Regulatory Matters - Utility Rates (Details)	Jan. 01, 2013	Jun. 01, 2011	Jun. 30, 2015 Customers Years MW
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%
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
PSCW 2012 Wisconsin Rate Case [Member] | Retail Customers [Member]
Public Utilities, General Disclosures [Line Items]
Approved Rate of Return on Common Equity	10.90%