Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Investments		$ 161.1	$ 118.7
Goodwill, Impairment Charge	[1]	0.0	0.0	$ 3.3
Intangible Assets, Carrying Amount		74.8	77.6
Intangible Assets, Indicators of Impairment		0.0	0.0	0.0
U.S. Water Services [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Goodwill, Carrying Amount		148.5	148.3	131.2
Goodwill, Impairment Charge		0.0	0.0	0.0
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]
Equity Investments		128.1	118.7	135.6
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]
Intangible Assets, Carrying Amount		74.8	77.6
Intangible Assets, 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		148.5	148.3
Goodwill, Impairment Charge		0.0	0.0	$ 0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | Corporate and Other [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Indicators of Impairment		0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | ATC [Member] | Regulated Operations [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Method Investment, Indicators of Impairment		$ 0.0	$ 0.0

[1] See Goodwill and Intangible Assets.