Borrowings and Other Financing Instruments Amended Credit Agreements (Details) $ in Thousands		6 Months Ended
		Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Term		5 years
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 3,100,000
Revolving Credit Facility [Member] | SPS
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 500,000	$ 400,000
Revolving Credit Facility [Member] | NSP-Wisconsin
Line of Credit Facility [Line Items]
Debt Instrument, Term		1 year
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 150,000
Revolving Credit Facility [Member] | Parent Company
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 1,250,000	$ 1,000,000
Revolving Credit Facility [Member] | Parent Company | NSP Minnesota, PSCo, and SPS [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Term		1 year
Number of extension you can request		2
Swingline Subfacility [Domain] | Parent Company
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	[1]	$ 75,000

[1] ^(a)  Expires in June 2024.