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Idaho Public Utilities Commission (IPUC) (Details) - IPUC [Member] - Natural gas rate proceeding [Member] - USD ($) $ in Millions | Sep. 14, 2017 | Aug. 17, 2017 | Apr. 28, 2017 | Jan. 17, 2017 | Aug. 12, 2016
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Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  | $ 1.2 |  |  | $ 10.2
Public utilities, requested rate increase (decrease), percentage |  | 1.36% |  |  | 4.10%
Public utilities, requested rate increase (decrease), amended, amount |  |  |  | $ 9.4 |
Public utilities, approved rate increase (decrease), amount | $ 6.7 |  | $ 4.1 |  |
Public utilities, approved rate increase (decrease), percentage |  |  | 1.60% |  |
Public utilities, approved return on equity, percentage |  |  | 9.50% |  |

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