INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS BALANCE SHEET INFORMATION (Details) - ATC - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investment in transmission affiliates
Accounts receivable for services provided to ATC	$ 2.4	$ 3.7
Accounts payable for services received from ATC	30.3	29.3
Amounts due from ATC for transmission infrastructure upgrades	$ 5.3	$ 4.6

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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