Equity Investments Equity Investment in Nobles 2 (Details) $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Equity Investments [Line Items]
Equity Investments		$ 197.6	$ 161.1
Proceeds from Equity Method Investment, Distribution, Return of Capital		(8.3)	0.0	$ 0.0
Cash Investments		$ 37.9	39.2	$ 7.8
Nobles 2 [Member]
Equity Investments [Line Items]
Equity Method Investment, Ownership Percentage		49.00%
Equity Investments		$ 56.0	$ 33.0
Cash Investments		31.3
Expected Additional Investment in 2020		$ 115.0
Resource Package [Member] | Wind Turbine Generators [Member] | Tenaska [Member] | MPUC [Member] | Minnesota Power [Member] | Nobles 2 PPA [Member]
Equity Investments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW	[1]	250
Contract Term (Years)		20 years
Minimum [Member] | Nobles 2 [Member]
Equity Investments [Line Items]
Estimated Project Cost		$ 350.0
Maximum [Member] | Nobles 2 [Member]
Equity Investments [Line Items]
Expected Tax Equity Financing		125.0
Estimated Project Cost		$ 400.0

[1] The agreement includes a fixed capacity charge and energy prices that escalate at a fixed rate annually over the term.