Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) (First Mortgage Bonds, USD $)	1 Months Ended			3 Months Ended		1 Months Ended
	Mar. 31, 2014 PSCo Series Due March 15, 2044	May 31, 2014 NSP-Minnesota Series Due May 15, 2044	Jun. 30, 2014 SPS Series No. 3 Due June 15, 2024	Jun. 30, 2014 SPS Issued Collateral on Series G Senior Note Due Dec. 1, 2018	Jun. 30, 2014 SPS Previously Issued Series G Senior Note Due Dec. 1, 2018	Jun. 30, 2014 NSP-Wisconsin Series Due June 15, 2024
Long-Term Borrowings and Other Financing Instruments [Abstract]
Face amount	$ 300,000,000	$ 300,000,000	$ 150,000,000	$ 250,000,000		$ 100,000,000
Interest rate, stated percentage (in hundredths)	4.30%	4.125%	3.30%	8.75%	8.75%	3.30%
Maturity date	Mar. 15, 2044	May 15, 2044	Jun. 15, 2024	Dec. 01, 2018	Dec. 01, 2018	Jun. 15, 2024