CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 199.7	$ 220.1
Property, plant, and equipment, accumulated depreciation and amortization	$ 9,799.3	$ 9,364.7
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	325,000,000	325,000,000
Common stock, shares outstanding	315,434,531	315,434,531
Regulatory assets (September 30, 2021 includes $102.9 related to WEPCo Environmental Trust)	$ 3,488.7	$ 3,524.1
WEPCo Environmental Trust
Regulatory assets (September 30, 2021 includes $102.9 related to WEPCo Environmental Trust)	102.9
Current portion of long-term debt (September 30, 2021 includes $8.5 related to WEPCo Environmental Trust)	8.5
Long-term debt (September 30, 2021 includes $107.0 related to WEPCo Environmental Trust)	$ 107.0

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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