Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended					3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011	Jun. 30, 2011 MPUC Approval of Deferral of PPACA [Member]	Sep. 30, 2011 MPUC Approval of Deferral of PPACA [Member]	Mar. 31, 2011 DTL Reversal, Revenue Receivable Forgone [Member]	Sep. 30, 2011 DTL Reversal, Revenue Receivable Forgone [Member]
Current Tax Expense [Abstract]
Federal	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]
State	0	[1]	(0.1)	[1]	0	[1]	0.1	[1]
Total Current Tax Expense (Benefit)	0		(0.1)		0		0.1
Deferred Tax Expense (Benefit) [Abstract]
Federal	10.5	[2]	8.5	[2]	24.2	[2]	19.3	[2]
State	(0.7)	[2]	4.5	[2]	(1.9)	[2]	6.0	[2]
Change in Valuation Allowance	0.7	[3]	0	[3]	1.7	[3]	0	[3]
Investment Tax Credit Amortization	(0.2)		(0.2)		(0.6)		(0.7)
Total Deferred Tax Expense	10.3		12.8		23.4		24.6			(2.9)		(6.2)
Total Income Tax Expense	10.3		12.7		23.4		24.7
Unusual or Infrequent Item [Line Items]
Deferred Income Tax Benefit	(10.3)		(12.8)		(23.4)		(24.6)			2.9		6.2
Effective Income Tax Rate Reconciliation [Abstract]
Effective Income Tax Rate					25.50%		24.90%
Effective Income Tax Rate Adjustment											2.90%		6.20%
Federal Statutory Income Tax Rate					41.00%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits	2.7				2.7				11.4
Decrease in Unrecognized Tax Benefits					8.7
Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate	$ 0.5				$ 0.5

[1] For the quarter and nine months ended September 30, 2012, the federal and state current tax expense of zero and zero, respectively, ($(0.1) million and $0.1 million for the quarter and nine months ended September 30, 2011) is due to a net operating loss (NOL) which resulted primarily from the bonus depreciation provision of the Tax Relief, Unemployment Insurance Reauthorization, and Job Creation Act of 2010. The 2012 and 2011 federal and state NOLs will be carried forward to offset future taxable income.

[2] For the quarter and nine months ended September 30, 2012, the state deferred tax benefit of $0.7 million and $1.9 million, respectively, is due to state renewable tax credits earned which will be carried forward to offset future state tax expense. The nine months ended September 30, 2011, included a second quarter income tax benefit of $2.9 million related to the MPUC approval of our request to defer the retail portion of the tax charge taken in 2010 resulting from the PPACA, and a first quarter benefit for the reversal of a $6.2 million deferred tax liability related to a revenue receivable that Minnesota Power agreed to forgo as part of a stipulation and settlement agreement in its 2010 rate case.

[3] For the quarter and nine months ended September 30, 2012, the valuation allowance is due to state renewable tax credits earned in 2012 which are not expected to be utilized within their allowable tax carryforward period.