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Basic and diluted earnings (loss) per share (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Numerator:    
Income (loss) from continuing operations attributable to Atlantic Power Corporation $ 5.6 $ (53.9)
Income from discontinued operations, net of tax 0.9 11.6
Net income (loss) attributable to Atlantic Power Corporation $ 6.5 $ (42.3)
Denominator:    
Weighted average basic shares outstanding 119.5 113.6
Dilutive potential shares:    
Convertible debentures (in shares) 27.7 13.3
LTIP notional units (in shares) 0.4 0.5
Potentially dilutive shares 147.6 127.4
Diluted loss per share from continuing operations attributable to Atlantic Power Corporation (in dollars per share) $ 0.04 $ (0.47)
Diluted earnings per share from discontinued operations (in dollars per share) $ 0.01 $ 0.10
Net income (loss) attributable to Atlantic Power Corporation (in dollars per share) $ 0.05 $ (0.37)