Commitments and Contingencies, Removal Costs (Details) - Plant Removal Costs - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,130	$ 953
NSP-Minnesota
Regulatory Liabilities [Line Items]
Regulatory liabilities	430	396
NSP-Wisconsin
Regulatory Liabilities [Line Items]
Regulatory liabilities	132	123
PSCo
Regulatory Liabilities [Line Items]
Regulatory liabilities	364	366
SPS
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 204	$ 68