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Pending Merger with Hydro One Pending Merger with Hydro One (Details) - Hydro One [Member] - USD ($) $ / shares in Units, $ in Millions |  |  |  |  |  | 9 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 10, 2018 | May 25, 2018 | Apr. 13, 2018 | Apr. 03, 2018 | Mar. 27, 2018 | Sep. 30, 2018 | Jul. 19, 2017
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business Acquisition, Share Price |  |  |  |  |  |  | $ 53
Total Financial Commitments Related to Merger Regulatory Settlements |  |  |  |  |  | $ 78.6 |
Termination Fee Payable By Company If Pending Merger Is Terminated Under Certain Circumstances |  |  |  |  |  |  | $ 103.0
WASHINGTON |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Rate Credit to Retail Customers Related to Merger |  |  |  |  | $ 31.0 |  |
Total Financial Commitments Related to Merger Regulatory Settlements |  |  |  |  |  | 42.0 |
ALASKA |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Rate Credit to Retail Customers Related to Merger |  |  |  | $ 1.0 |  |  |
IDAHO |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Rate Credit to Retail Customers Related to Merger |  |  | $ 16.0 |  |  |  |
Total Financial Commitments Related to Merger Regulatory Settlements |  |  |  |  |  | $ 21.0 |
OREGON |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Rate Credit to Retail Customers Related to Merger |  | $ 8.0 |  |  |  |  |
Total Financial Commitments Related to Merger Regulatory Settlements |  | $ 10.0 |  |  |  |  |
MONTANA |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Rate Credit to Retail Customers Related to Merger | $ 4.5 |  |  |  |  |  |

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