Rate Matters (Details) (USD $)	12 Months Ended							12 Months Ended																									3 Months Ended
	Dec. 31, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Feb. 29, 2012 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Nov. 30, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Aug. 31, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Jun. 30, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Jan. 02, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Nov. 30, 2010 NSP-Minnesota [Member] Electric Rate Case [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Dec. 31, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] North Dakota Public Service Commission (NDPSC) [Member]	Sep. 30, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] North Dakota Public Service Commission (NDPSC) [Member]	May 31, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] North Dakota Public Service Commission (NDPSC) [Member]	Feb. 18, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] North Dakota Public Service Commission (NDPSC) [Member]	Dec. 31, 2010 NSP-Minnesota [Member] Electric Rate Case [Member] North Dakota Public Service Commission (NDPSC) [Member]	Jan. 02, 2012 NSP-Minnesota [Member] Electric Rate Case [Member] South Dakota Public Utilities Commission (SDPUC) [Member]	Jun. 30, 2011 NSP-Minnesota [Member] Electric Rate Case [Member] South Dakota Public Utilities Commission (SDPUC) [Member]	Jan. 31, 2012 NSP-Minnesota [Member] Electric Conservation Improvement Program (CIP) Rider [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Oct. 31, 2010 NSP-Minnesota [Member] Electric Conservation Improvement Program (CIP) Rider [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Jan. 31, 2012 NSP-Minnesota [Member] Natural Gas Conservation Improvement Program (CIP) Rider [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Nov. 30, 2010 NSP-Minnesota [Member] Natural Gas Conservation Improvement Program (CIP) Rider [Member] Minnesota Public Utilities Commission (MPUC) [Member]	Dec. 31, 2011 NSP-Wisconsin [Member] Electric Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Jun. 30, 2011 NSP-Wisconsin [Member] Electric Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Dec. 31, 2011 NSP-Wisconsin [Member] Gas Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Jun. 30, 2011 NSP-Wisconsin [Member] Gas Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Dec. 31, 2011 NSP-Wisconsin [Member] Electric and Gas Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Jun. 30, 2011 NSP-Wisconsin [Member] Electric and Gas Rate Case [Member] Public Service Commission of Wisconsin (PSCW) [Member]	Jan. 27, 2012 PSCo [Member] Electric Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Jan. 12, 2012 PSCo [Member] Electric Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Nov. 30, 2011 PSCo [Member] Electric Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Aug. 31, 2011 PSCo [Member] Gas Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Mar. 31, 2011 PSCo [Member] Gas Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Dec. 31, 2010 PSCo [Member] Gas Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Feb. 28, 2011 PSCo [Member] Wholesale Electric Rate Case [Member] Colorado Public Utilities Commission (CPUC) [Member]	Dec. 31, 2011 PSCo [Member] Demand Side Management Cost Adjustment (DSMCA) [Member] Colorado Public Utilities Commission (CPUC) [Member]	Jun. 30, 2011 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Colorado Public Utilities Commission (CPUC) [Member]	Jan. 31, 2012 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Colorado Public Utilities Commission (CPUC) [Member]	Sep. 30, 2011 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Colorado Public Utilities Commission (CPUC) [Member]	May 31, 2011 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Colorado Public Utilities Commission (CPUC) [Member]	Jan. 31, 2012 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Customers [Member] Colorado Public Utilities Commission (CPUC) [Member]	Jun. 30, 2011 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Customers [Member] Colorado Public Utilities Commission (CPUC) [Member]	May 31, 2011 PSCo [Member] Renewable Energy Credit (REC) Sharing [Member] Customers [Member] Colorado Public Utilities Commission (CPUC) [Member]	Dec. 31, 2011 SPS [Member] Retail Rate Case [Member] New Mexico Public Regulation Commission (NMPRC) [Member]	Feb. 28, 2011 SPS [Member] Retail Rate Case [Member] New Mexico Public Regulation Commission (NMPRC) [Member]	Feb. 16, 2011 SPS [Member] Retail Rate Case [Member] Public Utility Commission of Texas (PUCT) [Member]	Nov. 30, 2010 SPS [Member] Retail Rate Case [Member] Public Utility Commission of Texas (PUCT) [Member]	May 31, 2010 SPS [Member] Retail Rate Case [Member] Public Utility Commission of Texas (PUCT) [Member]
Rate Matters [Abstract]
Entity's original requested rate increase (decrease) for 2011							$ 150,000,000					$ 19,800,000																		$ 27,500,000	$ 16,100,000													$ 71,500,000
Percentage increase in entity's original requested rate increase for 2011 (in hundredths)							5.62%					12.00%
Entity's additional requested rate increase (decrease) for 2012							48,300,000					4,200,000
Percentage increase in entity's additional requested rate increase for 2012 (in hundredths)							1.81%					2.60%
Entity's original requested percent return on equity (in hundredths)							11.25%					11.25%																			10.90%													11.35%
Entity's rate base in original requested rate increase (decrease)							5,600,000,000					328,000,000																			407,400,000													1,031,000,000
Entity's original requested equity ratio percent (in hundredths)							52.56%					52.56%																			57.10%													51.00%
Entity's interim rate increase implemented						123,000,000					17,400,000		12,700,000
Entity's revised requested rate increase (decrease) for 2011			58,000,000	122,000,000	122,800,000				13,700,000	18,000,000																			25,600,000
Entity's revised requested percent return on equity (in hundredths)			10.37%		10.85%				10.40%																				10.90%
Division of Energy Resources' revised recommendation for rate increase (decrease) for 2011					84,700,000
Division of Energy Resources' recommendation for additional rate increase (decrease) for 2012					34,000,000
