Debt (Narrative) (Details) (USD $)	3 Months Ended		3 Months Ended				3 Months Ended			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Minimum [Member]	Mar. 31, 2014 Maximum [Member]	Mar. 31, 2014 Unsecured Credit Agreement [Member]	Nov. 17, 2010 Unsecured Credit Agreement [Member] Initial Credit Agreement [Member]	Mar. 31, 2014 Unsecured Credit Agreement [Member] Amended Credit Agreement [Member]	Mar. 31, 2014 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member]	May 04, 2012 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member]	Mar. 31, 2014 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member] LIBOR [Member]	Mar. 31, 2014 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member] Facility Fee [Member]	Mar. 31, 2014 Term Loan Facility [Member]	Dec. 31, 2013 Term Loan Facility [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 50,000,000					$ 300,000,000			$ 100,000,000
Line of credit facility, outstanding amount	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.							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, 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							Nov. 17, 2016
Line of credit, initiation date					Nov. 17, 2010
Line of credit, amendment date							May 4, 2012	June 29, 2012
Commercial paper, basis points on variable rate										1.075%	0.175%
Commercial paper repricing term			1 day	270 days
Commercial paper	47,000,000	30,000,000
Debt, issuance date												Jan. 19, 2011
Debt outstanding												$ 28,600,000	$ 29,700,000