v2.4.0.6
Equity compensation plans (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Long-term incentive plan
      
Units      
Outstanding at the beginning of the period (in shares) 485,781,000 600,981,000 471,280,000
Granted (in shares) 233,752,000 216,110,000 305,112,000
Additional shares from dividends 38,667,000 36,204,000 46,854,000
Forfeitures (in shares) (28,932,000) (103,991,000)  
Vested and redeemed (in shares) (236,733,000) (263,523,000) (222,265,000)
Outstanding at the end of the period (in shares) 492,535,000 485,781,000 600,981,000
Grant Date Weighted Average Fair Value per Unit      
Outstanding at the beginning of the period (in dollars per share) $ 11.49 $ 10.28 $ 7.30
Granted (in dollars per share) $ 14.67 $ 14.02 $ 13.29
Additional shares from dividends (in dollars per share) $ 13.43 $ 11.04 $ 9.54
Forfeitures (in dollars per share) $ 13.63 $ 11.55  
Vested and redeemed (in dollars per share) $ 10.18 $ 9.40 $ 7.94
Outstanding at the end of the period (in dollars per share) $ 13.88 $ 11.49 $ 10.28
Fair value of outstanding notional units $ 6.3 $ 6.4  
Compensation expense related to LTIP 2.5 3.2 4.5
Cash payments made for vested notional units 1.1 1.5 2.8
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Weighted average risk free rate of return, minimum (as a percent) 0.05% 0.15%  
Weighted average risk free rate of return, maximum (as a percent) 0.28% 0.28%  
Dividend yield (as a percent) 10.10% 7.90%  
Expected volatility Atlantic Power (as a percent) 22.49% 22.20%  
Weighted average remaining measurement period 1 year 4 months 20 days 10 months 13 days  
Long-term incentive plan | Minimum
      
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Expected volatility of peer companies (as a percent) 11.90% 17.30%  
Long-term incentive plan | Maximum
      
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Expected volatility of peer companies (as a percent) 97.10% 112.90%  
Maximum number of common Shares to be issued under the incentive plan 1,300,000    
2012 Equity Incentive Plan
      
Grant Date Weighted Average Fair Value per Unit      
Compensation expense related to LTIP $ 0.1    
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Maximum number of common Shares to be issued under the incentive plan 10,000