Regulatory Matters - Conservation Improvement Program (CIP) (Details) - MPUC [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Regulatory Matters [Line Items]
Energy CIP Spending Requirement	1.50%
Annual Energy-Savings Goal	1.50%
CIP Triennial Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Investment Goal	$ 7.1	$ 6.9	$ 6.0
Actual Spending	6.6	7.2	6.4
CIP Annual Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Financial Incentive	$ 6.2	$ 8.7	$ 7.1