Regulatory Assets and Liabilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Regulatory Assets [Line Items]
Regulatory asset, current	$ 402,235,000		$ 388,541,000
Regulatory asset, noncurrent	2,389,008,000		2,151,460,000
Regulatory Liabilities [Line Items]
Regulatory liability, current	275,095,000		156,038,000
Regulatory liability, noncurrent	1,101,534,000		1,179,765,000
Plant Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Plant lives
Regulatory liability, current	0		0
Regulatory liability, noncurrent	945,377,000		979,666,000
Deferred Electric, Gas, and Steam Production Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	108,057,000		107,674,000
Regulatory liability, noncurrent	0		0
DOE Settlement [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	94,734,000		0
Regulatory liability, noncurrent	0		0
Investment Tax Credit Deferrals [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	0		0
Regulatory liability, noncurrent	61,710,000		65,856,000
Deferred Income Tax Adjustment [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	0		0
Regulatory liability, noncurrent	46,835,000		42,863,000
Conservation Programs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year	[1]
Regulatory liability, current	15,898,000	[1]	0	[1]
Regulatory liability, noncurrent	0	[1]	0	[1]
Contract Valuation Adjustments [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Term of related contract	[2]
Regulatory liability, current	25,268,000	[2]	6,684,000	[2]
Regulatory liability, noncurrent	15,450,000	[2]	19,743,000	[2]
Gain From Asset Sales [Member]
Regulatory Liabilities [Line Items]
Regulatory liability remaining amortization period, minimum	1Y
Regulatory liability remaining amortization period, maximum	3Y
Regulatory liability, current	5,780,000		4,281,000
Regulatory liability, noncurrent	18,696,000		25,492,000
Renewable Resources and Environmental Initiatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various	[3]
Regulatory liability, current	4,358,000	[3]	14,752,000	[3]
Regulatory liability, noncurrent	8,525,000	[3]	0	[3]
Low Income Discount Program [Member]
Regulatory Liabilities [Line Items]
Regulatory liability remaining amortization period, minimum	1Y
Regulatory liability remaining amortization period, maximum	2Y
Regulatory liability, current	8,696,000		7,062,000
Regulatory liability, noncurrent	347,000		4,032,000
Nuclear Refueling Outage Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability remaining amortization period	1Y
Regulatory liability, current	3,441,000		3,441,000
Regulatory liability, noncurrent	0		3,441,000
REC Margin Sharing [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, current	0	[4]	0	[4]
Regulatory liability, noncurrent	0	[4]	26,104,000	[4]
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	8,863,000		12,144,000
Regulatory liability, noncurrent	4,594,000		12,568,000
Pension and Retiree Medical Obligations [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various	[5]
Regulatory asset, current	130,764,000	[5]	115,218,000	[5]
Regulatory asset, noncurrent	1,299,399,000	[5]	1,209,879,000	[5]
Regulatory recognition of the NSP-Minnesota pension expense	365,300,000		400,200,000
PSCo unamortized prior service costs	3,900,000		7,800,000
Regulatory assets related to the non-qualified pension plan	27,200,000		20,400,000
Regulatory assets related to the non-qualified pension plan, current	12,100,000		2,200,000
Recoverable Deferred Taxes on AFUDC Recorded in Plant [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives	[3]
Regulatory asset, current	0	[3]	0	[3]
Regulatory asset, noncurrent	294,549,000	[3]	276,861,000	[3]
Contract Valuation Adjustments [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related contract	[2]
Regulatory asset, current	73,608,000	[2]	45,155,000	[2]
Regulatory asset, noncurrent	142,210,000	[2]	134,027,000	[2]
Net AROs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives	[6]
Regulatory asset, current	0	[6]	0	[6]
Regulatory asset, noncurrent	209,626,000	[6]	150,913,000	[6]
Conservation Programs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1Y	[1]
Regulatory asset, remaining amortization period, maximum	7Y	[1]
Regulatory asset, current	46,769,000	[1]	57,679,000	[1]
Regulatory asset, noncurrent	80,981,000	[1]	74,236,000	[1]
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various
Regulatory asset, current	2,309,000		3,561,000
Regulatory asset, noncurrent	109,720,000		98,725,000
Renewable Resources and Environmental Initiatives [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1Y	[3]
Regulatory asset, remaining amortization period, maximum	4Y	[3]
Regulatory asset, current	51,622,000	[3]	75,372,000	[3]
Regulatory asset, noncurrent	25,378,000	[3]	20,487,000	[3]
Depreciation Differences [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1Y	[3]
Regulatory asset, remaining amortization period, maximum	7Y	[3]
Regulatory asset, current	4,150,000	[3]	5,859,000	[3]
Regulatory asset, noncurrent	54,892,000	[3]	12,379,000	[3]
Purchased Power Contract Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related contract
Regulatory asset, current	0		0
Regulatory asset, noncurrent	54,471,000		44,464,000
Losses on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related debt
Regulatory asset, current	5,554,000		6,319,000
Regulatory asset, noncurrent	43,729,000		49,001,000
Nuclear Refueling Outage Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1Y
Regulatory asset, remaining amortization period, maximum	2Y
Regulatory asset, current	40,365,000		33,819,000
Regulatory asset, noncurrent	8,810,000		7,169,000
Gas Pipeline Inspection and Remediation Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Pending rate case
Regulatory asset, current	13,779,000		2,000,000
Regulatory asset, noncurrent	27,511,000		29,358,000
Recoverable Purchased Natural Gas and Electric Energy Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1Y
Regulatory asset, remaining amortization period, maximum	2Y
Regulatory asset, current	17,031,000		27,770,000
Regulatory asset, noncurrent	9,867,000		9,907,000
State Commission Adjustments [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives	[3]
Regulatory asset, current	311,000	[3]	0	[3]
Regulatory asset, noncurrent	9,399,000	[3]	9,235,000	[3]
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various
Regulatory asset, current	15,973,000		15,789,000
Regulatory asset, noncurrent	$ 18,466,000		$ 24,819,000

[1] Includes over- or under-recovered costs for DSM and conservation programs as well as incentives allowed in certain jurisdictions.

[2] Includes the fair value of certain long term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.

[3] Earns a return on investment in the ratemaking process. These amounts are amortized consistent with recovery in rates.

[4] As described in Note 12, in 2011 the CPUC determined that the customers' share of REC margins will be netted against the RESA regulatory asset balance. This is reflected in the Dec. 31, 2011 regulatory asset balance.

[5] Includes $365.3 million and $400.2 million for the regulatory recognition of the NSP Minnesota pension expense at Dec. 31, 2011 and Dec. 31, 2010, respectively. These amounts are offset by $3.9 million and $7.8 million for PSCo unamortized prior service costs at Dec. 31, 2011 and Dec. 31, 2010, respectively. Also included are $27.2 million and $20.4 million of regulatory assets related to the non-qualified pension plan of which $12.1 million and $2.2 million is included in the current asset at Dec. 31, 2011 and Dec. 31, 2010, respectively.

[6] Includes amounts recorded for future recovery of AROs, less amounts recovered through nuclear decommissioning accruals and gains from decommissioning investments.