Investment in ATC - FERC Complaints (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Loss Contingencies [Line Items]
Approved Return on Common Equity	12.20%
Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Basis Point Reduction on Approved Rate of Return on Common Equity	50
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%
ALLETE Transmission Holdings [Member]
Loss Contingencies [Line Items]
Reserve Reflected in Equity Earnings in ATC	$ 2.0
ALLETE Transmission Holdings [Member] | FERC Complaint 2 [Member]
Loss Contingencies [Line Items]
Reserve Reflected in Equity Earnings in ATC	1.1
After-tax [Member] | Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Annual Effect on Future Equity Earnings in ATC	0.5
Pre-tax [Member] | Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Annual Effect on Future Equity Earnings in ATC	$ 0.9