Goodwill and Intangibles - Intangible Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Period of amortization	5 years
Operating revenues
Amortization to be recorded in the next five years
2021	$ 7.2
2022	7.2
2023	7.2
2024	7.2
2025	7.2
Other operation and maintenance
Amortization to be recorded in the next five years
2021	0.2
2022	0.2
2023	0.2
2024	0.2
2025	0.2
WECI
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	88.4	$ 10.2
Accumulated Amortization	(1.6)	(0.8)
Net Carrying Amount	86.8	9.4
WECI | PPAs
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	76.1	0.0
Accumulated Amortization	0.0	0.0
Net Carrying Amount	76.1	0.0
WECI | Proxy revenue swap
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7.2	7.2
Accumulated Amortization	(1.3)	(0.6)
Net Carrying Amount	5.9	6.6
WECI | Interconnection agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5.1	3.0
Accumulated Amortization	(0.3)	(0.2)
Net Carrying Amount	$ 4.8	$ 2.8
Upstream Wind Energy LLC | Proxy revenue swap
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	8 years
Length of proxy revenue contract, in years	10 years
Blooming Grove Wind Energy Center LLC and Tatanka Ridge Wind, LLC | PPAs
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	12 years
Tatanka Ridge Wind LLC and Bishop Hill Energy III LLC | Interconnection agreements
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	20 years