Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) solar_projects	Dec. 31, 2018 USD ($)
Significant related party transactions
Charges to ATC for services and construction	$ 27.5	$ 25.9	$ 21.8
Charges from ATC for network transmission services	350.5	348.1	338.1
Net refund from ATC related to FERC ROE orders	10.7	0.0	0.0
Refund from ATC related to a FERC audit	0.0	0.0	$ 22.0
Balance sheet
Accounts receivable for services provided to ATC	3.7	3.5
Accounts payable for services received from ATC	29.3	29.0
Amounts due from ATC for transmission infrastructure upgrades	$ 4.6	$ 2.8
WPS
Balance sheet
Number of new solar projects \| solar_projects		2

Amount for accounts payable to related parties. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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