Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Charges from ATC for network transmission services
Charges to ATC for services and construction	$ 22.9	$ 27.5	$ 25.9
Charges from ATC for network transmission services	361.0	350.5	348.1
Net refund from ATC related to FERC ROE orders	7.3	10.7	$ 0.0
Balance sheet
Accounts receivable for services provided to ATC	2.0	3.7
Accounts payable for services received from ATC	30.2	29.3
Amounts due from ATC for transmission infrastructure upgrades	$ 13.0	$ 4.6