Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant related party transactions
Charges to ATC for services and construction	$ 25.9	$ 21.8	$ 17.1
Charges from ATC for network transmission services	348.1	338.1	349.3
Refund from ATC related to a FERC audit	0.0	22.0	0.0
Refund from ATC per FERC ROE order	0.0	0.0	$ 28.3
Balance Sheet
Accounts receivable for services provided to ATC	3.5	3.4
Accounts payable for services received from ATC	29.0	28.2
Amounts due from ATC for transmission infrastructure upgrades	$ 2.8	$ 29.4

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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