v3.5.0.2
Equity (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Increase (decrease) in shareholders' equity          
Balance at the beginning of the period       $ 435.2 $ 816.5
Net (loss) income       (109.8) 21.8
Realized and unrealized gain on hedging activities, net of tax   $ 0.2 $ (0.2) (0.1) (0.2)
Foreign currency translation adjustments   (22.0) (22.1) (2.6) (52.7)
Common share repurchases       (13.9)  
Stock-based compensation       1.4 2.1
Dividends paid to noncontrolling interests         (3.7)
Dividends declared on common shares         (8.5)
Dividends declared on preferred shares of a subsidiary company       (6.4) (6.7)
Derecognition of noncontrolling interests upon sale of subsidiaries         (224.3)
Balance at the end of the period $ 303.8 303.8 544.3 $ 303.8 544.3
Stock Repurchase Program          
Total Number of Shares Purchased 1,394,096     5,678,736  
Common stock repurchased, value       $ 13.9  
Total Atlantic Power Corporation Shareholder's Equity          
Increase (decrease) in shareholders' equity          
Balance at the beginning of the period       213.9 356.2
Net (loss) income       (116.2) 26.1
Realized and unrealized gain on hedging activities, net of tax       (0.1) (0.2)
Foreign currency translation adjustments       (2.6) (52.7)
Common share repurchases       (13.9)  
Stock-based compensation       1.4 2.1
Dividends declared on common shares         (8.5)
Balance at the end of the period $ 82.5 82.5 323.0 82.5 323.0
Preferred Shares of A Subsidiary Company          
Increase (decrease) in shareholders' equity          
Balance at the beginning of the period       221.3 221.3
Net (loss) income       6.4 6.7
Dividends declared on preferred shares of a subsidiary company       (6.4) (6.7)
Balance at the end of the period $ 221.3 $ 221.3 $ 221.3 $ 221.3 221.3
Noncontrolling Interests          
Increase (decrease) in shareholders' equity          
Balance at the beginning of the period         239.0
Net (loss) income         (11.0)
Dividends paid to noncontrolling interests         (3.7)
Derecognition of noncontrolling interests upon sale of subsidiaries         $ (224.3)