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Regulatory Matters - Minnesota Solar Energy Standard (Details) - MPUC [Member] - MW
Oct. 02, 2018
Sep. 30, 2018
Regulatory Matters [Line Items]    
Minnesota Solar Energy Standard - Overall Mandate Percentage   1.50%
Minnesota Solar Energy Standard [Member] | Minnesota Power [Member]    
Regulatory Matters [Line Items]    
Minnesota Solar Energy Standard - Percentage of Overall Mandate Expected to be Met with Current Filings or Projects   67.00%
Minnesota Solar Energy Standard - Camp Ripley Project [Member] | Minnesota Power [Member]    
Regulatory Matters [Line Items]    
Generating Capacity Owned (MW)   10
Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] | Minnesota Power [Member]    
Regulatory Matters [Line Items]    
Generating Capacity Counterparty Owned (MW)   1
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
Minimum [Member]    
Regulatory Matters [Line Items]    
Minnesota Solar Energy Standard - Overall Mandate Percentage   1.50%
Minnesota Solar Energy Standard - Small Scale Solar Mandate Percentage   10.00%
Maximum [Member]    
Regulatory Matters [Line Items]    
Minnesota Solar Energy Standard - Qualifying Capacity for Small Scale Solar Mandate (MW)   0.04
Subsequent Event [Member] | Minnesota Solar Energy Standard - Blanchard Solar Energy Facility [Member] | Minnesota Power [Member]    
Regulatory Matters [Line Items]    
Generating Capacity Counterparty Owned (MW) 10