INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Investment in Transmission Affiliates
Charges to ATC for services and construction	$ 4.6	$ 4.2
Charges from ATC for network transmission services	84.5	87.3
Refund from ATC per FERC ROE order	$ 0.0	$ 28.3

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Refund of previous year's revenue that wasn't distributed by ATC. This resulted from an order by the FERC.

+ References

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- Definition

Charges to related parties for services, construction, and/or operations (excluding transactions that are eliminated in consolidated or combined financial statements) during the period.

+ References

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