] | Commercial Paper [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity							$ 300,000,000
Line of credit, commencement date		Nov. 17, 2010
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 CoBank Note [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term Obligations		$ 0	$ 24,941,165
Repayments of long-term debt		22,200,000
2016 CoBank Note [Member]
Debt Instrument [Line Items]
Bonds issuance amount		$ 45,600,000
Bonds maturity date		Mar. 15, 2031
Long-term Obligations		$ 43,776,000	$ 0
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
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%