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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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Percentage of public utility's approved equity to capital, including debt and equity.

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Percentage of public utility's approved rate increase (decrease) by regulatory agency.

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Percentage of public utility's approved return on equity.

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The amount of interim rates refunded to customers.

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