v3.4.0.3
Derivative Instruments - Derivative assets and liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Derivative Asset    
Other current derivative assets $ 5.3 $ 8.8
Other long-term derivative assets 0.8 1.1
Derivative asset 6.1 9.9
Derivative Liability    
Other current derivative liabilities 42.3 49.0
Other long-term derivative liabilities 6.7 10.0
Derivative liability 49.0 59.0
Natural gas contracts    
Derivative Asset    
Other current derivative assets 1.7 2.6
Other long-term derivative assets 0.6 0.5
Derivative Liability    
Other current derivative liabilities 28.7 38.5
Other long-term derivative liabilities 1.8 3.3
Petroleum products contracts    
Derivative Asset    
Other current derivative assets 0.9 0.9
Other long-term derivative assets 0.2 0.3
Derivative Liability    
Other current derivative liabilities 3.3 3.8
Other long-term derivative liabilities 0.5 1.1
FTRs    
Derivative Asset    
Other current derivative assets 1.1 3.6
Derivative Liability    
Other current derivative liabilities 0.0 0.0
Coal contracts    
Derivative Asset    
Other current derivative assets 1.6 1.7
Other long-term derivative assets 0.0 0.3
Derivative Liability    
Other current derivative liabilities 10.3 6.7
Other long-term derivative liabilities $ 4.4 $ 5.6