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REGULATORY ENVIRONMENT - MERC (Details) - 2018 rates - Minnesota Public Utilities Commission (MPUC) - MERC - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2019
Public Utilities, General Disclosures [Line Items] |  |
Approved rate increase | $ 3.1 |
Approved rate increase (as a percent) | 1.26% |
Approved return on equity (as a percent) | 9.70% |
Approved common equity component average (as a percent) | 50.90% |
Interim rates refunded to customers |  | $ 8.0

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