Regulatory Matters - Conservation Improvement Program (CIP) (Details) - MPUC [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Regulatory Matters [Line Items]
Annual Energy-Savings Goal	1.50%
Minimum [Member]
Regulatory Matters [Line Items]
CIP Spending Requirement	1.50%
CIP Triennial Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Investment Goal	$ 7.3	$ 7.1	$ 6.9
Actual Spending	7.4	6.6	7.2
Investment Goal in 2017	10.6
Investment Goal in 2018	10.8
Investment Goal in 2019	10.9
CIP Consolidated Filing [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Financial Incentive	$ 7.5	$ 6.2	$ 8.7