v2.4.0.6
Employee incentive programs (Details) (Long-term incentive plan, USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Units    
Outstanding at the beginning of the period (in shares) 492,535  
Granted (in shares) 482,384  
Additional units from dividends (in shares) 9,906  
Forfeited (in shares) (42,000)  
Vested (in shares) (200,697)  
Outstanding at the end of the period (in shares) 742,128 492,535
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) $ 13.8  
Forfeited (in dollars per share) $ 12.3  
Vested (in dollars per share) $ 13.6  
Outstanding at the end of the period (in dollars per share) $ 8.2 $ 13.9
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model    
Weighted average risk free rate of return (as a percent) 0.10%  
Weighted average risk free rate of return, minimum (as a percent)   0.10%
Weighted average risk free rate of return, maximum (as a percent)   0.30%
Dividend yield (as a percent) 8.00% 10.10%
Expected volatility Atlantic Power (as a percent) 48.60% 22.50%
Weighted average remaining measurement period 10 months 24 days 1 year 4 months 24 days
Cash payments made for vested notional units $ 0.9  
Minimum
    
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model    
Expected volatility of peer companies (as a percent) 11.90% 11.90%
Maximum
    
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model    
Expected volatility of peer companies (as a percent) 72.80% 97.10%