v2.4.0.8
Employee incentive programs (Details) (Long-term incentive plan, USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Dec. 31, 2012
Units      
Outstanding at the beginning of the period (in shares)   492,535  
Granted (in shares)   587,201  
Additional shares from dividends (in shares)   27,151  
Forfeited (in shares)   (42,000)  
Vested (in shares)   (202,696)  
Outstanding at the end of the period (in shares) 862,191 862,191 492,535
Grant Date Weighted Average Fair Value per Unit      
Outstanding at the beginning of the period (in dollars per share)   $ 13.9  
Granted (in dollars per share)   $ 4.9  
Additional shares from dividends (in dollars per share)   $ 10.0  
Forfeited (in dollars per share)   $ 12.3  
Vested (in dollars per share)   $ 13.5  
Outstanding at the end of the period (in dollars per share) $ 7.8 $ 7.8 $ 13.9
Cash payments made for vested notional units   $ 0.9  
Compensation expense $ 0.8 $ 1.2  
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Weighted average risk free rate of return, minimum (as a percent)   0.10% 0.10%
Weighted average risk free rate of return, maximum (as a percent)   0.60% 0.30%
Dividend yield (as a percent)   9.70% 10.10%
Expected volatility Atlantic Power (as a percent)     22.50%
Weighted average remaining measurement period   2 years 2 months 12 days 1 year 4 months 24 days
Minimum
      
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Expected volatility Atlantic Power (as a percent)   37.90%  
Expected volatility of peer companies (as a percent)   14.70% 11.90%
Maximum
      
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model      
Expected volatility Atlantic Power (as a percent)   62.10%  
Expected volatility of peer companies (as a percent)   82.70% 97.10%