Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment in Transmission Affiliates
Charges to ATC for services and construction	$ 21.8	$ 17.1	$ 18.5
Charges from ATC for network transmission services	338.1	349.3	357.3
Refund from ATC related to a FERC audit	22.0	0.0	0.0
Refund from ATC per FERC ROE order	0.0	28.3	0.0
Accounts receivable for services provided to ATC	3.4	1.5
Dividends Receivable	0.0	39.9	$ 35.2
Accounts payable for services received from ATC	$ 28.2	$ 31.2

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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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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The amount of the refund received from ATC due to a FERC audit.

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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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