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Regulatory Matters - Minnesota Solar Energy Standard (Details) - MW	Dec. 31, 2017	Dec. 31, 2016
Regulatory Matters [Line Items]
Solar Energy Standard Mandate - Qualifying Capacity for Small Scale Solar Mandate (MW)	40
MPUC [Member]
Regulatory Matters [Line Items]
Solar Energy Standard Mandate - Overall Mandate Percentage	1.50%
MPUC [Member] | Minimum [Member]
Regulatory Matters [Line Items]
Solar Energy Standard Mandate - Overall Mandate Percentage	1.50%
Solar Energy Standard Mandate - Small Scale Solar Mandate Percentage	10.00%
MPUC [Member] | Maximum [Member]
Regulatory Matters [Line Items]
Solar Energy Standard Mandate - Qualifying Capacity for Small Scale Solar Mandate (MW)		0.02
MPUC [Member] | Minnesota Solar Energy Standard - Camp Ripley Project [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Generating Capacity Owned (MW)	10
MPUC [Member] | Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Generating Capacity Counterparty Owned (MW)	1
MPUC [Member] | Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Generating Capacity Owned (MW)	0.04

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Generating Capacity Counterparty Owned (MW)

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Capacity of a generating facility, unit, or project in megawatts.

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Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020.

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The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard.

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Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate.

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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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