Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				6 Months Ended		1 Months Ended				6 Months Ended
	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury and Air Toxics Standards (MATS) Rule [Member] MW	Jun. 30, 2012 Other MGP Sites [Member]	Dec. 31, 2011 Other MGP Sites [Member]	Jun. 30, 2012 NSP-Wisconsin [Member] Capital Commitments [Member] Cross-State Air Pollution Rule (CSAPR) [Member]	Jun. 30, 2012 NSP-Wisconsin [Member] Ashland MGP Site [Member]	Dec. 31, 2011 NSP-Wisconsin [Member] Ashland MGP Site [Member]	Jun. 30, 2012 NSP-Wisconsin [Member] Ashland MGP Site - Phase I Project Area [Member] acre	Jun. 30, 2012 NSP-Wisconsin [Member] Ashland MGP Site - Sediments [Member]	Jun. 30, 2012 SPS [Member] New Mexico Greenhouse Gas (GHG) Regulations [Member]	Mar. 31, 2012 SPS [Member] New Mexico Greenhouse Gas (GHG) Regulations [Member]	Jun. 30, 2012 SPS [Member] Capital Commitments [Member] Cross-State Air Pollution Rule (CSAPR) [Member]	May 31, 2012 NSP-Minnesota [Member] Regional Haze Rules [Member]	Jun. 30, 2012 NSP-Minnesota [Member] Capital Commitments [Member] Cross-State Air Pollution Rule (CSAPR) [Member]	Jun. 30, 2012 NSP-Minnesota [Member] Capital Commitments [Member] Regional Haze Rules [Member]	Mar. 31, 2010 PSCo [Member] Regional Haze Rules [Member]
Manufactured Gas Plant (MGP) Sites [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own					2
Liability for estimated cost of remediating sites		$ 4.0	$ 3.9		$ 104.3	$ 104.3	$ 40.0
Number of acres of land to be conveyed to the State of Wisconsin and tribes (in acres)							1,390
Number of days in public comment period (in days)							30D
Estimated cost of remediating site, low end of range								63.3
Estimated cost of remediating site, high end of range								77.1
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)								50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)								(30.00%)
Liability for estimated cost of remediating sites, current					26.6	26.6
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)					4Y
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)					6Y
Number of identified MGP sites under current investigation and/or remediation		8
Environmental Requirements [Abstract]
Number of regulations in New Mexico to limit GHG emissions which were repealed in 2012										2
Number of appeals in which intervention has been granted									1
Number of petitions filed to intervene in the second appeal									1
Liability for estimated cost to comply with regulation				0.2							470.0		10.0	50.0
Number of years in which capital expenditures are expected to be incurred (in years)											5Y
Generating capacity (in MW)	25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)	3Y
Number of years before affected facilities must demonstrate compliance, high end of range (in years)	4Y
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
Estimated amount spent on projects to reduce NOx emissions on Sherco Units 1 and 2														$ 20
Number of days provided in the notice of intent to sue (in days)												180D