Commitments and Contingencies, Legal Contingencies (Details) (USD $)	3 Months Ended		6 Months Ended							1 Months Ended		6 Months Ended			6 Months Ended	13 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	May 15, 2014 NSP-Minnesota Nuclear Waste Disposal Litigation [Member]	Jun. 30, 2014 SPS Exelon Wind Complaint [Member] Dispute Site	Jun. 30, 2014 PSCo Pacific Northwest FERC Refund Proceeding [Member] Factor	Jun. 30, 2001 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			20,000,000						34,000,000
Number of main areas of dispute														2
Number of wind facilities														12
Sales to the City of Seattle	2,685,096,000	2,578,913,000	5,887,700,000	5,361,762,000												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
Claim submitted for storage costs for spent nuclear fuel													$ 33,600,000