Commitments and Contingencies, Legal Contingencies (Details) (USD $)	3 Months Ended		9 Months Ended					1 Months Ended			9 Months Ended			9 Months Ended	13 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	May 31, 2011 NSP-Minnesota Merricourt Wind Project Litigation [Member]	Apr. 30, 2011 NSP-Minnesota Merricourt Wind Project Litigation [Member] MW	Sep. 30, 2014 NSP-Minnesota Fibrominn Fuel Handling Dispute [Member]	Aug. 31, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Jul. 31, 2011 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Sep. 30, 2007 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Sep. 30, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	May 15, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Sep. 30, 2014 SPS Exelon Wind Complaint [Member] Site Dispute	Sep. 30, 2014 PSCo Pacific Northwest FERC Refund Proceeding [Member] Factor	Jun. 30, 2001 PSCo Pacific Northwest FERC Refund Proceeding [Member]	Sep. 30, 2014 Public Utility Commission of Texas (PUCT) SPS Exelon Wind Complaint [Member] Site
Legal Contingencies [Abstract]
Accrual for legal contingency							$ 0						$ 0	$ 0
Generating capacity (in MW)						150
Minimum amount of damages claimed by plaintiff					240,000,000		20,000,000							34,000,000
Number of main areas of dispute													2
Number of wind facilities													12			6
Sales to the City of Seattle	2,869,807,000	2,822,338,000	8,757,507,000	8,184,100,000											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								32,800,000		116,500,000
Storage costs for spent nuclear fuel											100,000,000
Cash payment received under settlement agreement									100,000,000		181,900,000
Claim submitted for storage costs for spent nuclear fuel												$ 33,600,000