Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Charges from ATC for network transmission services
Charges to ATC for services and construction	$ 18.9	$ 22.9	$ 27.5
Charges from ATC for network transmission services	363.7	361.0	350.5
Net refund (payment) from (to) ATC related to FERC ROE orders	(0.1)	7.3	$ 10.7
Balance sheet
Accounts receivable for services provided to ATC	1.2	2.0
Accounts payable for services received from ATC	30.4	30.2
Amounts due from ATC for transmission infrastructure upgrades (1)	$ 26.6	$ 13.0