v3.5.0.2
Long-term debt (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2016
Dec. 31, 2015
May 05, 2014
Long-term debt      
Less: unamortized discount $ (18.5)    
Less: current maturities (101.4) $ (15.8)  
Total long-term debt 778.9 682.7  
Interest rate swaps      
Long-term debt      
Notional amount 520.4 302.3  
London Interbank Offered Rate (LIBOR) | Interest rate swaps      
Long-term debt      
Aggregate principal amount of debt 654.9    
Long-term debt excluding debentures      
Long-term debt      
Less: unamortized deferred financing costs (16.6) (34.8)  
Less: current maturities (101.4) (15.8)  
Total long-term debt 778.9 682.7  
Long-term debt excluding debentures | Senior secured term loan facility, due 2021      
Long-term debt      
Long-term debt   473.2  
Less: current maturities   (4.7)  
Long-term debt excluding debentures | Senior secured term loan facility, due 2023      
Long-term debt      
Long-term debt 654.9    
Less: current maturities $ (90.0)    
Applicable margin (as a percent) 5.00%    
Long-term debt excluding debentures | Senior unsecured notes, due June 2036 (Cdn$210.0)      
Long-term debt      
Long-term debt $ 160.1 151.7  
Interest rate (as a percent) 5.95%    
Long-term debt excluding debentures | Epsilon Power Partners term facility, due 2019      
Long-term debt      
Long-term debt $ 15.0 19.5  
Less: current maturities (6.1) (6.0)  
Long-term debt excluding debentures | Cadillac term loan, due 2025      
Long-term debt      
Long-term debt 27.7 29.5  
Less: current maturities (2.9) (2.5)  
Long-term debt excluding debentures | Piedmont term loan, due 2018      
Long-term debt      
Long-term debt 57.5 59.0  
Less: current maturities $ (2.2) (2.4)  
Applicable margin (as a percent) 8.47%    
Long-term debt excluding debentures | Other long-term debt      
Long-term debt      
Long-term debt $ 0.2 0.4  
Less: current maturities $ (0.2) $ (0.2)  
Long-term debt excluding debentures | Minimum | Other long-term debt      
Long-term debt      
Interest rate (as a percent) 5.50%    
Long-term debt excluding debentures | Maximum | Other long-term debt      
Long-term debt      
Interest rate (as a percent) 6.70%    
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2021      
Long-term debt      
Minimum percentage of variable rate base 1.00%    
Applicable margin (as a percent) 3.75%    
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Epsilon Power Partners term facility, due 2019      
Long-term debt      
Applicable margin (as a percent) 3.125%    
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Cadillac term loan, due 2025      
Long-term debt      
Applicable margin (as a percent) 1.37%    
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Interest rate swaps | Senior secured term loan facility, due 2021      
Long-term debt      
Notional amount     $ 434.7