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Regulatory Matters - Utility Rates (Details) | Jan. 01, 2013 | Jun. 01, 2011 | Sep. 30, 2015 Customers Years MW
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Bison Wind Energy Center [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Generating Capacity (MW) | MW |  |  | 497
MPUC 2010 Minnesota Rate Case [Member] | Electric Rates [Member] | Retail Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved Rate of Return on Common Equity |  | 10.38% |
Approved Percentage of Capital Structure Related to Equity |  | 54.29% |
MPUC 2010 Minnesota Rate Case [Member] | Electric Rates [Member] | Municipal Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved Rate of Return on Common Equity |  | 10.38% |
FERC-Approved Wholesale Rates [Member] | Electric Rates [Member] | Municipal Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of Customers |  |  | 16
Length of Notice Required to Terminate (Years) | Years |  |  | 3
FERC-Approved Wholesale Rates [Member] | Electric Rates [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of Customers |  |  | 14
FERC-Approved Wholesale Rates [Member] | Electric Rates [Member] | Municipal Customers [Member] | Minimum [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Annual Change in Capacity Charge, Percent |  |  | (1.00%)
FERC-Approved Wholesale Rates [Member] | Electric Rates [Member] | Municipal Customers [Member] | Maximum [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Annual Change in Capacity Charge, Percent |  |  | 2.00%
PSCW 2012 Wisconsin Rate Case [Member] | Retail Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved Rate of Return on Common Equity | 10.90% |  |

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