Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment in Transmission Affiliates
Charges to ATC for services and construction	$ 17.1	$ 18.5	$ 15.4
Charges from ATC for network transmission services	349.3	357.3	289.2
Refund from ATC per FERC ROE order	(28.3)	0.0	$ 0.0
Accounts receivable for services provided to ATC	1.5	2.2
Dividends Receivable	39.9	35.2
Accounts payable for services received from ATC	$ 31.2	$ 28.7

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).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-Subparagraph (SX 210.5-02.3(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Carrying amount as of the balance sheet date of dividends declared but not received.

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

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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.

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

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

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