Commitments and Contingent Liabilities, Removal Costs (Details) (Plant Removal Costs [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Regulatory Liabilities [Line Items]
Regulatory liability	$ 945	$ 980
NSP-Minnesota [Member]
Regulatory Liabilities [Line Items]
Regulatory liability	382	400
NSP-Wisconsin [Member]
Regulatory Liabilities [Line Items]
Regulatory liability	109	107
PSCo [Member]
Regulatory Liabilities [Line Items]
Regulatory liability	380	385
SPS [Member]
Regulatory Liabilities [Line Items]
Regulatory liability	$ 74	$ 88