Investment in ATC - FERC Complaints (Details) - Jun. 30, 2015 $ in Millions	USD ($)
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%
Rainy River Energy [Member]
Loss Contingencies [Line Items]
Loss Contingency, Accrual Reflected in Equity Earnings	$ 1.7
Rainy River Energy [Member] | FERC Complaint 2 [Member]
Loss Contingencies [Line Items]
Loss Contingency, Accrual Reflected in Equity Earnings	1.1
After-tax [Member] | Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Effect on Future Earnings, Amount	0.5
Pre-tax [Member] | Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Effect on Future Earnings, Amount	$ 0.9