Regulatory Matters - MISO Return on Equity Complaints (Details) - Jun. 30, 2015 - USD ($) $ in Millions	Total
Loss Contingencies [Line Items]
Estimated Refund Obligation	$ 5.6
Estimated Refund	$ 3.7
FERC Complaint 1 [Member]
Loss Contingencies [Line Items]
Equity Method Investment, Proposed Return on Common Equity	9.15%
FERC Complaint 2 [Member]
Loss Contingencies [Line Items]
Equity Method Investment, Proposed Return on Common Equity	8.67%
Minnesota Power [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	$ 1.9
Minnesota Power [Member] | FERC Complaint 2 [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	$ 1.5