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Investment in ATC - FERC Complaint (Details) $ in Millions | 12 Months Ended
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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

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