Borrowings and Other Financing Instruments, Intercompany Borrowing Arrangement and Other Short-Term Borrowings (Details) (USD $)			3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2011 Intercompany Borrowing Arrangement [Member] NSP-Minnesota [Member]	Dec. 31, 2010 Intercompany Borrowing Arrangement [Member] NSP-Minnesota [Member]	Dec. 31, 2012 Notes Payable [Member]	Dec. 31, 2011 Notes Payable [Member]
Short-term Debt [Line Items]
Borrowings during the period			$ 0
Borrowing limit				100,000,000
Amount outstanding at period end	39,000,000	66,000,000		37,000,000
Average amount outstanding				11,000,000
Maximum amount outstanding				59,000,000
Notes payable to affiliates	$ 550,000	$ 500,000			$ 550,000	$ 500,000
Weighted average interest rate, computed on a daily basis (in hundredths)				0.33%
Weighted average interest rate at end of period (in hundredths)				0.38%	0.33%	0.46%