v3.3.1.900
Power purchase agreements and other intangible assets and liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Components of intangible assets      
Gross balance at the end of the period $ 546.9 $ 577.0  
Less: accumulated amortization (238.0) (199.9)  
Net balance at the end of the period 308.9 377.1  
Gross balances at the end of the period (41.0) (44.8)  
Less: accumulated amortization 14.0 11.4  
Net balance at the end of the period (27.0) (33.4)  
Amortization of intangible assets (liabilities) 50.1 56.4 $ 59.4
Power Purchase Agreements      
Components of intangible assets      
Gross balance at the end of the period 534.0 563.6  
Less: accumulated amortization (225.4) (187.4)  
Net balance at the end of the period 308.6 376.2  
Gross balances at the end of the period (28.4) (32.2)  
Less: accumulated amortization 9.1 7.7  
Net balance at the end of the period (19.3) (24.5)  
Amortization of intangible assets (liabilities) 51.3 57.6 60.6
Development Costs      
Components of intangible assets      
Gross balance at the end of the period 12.9 13.4  
Less: accumulated amortization (12.6) (12.5)  
Net balance at the end of the period 0.3 0.9  
Fuel Supply Agreements      
Components of intangible assets      
Gross balances at the end of the period (12.6) (12.6)  
Less: accumulated amortization 4.9 3.7  
Net balance at the end of the period (7.7) (8.9)  
Amortization of intangible assets (liabilities) $ (1.2) $ (1.2) $ (1.2)