Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Current Tax Expense [Abstract]
Current Tax Expense - Federal	$ 0	[1]	$ 0	[1]
Current Tax Expense - State	0	[1]	0.1	[1]
Total Current Tax Expense	0		0.1
Deferred Tax Expense (Benefit) [Abstract]
Deferred Tax Expense - Federal	8.7	[2]	6.8	[2]
Deferred Tax Expense (Benefit) - State	(0.8)	[2]	1.5	[2]
Change in Valuation Allowance	0.6	[3]	0	[3]
Investment Tax Credit Amortization	(0.2)		(0.2)
Total Deferred Tax Expense	8.3		8.1
Total Income Tax Expense	8.3		8.2
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Effective Income Tax Rate, Continuing Operations	25.40%		18.10%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate	41.00%
Uncertain Tax Positions [Abstract]
Unrecognized Tax Benefits	11.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0.5
DTL Reversal, Revenue Receivable Forgone [Member]
Income Tax Expense, Unusual/Nonrecurring Items [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions			$ 6.2
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Effective Income Tax Rate Reconciliation, Other Adjustments			31.80%

[1] For the quarter ended March 31, 2012, the federal and state current tax expense (benefit) of zero and zero, respectively, (zero and $0.1 million for the quarter ended March 31, 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 2011 and 2012 federal and state NOLs will be carried forward to offset future taxable income.

[2] The quarter ended March 31, 2011, includes a 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] The increase in valuation allowance in 2012 is from renewable tax credits earned in 2012 which are not expected to be utilized within their allowable tax carryforward period.