Fair Value of Financial Assets and Liabilities (Details) (USD $)	3 Months Ended		9 Months Ended							3 Months Ended		9 Months Ended																		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended			9 Months Ended						1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Carrying Amount [Member]	Dec. 31, 2011 Carrying Amount [Member]	Sep. 30, 2012 Fair Value [Member]	Dec. 31, 2011 Fair Value [Member]	Sep. 30, 2012 Commodity Derivatives, Net [Member]	Sep. 30, 2011 Commodity Derivatives, Net [Member]	Sep. 30, 2012 Commodity Derivatives, Net [Member]	Sep. 30, 2011 Commodity Derivatives, Net [Member]	Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member]		Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member]		Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member] Level 1 [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 1 [Member]	Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member] Level 2 [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 2 [Member]	Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member] Level 3 [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 3 [Member]	Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member] Fair Value Total [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Fair Value Total [Member]	Sep. 30, 2012 Fair Value Measured on a Recurring Basis [Member] Counterparty Netting [Member]		Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Counterparty Netting [Member]		Sep. 30, 2012 Cash Flow Hedges [Member]	Sep. 30, 2011 Cash Flow Hedges [Member]	Sep. 30, 2012 Cash Flow Hedges [Member]	Sep. 30, 2011 Cash Flow Hedges [Member]	Sep. 30, 2012 Cash Flow Hedges [Member] Interest Rate [Member]		Sep. 30, 2011 Cash Flow Hedges [Member] Interest Rate [Member]		Sep. 30, 2012 Cash Flow Hedges [Member] Interest Rate [Member]		Sep. 30, 2011 Cash Flow Hedges [Member] Interest Rate [Member]		Sep. 30, 2012 Cash Flow Hedges [Member] Vehicle Fuel And Other Commodity [Member]		Sep. 30, 2011 Cash Flow Hedges [Member] Vehicle Fuel And Other Commodity [Member]		Sep. 30, 2012 Cash Flow Hedges [Member] Vehicle Fuel And Other Commodity [Member]		Sep. 30, 2011 Cash Flow Hedges [Member] Vehicle Fuel And Other Commodity [Member]		Sep. 30, 2012 Other Derivative Instruments [Member]	Sep. 30, 2011 Other Derivative Instruments [Member]	Sep. 30, 2012 Other Derivative Instruments [Member]	Sep. 30, 2011 Other Derivative Instruments [Member]	Sep. 30, 2012 Other Derivative Instruments [Member] Trading Commodity [Member]		Sep. 30, 2011 Other Derivative Instruments [Member] Trading Commodity [Member]		Sep. 30, 2012 Other Derivative Instruments [Member] Trading Commodity [Member]		Sep. 30, 2011 Other Derivative Instruments [Member] Trading Commodity [Member]		Sep. 30, 2012 Other Derivative Instruments [Member] Natural Gas Commodity [Member]	Sep. 30, 2011 Other Derivative Instruments [Member] Natural Gas Commodity [Member]		Sep. 30, 2012 Other Derivative Instruments [Member] Natural Gas Commodity [Member]		Sep. 30, 2011 Other Derivative Instruments [Member] Natural Gas Commodity [Member]		Sep. 30, 2012 Other Derivative Instruments [Member] Natural Gas Commodity for Electric Generation [Member]	Sep. 30, 2011 Other Derivative Instruments [Member] Natural Gas Commodity for Electric Generation [Member]	Sep. 30, 2012 Interest Rate Swap [Member]	Sep. 30, 2012 Electric Commodity [Member] MWh		Dec. 31, 2011 Electric Commodity [Member] MWh		Sep. 30, 2012 Natural Gas Commodity [Member] MMBTU		Dec. 31, 2011 Natural Gas Commodity [Member] MMBTU		Sep. 30, 2012 Vehicle Fuel Commodity [Member] gal		Dec. 31, 2011 Vehicle Fuel Commodity [Member] gal
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months																																																																							$ 500,000
Notional amount settled																																																																							250,000,000
Cash payments to settle interest rate hedging instruments																																																																							44,700,000
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount																																																																								973,000	[1],[2]	1,299,000	[1],[2]	6,932,000	[1],[2]	32,053,000	[1],[2]	329,000	[1],[2]	270,000	[1],[2]
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive (loss) income related to cash flow hedges at beginning of period	(18,865,000)	6,793,000	(12,377,000)	7,457,000
After-tax net unrealized losses related to derivatives accounted for as hedges	(3,574,000)	(14,428,000)	(9,311,000)	(14,346,000)
After-tax net realized gains on derivative transactions reclassified into earnings	(304,000)	(381,000)	(1,055,000)	(1,127,000)
