Commitments and Contingent Liabilities, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 MW	Dec. 31, 2011 NSP-Minnesota [Member]	Dec. 31, 2011 NSP-Minnesota [Member] Capital Commitments [Member] CSAPR [Member]	Dec. 31, 2011 SPS [Member] Capital Commitments [Member] CSAPR [Member]	Dec. 31, 2011 NSP-Wisconsin [Member] Capital Commitments [Member] CSAPR [Member]	Dec. 31, 2011 Ashland MGP Site [Member]	Dec. 31, 2010 Ashland MGP Site [Member]	Dec. 31, 2011 Other MGP Sites [Member]	Dec. 31, 2010 Other MGP Sites [Member]
Site Remediation [Abstract]
Liability for estimated cost of remediating sites						$ 104.3	$ 97.5	$ 3.9	$ 3.2
Liability for estimated cost of remediating sites, current						26.6	4.8
Amortization period for recovery of remediation costs in natural gas rates, lower range (in years)						4Y
Amortization period for recovery of remediation costs in natural gas rates, upper range (in years)						6Y
Number of identified MGP sites under current investigation and/or remediation								8
Minimum number of subsidiaries which could be alleged to be potentially responsible party in an environmental contingency	1
Number of properties included in superfund site which NSP-Wisconsin does not own	2
Regional Haze Rules [Abstract]
Number of environmental groups who petitioned the U.S. Department of the Interior	2
Number of coal-fired boilers in Colorado	12
Number of coal-fired cement kilns in Colorado	1
Number of PSCo plants named in the petition	4
Estimated cost for meeting the BART and other CAA requirements		50
Estimated amount spent on projects to reduce NOx emissions on Sherco Units 1 and 2		20
Other Environmental Requirements [Abstract]
Number of regulations in New Mexico to limit GHG emissions	2
Estimated costs to comply with the Cross-State Air Pollution Rule (CSAPR)			10.0	470.0	0.2
Number of years in which capital expenditures are expected to be incurred (in years)				4Y
Electric Generating Unit (EGU) Maximum Achievable Control Technology (MACT) Rule [Abstract]
Generating capacity (in MW)	25
Minimum number of years to amortize recovered remediation costs for manufactured gas plants (in years)	3Y
Maximum number of years to amortize recovered remediation costs for manufactured gas plants (in years)	4Y
Colorado Mercury Regulation [Abstract]
Minimum percent of mercury which must be captured (in hundredths)	80.00%
Estimated capital cost	1.1
Estimated annual sorbent expense	0.5
Minnesota Mercury Legislation [Abstract]
Study costs incurred	1.5
Amount spent on testing and studying of technologies	0.6
Estimated annual testing and study cost	0.5
Projected installation costs for the two mercury control units	12.0
Projected O&M expense per year beginning in 2014	10.0
Industrial Broiler (IB) MACT Rules [Abstract]
Estimated capital cost per unit to comply with IB MACT rules	9.0
Colorado Proposed Surface Impoundment Regulations (Section 9) [Abstract]
Estimated costs for compliance with proposed new solid waste regulations in Colorado	18
Cunningham Compliance Order [Abstract]
Negotiated penalty included in final compliance order	$ 0.8