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Equity Investments - Investment in Nobles 2 (Details) $ in Millions		3 Months Ended
		Mar. 31, 2021 USD ($) MW	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investment in Nobles 2 [Line Items]
Equity Investment Balance		$ 301.5		$ 301.2
Cash Investments		1.0	$ 27.8
Equity Earnings		4.8	5.2
Net Loss Attributable to Non-Controlling Interest		$ (8.4)	$ (1.8)
Nobles 2 [Member]
Investment in Nobles 2 [Line Items]
Ownership Percentage		49.00%
Equity Investment Balance		$ 151.2		152.2
Cash Investments		1.0
Distributed ATC Earnings		(1.5)
Equity Earnings	[1]	(0.5)
Net Loss Attributable to Non-Controlling Interest		$ 1.4
ATC [Member]
Investment in Nobles 2 [Line Items]
Ownership Percentage		8.00%
Equity Investment Balance		$ 150.3		$ 149.0
Distributed ATC Earnings		(4.3)
Equity Earnings		$ 5.3
Wind Turbine Generators [Member] | Tenaska [Member] | Minnesota Power [Member] | Tenaska PPA [Member]
Investment in Nobles 2 [Line Items]
Generating Capacity Counterparty Owned (MW) | MW		250
Long-term Contract for Purchase of Electric Power, Term of Contract (Years)		20 years
[1] The Company also recorded net loss attributable to non-controlling interest of $1.4 million related to its investment in Nobles 2.

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