v3.5.0.2
Accounting for derivative instruments and hedging activities (Details)
MMBTU in Millions, $ in Millions
1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Apr. 13, 2016
USD ($)
May 05, 2014
USD ($)
Jun. 30, 2014
MMBTU
Sep. 30, 2016
USD ($)
item
Sep. 30, 2015
USD ($)
Mar. 31, 2017
MMBTU
Sep. 30, 2016
USD ($)
MMBTU
item
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
MMBTU
Mar. 31, 2020
USD ($)
Sep. 30, 2019
USD ($)
Feb. 29, 2016
Mar. 31, 2015
Feb. 14, 2014
USD ($)
Derivative instruments                            
Derivative Number of Contracts Designated as Cash Flow Hedges | item       1     1              
Minimum fixed interest rate (as a percent) 6.00%                          
Convertible Debentures                            
Derivative instruments                            
Aggregate principal amount of debt       $ 101.4     $ 101.4   $ 277.7          
Senior secured term loan facility, due 2021 | Long-term debt excluding debentures                            
Derivative instruments                            
Long-term debt                 473.2          
Piedmont term loan, due 2018 | Long-term debt excluding debentures                            
Derivative instruments                            
Long-term debt       57.5     $ 57.5   $ 59.0          
Applicable margin (as a percent)             8.47%              
London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2021 | Long-term debt excluding debentures                            
Derivative instruments                            
Minimum percentage of variable rate base             1.00%              
Applicable margin (as a percent)             3.75%              
Adjusted Eurodollar Rate                            
Derivative instruments                            
Minimum percentage of variable rate base   1.00%                        
Applicable margin (as a percent)             5.00%              
Gas purchase agreements | Ontario project                            
Derivative instruments                            
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando) | MMBTU     2.9                      
Gas purchase agreements | Morris Project | Forecast                            
Derivative instruments                            
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando) | MMBTU           1.3                
Expected uncontracted gas requirements for 2015 | Ontario project                            
Derivative instruments                            
Percentage of requirements whose price is fixed by derivative             100.00%              
Expected uncontracted gas requirements for 2016 | Ontario project                            
Derivative instruments                            
Percentage of requirements whose price is fixed by derivative             35.00%              
Expected uncontracted gas requirements for 2017 | Ontario project                            
Derivative instruments                            
Percentage of requirements whose price is fixed by derivative             30.00%              
Natural gas swaps                            
Derivative instruments                            
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando) | MMBTU             4.7   2.8          
Natural gas swaps | Derivative instruments not designated as cash flow hedges                            
Derivative instruments                            
Gain (Loss) on Sale of Derivatives       0.6 $ 1.5   $ 4.0 $ 4.3            
Natural gas swaps during remainder of 2014 | Orlando project                            
Derivative instruments                            
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando) | MMBTU             5.7              
Percentage of the entity's share in required natural gas purchases hedge             95.00%              
Interest rate swaps                            
Derivative instruments                            
Minimum fixed interest rate (as a percent) 6.00%                          
Notional amount       520.4     $ 520.4   $ 302.3          
Interest rate swaps | Derivative instruments not designated as cash flow hedges                            
Derivative instruments                            
Gain (Loss) on Sale of Derivatives       1.0 $ 0.5   2.7 $ 1.7            
Interest rate swaps | London Interbank Offered Rate (LIBOR)                            
Derivative instruments                            
Aggregate principal amount of debt       654.9     654.9              
Interest rate swaps | London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2021 | Long-term debt excluding debentures                            
Derivative instruments                            
Notional amount   $ 434.7                        
Interest rate swaps | Adjusted Eurodollar Rate                            
Derivative instruments                            
Notional amount   199.0   124.7     124.7              
Interest rate swaps | Adjusted Eurodollar Rate | Senior secured term loan facility, due 2021 | Long-term debt excluding debentures                            
Derivative instruments                            
Notional amount $ 509.0                          
Aggregate principal amount of debt   $ 600.0                        
Interest rate swaps | Minimum | Adjusted Eurodollar Rate                            
Derivative instruments                            
Minimum percentage of variable rate base 1.00%                          
Interest rate swaps | APLP Holdings | Adjusted Eurodollar Rate | Senior secured term loan facility, due 2021                            
Derivative instruments                            
Notional amount $ 310.0     124.7     124.7              
Aggregate principal amount of debt $ 700.0     $ 654.9     $ 654.9              
Interest rate swaps | APLP Holdings | Forecast                            
Derivative instruments                            
Remaining notional amount to terminate                   $ 150.0 $ 160.0      
Interest rate swaps | Piedmont                            
Derivative instruments                            
Long-term debt                           $ 68.5
Interest rate swaps | Swaption interest rate until February 15, 2015 | Cadillac Project                            
Derivative instruments                            
Swaption interest rate (as a percent)                         6.00%  
Interest rate swaps | Swaption interest rate from February 16, 2015 to February 15, 2019 | Cadillac Project                            
Derivative instruments                            
Swaption interest rate (as a percent)       6.10%     6.10%              
Interest rate swaps | Swaption interest rate from February 16, 2019 to February 15, 2023 | Cadillac Project                            
Derivative instruments                            
Swaption interest rate (as a percent)       6.30%     6.30%              
Interest rate swaps | Swaption interest rate after February 15, 2023 | Cadillac Project                            
Derivative instruments                            
Swaption interest rate (as a percent)       6.40%     6.40%              
Interest rate swaps | Swaption interest rate until February 29, 2016 | Piedmont                            
Derivative instruments                            
Swaption interest rate (as a percent)       1.70%     1.70%              
Interest rate swaps | Swaption interest rate until February 29, 2016 | Piedmont | Minimum                            
Derivative instruments                            
Applicable margin of derivative (as a percent)                       3.50%    
Interest rate swaps | Swaption interest rate until February 29, 2016 | Piedmont | Maximum                            
Derivative instruments                            
Applicable margin of derivative (as a percent)                       3.80%    
Interest rate swaps | Swaption interest rate From February, 2016 to August, 2018 | Piedmont                            
Derivative instruments                            
Swaption interest rate (as a percent)       4.47%     4.47%              
Applicable margin of derivative (as a percent)       4.00%     4.00%              
Swaption all in one interest rate (as a percent)       8.50%     8.50%              
Interest rate swaps | Swaption interest rate From August, 2018 to November, 2030 | Piedmont                            
Derivative instruments                            
Swaption interest rate (as a percent)       4.47%     4.47%