Regulatory Matters - Utility Rates (Details)	0 Months Ended
	Jun. 01, 2011	Jan. 01, 2013	Mar. 31, 2015 MW
Bison Wind Energy Center [Member]
Public Utilities, General Disclosures [Line Items]
Generating Capacity (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)			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%