Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Investments	$ 301.2	$ 197.6
Taconite Harbor [Member] | Resource Package [Member] | MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	50.0
Boswell Unit 3 [Member] | 2021 Integrated Resource Plan [Member] | MPUC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Net Book Value	255.0
ATC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Investments	149.0	141.6	$ 128.1
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]
Impairment of Long-Lived Assets Held-for-use	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