Accumulated other comprehensive loss related to cash flow hedges at end of period	(22,743,000)	(8,016,000)	(22,743,000)	(8,016,000)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Fair value gains (losses) recognized during the period in accumulated other comprehensive income (loss)																														(5,771,000)	(23,267,000)	(15,021,000)	(23,134,000)	(5,836,000)		(23,178,000)		(15,082,000)		(23,178,000)		65,000		(89,000)		61,000		44,000		0	0	0	0	0		0		0		0		0	0		0		0
Fair value gains (losses) recognized during the period in regulatory (assets) and liabilities																														0	0	0	0	0		0		0		0		0		0		0		0		1,109,000	(31,802,000)	(5,837,000)	(44,948,000)	0		0		0		0		1,109,000	(31,802,000)		(5,837,000)		(44,948,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive income (loss)																														(490,000)	(614,000)	(1,701,000)	(1,818,000)	(470,000)	[3]	(589,000)	[3]	(1,635,000)	[3]	(1,748,000)	[3]	(20,000)	[4]	(25,000)	[4]	(66,000)	[4]	(70,000)	[4]	0	0	0	0	0		0		0		0		0	0		0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)																														0	0	0	0	0		0		0		0		0		0		0		0		0	308,000	61,858,000	45,527,000	0		0		0		0		0	308,000	[5]	61,858,000	[5]	45,527,000	[5]	5,000,000	9,900,000
Pre-tax gains (losses) recognized during the period in income																														0	0	0	0	0		0		0		0		0		0		0		0		1,000	(138,000)	(107,000)	(43,000)	1,000	[6]	(12,000)	[6]	2,000	[6]	83,000	[6]	0	(126,000)	[7]	(109,000)	[7]	(126,000)	[7]
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	0	0	0	0
Recognized gains (losses) from fair value hedges or related hedged transactions	0	0	0	0
Credit Related Contingent Features [Abstract]
Derivative instruments in a gross liability position	5,400,000		5,400,000		6,900,000
Payments required if credit ratings were downgraded below investment grade	5,400,000		5,400,000		9,200,000
Collateral posted on derivative instruments	0		0		0
Collateral posted related to adequate assurance clauses in derivative contracts	0		0		0
Derivatives Designated as Cash Flow Hedges [Abstract]
Vehicle fuel and other commodity														60,000		0		0	0	60,000	76,000	0	0	60,000	76,000	0	[8]	(76,000)	[8]
Other derivative instruments [Abstract]
Trading commodity														2,749,000		2,838,000		0	0	5,899,000	6,550,000	0	0	5,899,000	6,550,000	(3,150,000)	[8]	(3,712,000)	[8]
Natural gas commodity														1,878,000				0		1,878,000		0		1,878,000		0	[8]
Total current derivative assets																		0	0	7,837,000	6,626,000	0	0	7,837,000	6,626,000	(3,150,000)	[8]	(3,788,000)	[8]
Total current derivative assets														4,687,000		2,838,000
Purchased power agreements														1,716,000	[9]	2,092,000	[9]
Current derivative instruments	6,403,000		6,403,000		4,930,000									6,403,000		4,930,000
Derivatives designated as cash flow hedges [Abstract]
Vehicle fuel and other commodity														62,000		48,000		0	0	62,000	48,000	0	0	62,000	48,000	0	[8]	0	[8]
Other derivative instruments [Abstract]
Trading commodity														2,858,000		4,987,000		0	0	4,710,000	8,292,000	0	0	4,710,000	8,292,000	(1,852,000)	[8]	(3,305,000)	[8]
Total noncurrent derivative assets																		0	0	4,772,000	8,340,000	0	0	4,772,000	8,340,000	(1,852,000)	[8]	(3,305,000)	[8]
Total noncurrent derivative assets														2,920,000		5,035,000
Purchased power agreements														9,037,000	[9]	10,322,000	[9]
Noncurrent derivative instruments	11,957,000		11,957,000		15,357,000									11,957,000		15,357,000
Derivatives designated as cash flow hedges [Abstract]
Interest rate																29,630,000			0		29,630,000		0		29,630,000			0	[8]
Other derivative instruments [Abstract]
Trading commodity														2,980,000		3,230,000		0	0	5,425,000	6,076,000	0	0	5,425,000	6,076,000	(2,445,000)	[8]	(2,846,000)	[8]
Natural gas commodity																47,115,000			0		54,525,000		0		54,525,000			(7,410,000)	[8]
Total current derivative liabilities																		0	0	5,425,000	90,231,000	0	0	5,425,000	90,231,000	(2,445,000)	[8]	(10,256,000)	[8]
Total current derivative liabilities														2,980,000		79,975,000
Purchased power agreements														5,428,000	[9]	5,543,000	[9]
Current derivative instruments	8,408,000		8,408,000		85,518,000									8,408,000		85,518,000
Other derivative instruments [Abstract]
Trading commodity														2,385,000		4,197,000		0	0	4,237,000	7,502,000	0	0	4,237,000	7,502,000	(1,852,000)	[8]	(3,305,000)	[8]
Total noncurrent derivative liabilities																		0	0	4,237,000	7,502,000	0	0	4,237,000	7,502,000	(1,852,000)	[8]	(3,305,000)	[8]
Total noncurrent derivative liabilities														2,385,000		4,197,000
Purchased power agreements														30,058,000	[9]	34,128,000	[9]
Noncurrent derivative instruments	32,443,000		32,443,000		38,325,000									32,443,000		38,325,000
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers into Level 3										0	0	0	0
Transfers out of Level 3										0	0	0	0
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion						$ 4,280,745,000	$ 3,486,275,000	$ 4,825,506,000	$ 4,020,083,000

