Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) (Pension Plans [Member], USD $)	1 Months Ended	12 Months Ended
	Jan. 31, 2013	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation at Dec. 31		$ 416,808,000		$ 368,585,000
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	454,184,000	403,367,000		370,587,000
Service cost		8,520,000		7,690,000		7,008,000
Interest cost		19,697,000		20,036,000		19,997,000
Plan amendments		98,000		0
Actuarial loss		45,881,000		27,512,000
Benefit payments		(23,379,000)		(22,458,000)
Obligation at Dec. 31		454,184,000		403,367,000		370,587,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	376,138,000	350,054,000		337,421,000
Actual return on plan assets		36,403,000		29,913,000
Employer contributions		13,060,000		5,178,000
Benefit payments		(23,379,000)		(22,458,000)
Fair value of plan assets at Dec. 31		376,138,000		350,054,000		337,421,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(78,046,000)	[1]	(53,313,000)	[1]
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss		244,412,000		222,849,000
Prior service cost		963,000		2,401,000
Total		245,375,000		225,250,000
Amounts Related to the Funded Status of the Plans Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		14,877,000		14,761,000
Noncurrent regulatory assets		230,498,000		210,489,000
Total		245,375,000		225,250,000
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Measurement date		2012-12-31		2011-12-31
Discount rate for year-end valuation (in hundredths)		4.00%		5.00%
Expected average long-term increase in compensation level (in hundredths)		3.75%		4.00%
Mortality table		RP 2000		RP 2000
Cash Flows [Abstract]
Total contributions to Xcel Energy's pension plans during the period	21,900,000	13,100,000		5,200,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		8,520,000		7,690,000		7,008,000
Interest cost		19,697,000		20,036,000		19,997,000
Expected return on plan assets		(24,928,000)		(26,316,000)		(27,542,000)
Amortization of prior service cost		1,438,000		1,505,000		1,504,000
Amortization of net loss		12,897,000		9,046,000		4,826,000
Net periodic benefit cost		17,624,000		11,961,000		5,793,000
Costs not recognized due to effects of regulation		(4,300,000)		(2,300,000)		0
Net benefit cost recognized for financial reporting		13,324,000		9,661,000		5,793,000
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (in hundredths)		5.00%		5.50%		6.00%
Expected average long-term increase in compensation level (in hundredths)		4.00%		4.00%		4.00%
Expected average long-term rate of return on assets (in hundredths)		6.68%		6.80%		6.80%
Allocated costs for pension plans sponsored by Xcel Energy Services Inc.		4,100,000		2,900,000		2,200,000
Expected average long-term rate of return on assets for next fiscal year (in hundredths)		6.49%
Number of years fair market value of plan assets is adjusted using calculated value method (in years)		5 years
Annual adjustment rate used in calculated value method (in hundredths)		20.00%
Xcel Energy Inc. [Member]
Cash Flows [Abstract]
Total contributions to Xcel Energy's pension plans during the period	$ 191,500,000	$ 198,100,000		$ 137,300,000
Number of pension plans to which contributions were made	4	4		3

[1] Amounts are recognized in noncurrent liabilities on SPS' balance sheet.