Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Method Investment, Carrying Amount		$ 135.6		$ 124.5
Goodwill, Carrying Amount		131.2		130.6	$ 2.9
Goodwill, Impairment Charge	[1]	3.3	[2]	0.0	0.0
Intangible Assets, Carrying Amount		82.2		84.6
Intangible Assets, Indicators of Impairment		0.0		0.0
Net Book Value		3,741.2		3,669.1
Taconite Harbor [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value		90.0
Boswell Units 1 and 2 [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value		30.0
U.S. Water Services [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Goodwill, Carrying Amount		131.2		127.3	0.0
Goodwill, Impairment Charge		0.0	[2]	0.0
Net Book Value		12.6		12.2
ALLETE Clean Energy [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Goodwill, Carrying Amount		0.0		3.3	2.9
Goodwill, Impairment Charge		3.3	[2]	0.0	0.0
Net Book Value		532.3		447.3
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Method Investment, Indicators of Impairment				0.0
Intangible Assets, Carrying Amount		82.2		84.6
Intangible Assets, Indicators of Impairment		0.0
Property, Plant and Equipment, Indicators of Impairment		0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | U.S. Water Services [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Goodwill, Carrying Amount		131.2		130.6
Goodwill, Impairment Charge		0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | ALLETE Clean Energy [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Goodwill, Impairment Charge		$ 3.3
ATC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Ownership Percentage		8.00%
Equity Method Investment, Carrying Amount		$ 135.6		124.5	$ 121.1
ATC [Member] | Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Ownership Percentage		8.00%
Equity Method Investment, Carrying Amount		$ 135.6		$ 124.5
Equity Method Investment, Indicators of Impairment		$ 0.0

[1] See Goodwill and Intangible Assets.

[2] The facts and circumstances that led to an impairment of ALLETE Clean Energy’s goodwill primarily relate to lower estimated energy prices in periods not under PSAs. Impairment Charge is included in Operating Expenses – Other on the Consolidated Statement of Income. (See Note 1. Operations and Significant Accounting Policies.) ALLETE Clean Energy’s goodwill was primarily related to the acquisition of Storm Lake II in January 2014.