Commitments and Contingencies, Legal Contingencies (Details) (USD $)	3 Months Ended								1 Months Ended		3 Months Ended		13 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Pacific Northwest FERC Refund Proceeding [Member] Factor	Mar. 31, 2014 NSP-Minnesota Merricourt Wind Project Litigation [Member]	May 31, 2011 NSP-Minnesota Merricourt Wind Project Litigation [Member]	Apr. 30, 2011 NSP-Minnesota Merricourt Wind Project Litigation [Member] MW	Mar. 31, 2011 NSP-Minnesota Merricourt Wind Project Litigation [Member]	Mar. 31, 2014 NSP-Minnesota Fibrominn Fuel Handling Dispute [Member]	Jul. 31, 2011 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Sep. 30, 2007 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Mar. 31, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Mar. 31, 2014 SPS Exelon Wind Complaint [Member] Site Dispute	Jun. 30, 2001 PSCo Pacific Northwest FERC Refund Proceeding [Member]	Mar. 31, 2014 PSCo Pacific Northwest FERC Refund Proceeding [Member]
Legal Contingencies [Abstract]
Accrual for legal contingency				$ 0				$ 0				$ 0		$ 0
Generating capacity (in MW)						150
Merricourt deposit							101,000,000
Minimum amount of damages claimed by plaintiff					240,000,000			19,000,000						34,000,000
Number of main areas of dispute												2
Number of wind facilities												12
Sales to the City of Seattle	3,202,604,000	2,782,849,000											50,000,000
Estimated City of Seattle's claim for refunds not including interest														28,000,000
Number of factors considered in assessment			2
Damages awarded										116,500,000
Storage costs for spent nuclear fuel											100,000,000
Cash payment received under settlement agreement									$ 100,000,000		$ 181,900,000