The proposed rate credit that will be refunded to customers related to the proposed merger with Hydro One.

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Termination fee payable by company or acquirer if pending merger is terminated under certain circumstances.

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Total Financial Commitments Related to Merger Regulatory Settlements

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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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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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Pending Merger with Hydro One Pending Merger with Hydro One (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended	3 Months Ended
	Apr. 13, 2018	Apr. 03, 2018	Mar. 27, 2018	Sep. 30, 2017	Mar. 31, 2018	Jul. 19, 2017
Business Acquisition [Line Items]
Total Financial Commitments Related to Merger Regulatory Settlements					$ 74.0
Hydro One [Member]
Business Acquisition [Line Items]
Business Acquisition, Share Price						$ 53
Rate Credit to Retail Customers Related to Merger				$ 32.0
Termination Fee Payable By Company If Pending Merger Is Terminated Under Certain Circumstances						$ 103.0
WASHINGTON
Business Acquisition [Line Items]
Total Financial Commitments Related to Merger Regulatory Settlements					44.0
WASHINGTON \| Hydro One [Member]
Business Acquisition [Line Items]
Rate Credit to Retail Customers Related to Merger			$ 31.0	$ 20.0
IDAHO
Business Acquisition [Line Items]
Total Financial Commitments Related to Merger Regulatory Settlements					$ 22.0
Subsequent Event [Member] \| ALASKA \| Hydro One [Member]
Business Acquisition [Line Items]
Rate Credit to Retail Customers Related to Merger		$ 1.0
Subsequent Event [Member] \| IDAHO \| Hydro One [Member]
Business Acquisition [Line Items]
Rate Credit to Retail Customers Related to Merger	$ 16.0