Borrowings and Other Financing Instruments Dividend and Other Capital-Related Restrictions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
NSP Minnesota
Debt Instrument [Line Items]
Equity to total capitalization ratio, low end of range (in hundredths)	47.10%
Equity to total capitalization ratio, high end of range (in hundredths)	57.50%
Equity to total capitalization ratio	52.30%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 1,147
Capitalization, Short term debt, long term debt and equity	11,634
Maximum total capitalization	12,700
Maximum additional short term debt authorized for issuance	$ 1,905	[1]
Maximum percentage of short term debt to total capitalization (in hundredths)	15.00%
Maximum Percentage of long term debt to total capitalization	52.93%	[1]
NSP-Wisconsin
Debt Instrument [Line Items]
Minimum calendar year average equity to total capitalization ratio authorized by state commission	51.50%
Equity to total capitalization ratio	51.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 12	[2]
Capitalization, Short term debt, long term debt and equity	1,827
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met	55
Maximum additional long term debt authorized for issuance	0	[3]
Maximum additional short term debt authorized for issuance	$ 150
SPS
Debt Instrument [Line Items]
Equity to total capitalization ratio (excluding short-term debt), low end of range (in hundredths)	45.00%	[4]
Equity to total capitalization ratio (excluding short-term debt), high end of range (in hundredths)	55.00%	[4]
Equity to total capitalization ratio (excluding short-term debt) (in hundredths)	54.40%	[4]
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 535	[5]
Capitalization, Short term debt, long term debt and equity	5,304
Maximum additional long term debt authorized for issuance	0	[6]
Maximum additional short term debt authorized for issuance	600
PSCo
Debt Instrument [Line Items]
Maximum additional long term debt authorized for issuance	150
Maximum additional short term debt authorized for issuance	$ 800

[1] NSP-Minnesota has authorization to issue long-term securities provided the equity-to-total capitalization remains within the required range, and to issue short-term debt provided it does not exceed 15% of total capitalization.

[2] Cannot pay annual dividends in excess of approximately $55 million if its average equity-to-total capitalization ratio falls below the commission authorized level.

[3] NSP-Wisconsin filed for additional long-term debt authorization in December 2019.

[4] Excludes short-term debt.

[5] May not pay a dividend that would cause a loss of its investment grade bond rating.

[6] SPS filed for additional long-term debt authorization in February 2020.