Debt (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commercial paper	$ 51,000,000	$ 68,200,000
Balance outstanding	$ 459,795,331	445,605,998
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
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	2.75%	2.50%
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	Sep. 29, 2022
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 London Interbank Offered Rate (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%
2016 CoBank Note [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 20, 2031
Interest rate	2.58%
Average Life (Years)	15 years
Balance outstanding	$ 41,200,000	$ 43,800,000
2017 Series A Bonds [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