Commitments and Contingencies, Removal Costs (Details) - Plant Removal Costs - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,135	$ 1,130
NSP-Minnesota
Regulatory Liabilities [Line Items]
Regulatory liabilities	419	430
PSCo
Regulatory Liabilities [Line Items]
Regulatory liabilities	367	364
SPS
Regulatory Liabilities [Line Items]
Regulatory liabilities	209	204
NSP-Wisconsin
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 140	$ 132