INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS BALANCE SHEET INFORMATION (Details) - ATC - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investment in transmission affiliates
Accounts receivable for services provided to ATC	$ 2.0	$ 3.4
Accounts payable for services received from ATC	$ 29.0	$ 28.2

Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant