Investment in ATC - FERC Complaint (Details) $ in Millions	12 Months Ended
	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%
ATC [Member]
Loss Contingencies [Line Items]
Approved Return on Common Equity	12.20%
Sensitivity Analysis [Member] | ATC [Member]
Sensitivity Analysis [Line Items]
Basis Point Reduction on Approved Rate of Return on Common Equity	50
Sensitivity Analysis [Member] | ATC [Member] | After-tax [Member]
Sensitivity Analysis [Line Items]
Annual Effect on Future Equity Earnings in ATC	$ 0.5
Sensitivity Analysis [Member] | ATC [Member] | Pre-tax [Member]
Sensitivity Analysis [Line Items]
Annual Effect on Future Equity Earnings in ATC	0.9
ALLETE Transmission Holdings [Member] | ATC [Member]
Loss Contingencies [Line Items]
Reserve Reflected in Equity Earnings in ATC	5.2
Reserve Reflected in Equity Earnings in ATC, Portion Attributable to Prior Years	$ 2.4