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are also included on a gross basis, but are weighted for the probability of exercise.

[3] Amounts are recorded to interest charges.

[4] Amounts are recorded to operating and maintenance (O&M) expenses.

[5] Amounts for the nine months ended Sept. 30, 2012 and 2011 include $5.0 million and $9.9 million of settlement losses, respectively, on derivatives utilized to mitigate natural gas price risk for electric generation, recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. The remaining settlement losses for the nine months ended Sept. 30, 2012 and 2011, and all settlement losses for the three months ended Sept. 30, 2012 and 2011, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These losses are subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate.

[6] Amounts are recorded to electric operating revenues. Portions of these gains and losses are shared with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[7] Amounts are recorded to electric fuel and purchased power.

[8] The accounting guidance for derivatives and hedging permits the netting of receivables and payables for derivatives and related collateral amounts when a legally enforceable master netting agreement exists between PSCo and a counterparty. A master netting agreement is an agreement between two parties who have multiple contracts with each other that provides for the net settlement of all contracts in the event of default on or termination of any one contract.

[9] In 2003, as a result of implementing new guidance on the normal purchase exception for derivative accounting, PSCo began recording several long-term purchased power agreements at fair value due to accounting requirements related to underlying price adjustments. As these purchases are recovered through normal regulatory recovery mechanisms, the changes in fair value for these contracts were offset by regulatory assets and liabilities. During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.