Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2015 USD ($) Plant	Oct. 30, 2015 Period	May. 31, 2015 Parties	Dec. 31, 2014 USD ($)
Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of phases under a consent decree which the EPA is requiring states to evaluate areas for attainment		3
Number of months in which the state would have to submit an implementation plan for the respective nonattaiment areas		18 months
Number of years for the state to achieve the designated attainment standard		5 years
National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of hours measured for standard | Period			8
Current level of air quality concentrations (in parts per billion)			75
Proposed level of air quality concentrations (in parts per billion)			70
NSP-Minnesota | Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Minimum number of plants which could be required to make improvements to reduce entrainment | Plant		6
NSP-Minnesota | Regional Haze Rules
Environmental Requirements [Abstract]
Amount spent on installation of emission controls					$ 46.9
NSP-Minnesota | Reasonably Attributable Visibility Impairment
Environmental Requirements [Abstract]
Number of environmental advocacy organizations who filed a settlement agreement in the Minnesota District Court with NSP-Minnesota and the EPA | Parties				6
Number of months the EPA has to recommend and adopt a rule which will set the agreed-upon sulfur dioxide emissions	7 months
SPS | Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Proposed percent reduction related to the ozone season emission budget for Nitrogen Oxide		14.00%
SPS | National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Proposed level of air quality concentrations (in parts per billion)			70
PSCo | Regional Haze Rules
Environmental Requirements [Abstract]
Amount spent on installation of emission controls		$ 75.2
Capital Commitments | NSP-Minnesota | Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		49.0
Capital Commitments | SPS | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		$ 600.0
Minimum | SPS | Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of units which sulfur dioxide controls could be required		1
Minimum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		$ 9.0
Maximum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		21.0
Maximum | Capital Commitments | NSP-Minnesota | Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Liability for estimated cost to comply with entrainment regulation		$ 194.0