Division of Energy Resources' recommendation for percent return on equity (in hundredths)					10.37%
Entity's revised requested rate increase (decrease) for 2012			14,800,000	29,000,000					2,000,000	2,400,000
Entity's adjustment related to depreciation			(30,000,000)
Entity's adjustment related to employee compensation			(7,500,000)
Entity's adjustment related to property taxes			(28,000,000)
Entity's recorded provision for revenue subject to refund	67,400,000							2,400,000
Entity's recorded increase (decrease) in depreciation expense	(30,000,000)
Entity's revised interim rate increase implemented		72,800,000																																									63,700,000
Percentage increase in entity's revised requested rate increase for 2011 (in hundredths)										11.00%
Percentage increase in entity's revised requested rate increase for 2012 (in hundredths)										1.40%
Entity's original requested rate increase (decrease) for 2012														14,600,000						29,200,000		8,000,000					141,900,000														19,900,000
Amounts currently recovered in automatic recovery mechanisms included in requested rate increase (decrease) for 2012														700,000
Entity's recovery of CIP expenses through rider															74,700,000	84,400,000	10,600,000	27,400,000
Entity's recovery of CIP expenses through base rates															48,300,000	60,900,000	3,800,000	4,400,000
Percentage increase in entity's rate increase approved to be implemented (in hundredths)																			2.10%		2.40%
Entity's percent return on equity implemented (in hundredths)																							10.40%					10.10%
Entity's equity ratio percent implemented (in hundredths)																							52.59%					56.00%
Entity's rate base in revised requested rate increase (decrease)																													1,100,000,000
Entity's revised requested equity ratio percent (in hundredths)																													57.10%
Entity's requested recovery of specific pipeline integrity costs																													23,200,000
Entity's approved rate increase (decrease) due to recovery through automatic recovery mechanisms																												(10,000,000)
Entity's request for deferred accounting of annual revenue requirement associated with Black Hills contract																									42,000,000
Percentage increase in entity's original requested rate increase for 2012 (in hundredths)																				5.10%		6.60%
Entity's original requested percent return on equity for 2012 (in hundredths)														11.00%										10.75%			10.75%														11.25%
Entity's rate base in original requested rate increase (decrease) for 2012														323,400,000,000						718,000,000		84,000,000					5,400,000,000														390,300,000
Entity's original requested equity ratio percent for 2012 (in hundredths)														52.48%										52.54%			56.00%														51.11%
Increase in savings goals over current goals approved (in hundredths)																																130.00%
Number of installments to be awarded in the year following plan achievements																																1
Percent of net economic benefits to be earned at an achievement level of 130 percent																																11.00%
Maximum annual incentive to be earned																																30,000,000
Electric demand side management (DSM) budget approved																																77,300,000
Gas demand side management (DSM) budget approved																																12,200,000
Electricity demand response programs recovered through demand side management cost adjustment (DSMCA)																																29,400,000
Initial percentage of margin associated with stand-alone REC transactions (in hundredths)																																				20.00%			80.00%
Maximum percentage of margin associated with stand-alone REC transactions (in hundredths)																																				10.00%			90.00%
Customers' share of margins credited against RESA regulatory asset balance																																	37,000,000
Percentage of margin associated with Colorado RECs bundled with California energy for deliveries outside of California (in hundredths)																																	30.00%					70.00%
Percentage of margin associated with Colorado RECs bundled with California energy for deliveries inside of California (in hundredths)																																	40.00%					60.00%
Percentage of margin associated with RECs bundled with on-system energy and aggregated with other trading margins (in hundredths)																																	20.00%					80.00%
Minimum requested percentage of margin sharing associated with Colorado RECs bundled with California energy for deliveries outside of California (in hundredths)																																			8.00%
Maximum requested percentage of margin sharing associated with Colorado RECs bundled with California energy for deliveries outside of California (in hundredths)																																			25.00%
Minimum requested percentage of margin sharing associated with Colorado RECs bundled with California energy for deliveries inside of California (in hundredths)																																			8.00%
Maximum requested percentage of margin sharing associated with Colorado RECs bundled with California energy for deliveries inside of California (in hundredths)																																			35.00%
First amount of margins on hybrid REC trades to be shared																																		20,000,000
Percentage of margin on hybrid RECs approved for first $20 million of margins (in hundredths)																																		20.00%			80.00%
Percentage of margin on hybrid RECs approved for margins in excess of $20 million (in hundredths)																																		10.00%			90.00%
Entity's rate increase implemented																												12,800,000														39,400,000
Amounts currently recovered in automatic recovery mechanisms included in requested rate increase (decrease) for 2011																																										16,900,000
Rates increase approved for implementation in 2012																			12,200,000		2,900,000																			13,500,000		13,100,000
Required reduction in calculated TCRF revenue requirement if TCFR application is filed before effective date of rates in next rate case																																										(12,200,000)
Entity's interim rate increase requested																										$ 42,000,000	$ 100,000,000