Regulatory Matters - MISO Return on Equity Complaints (Details) - FERC [Member] $ in Millions	Dec. 31, 2015 USD ($)
Return on Equity Complaint 1 [Member]
Loss Contingencies [Line Items]
Proposed Return on Common Equity	9.15%
Return on Equity Complaint 2 [Member]
Loss Contingencies [Line Items]
Proposed Return on Common Equity	8.67%
Return on Equity Complaints [Member]
Loss Contingencies [Line Items]
Proposed Return on Common Equity	10.32%
Return on Equity Complaints [Member] | Minnesota Power [Member]
Loss Contingencies [Line Items]
Estimated Refund Obligation for MISO Revenue	$ 7.2
Estimated Refund for MISO Transmission Expense	4.5
Reserve	2.7
Reserve, Portion Attributable to Prior Years	$ 1.5