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		$ 169,939,000		$ 150,405,000
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	179,995,000	159,766,000		154,147,000
Service cost		4,568,000		4,271,000		4,260,000
Interest cost		7,765,000		8,031,000		8,311,000
Plan amendments		216,000		0
Actuarial loss		21,083,000		7,430,000
Benefit payments		(13,403,000)		(14,113,000)
Obligation at Dec. 31		179,995,000		159,766,000		154,147,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	136,546,000	121,348,000		123,541,000
Actual return on plan assets		16,079,000		5,474,000
Employer contributions		12,522,000		6,446,000
Benefit payments		(13,403,000)		(14,113,000)
Fair value of plan assets at Dec. 31		136,546,000		121,348,000		123,541,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(43,449,000)	[1]	(38,418,000)	[1]
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss		99,338,000		89,730,000
Prior service cost		2,290,000		4,061,000
Total		101,628,000		93,791,000
Amounts Related to the Funded Status of the Plans Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		6,895,000		7,530,000
Noncurrent regulatory assets		94,733,000		86,261,000
Total		101,628,000		93,791,000
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	11,300,000	12,500,000		6,400,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		4,568,000		4,271,000		4,260,000
Interest cost		7,765,000		8,031,000		8,311,000
Expected return on plan assets		(10,489,000)		(11,484,000)		(11,800,000)
Amortization of prior service cost		1,771,000		1,895,000		1,629,000
Amortization of net loss		6,004,000		4,070,000		2,463,000
Net periodic benefit cost		9,619,000		6,783,000		4,863,000
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)		7.50%		8.00%		8.00%
Allocated costs for pension plans sponsored by Xcel Energy Inc.		1,800,000		1,300,000		1,000,000
Expected average long-term rate of return on assets for next fiscal year (in hundredths)		7.25%
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 NSP-Wisconsin's consolidated balance sheet.