Debt (Narrative) (Details) (USD $)	3 Months Ended		3 Months Ended				3 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2013 Unsecured Credit Agreement [Member] Initial Credit Agreement [Member]	Nov. 17, 2010 Unsecured Credit Agreement [Member] Initial Credit Agreement [Member]	Mar. 31, 2013 Unsecured Credit Agreement [Member] Amended Credit Agreement [Member]	May 04, 2012 Unsecured Credit Agreement [Member] Amended Credit Agreement [Member]	Mar. 31, 2013 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member]	Jun. 29, 2012 Unsecured Credit Agreement [Member] Amended And Extended Credit Agreement [Member]	Mar. 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
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, borrowing rate	2.90%	2.90%
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-/A3 unsecured debt rating)
Line of credit facility, outstanding amount	0	0
Line of credit, basis spread on variable rate										1.075%
Line of credit facility, basis points facility fee										0.175%
Line of credit, initiation date					Nov. 17, 2010
Line of credit, amendment date							May 4, 2012		June 29, 2012
Line of credit, expiration date	Oct. 12, 2017								Nov. 17, 2016
Commercial paper	40,000,000	11,500,000
Commercial paper repricing term			1 day	270 days
Debt, issuance date											Jan. 19, 2011
Debt outstanding											$ 30,800,000