Basic and diluted earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Loss from continuing operations attributable to Atlantic Power Corporation	$ (18.5)	$ (22.3)	$ (33.5)	$ 0.1
Income from discontinued operations, net of tax		37.0		32.1
Net (loss) income attributable to Atlantic Power Corporation	$ (18.5)	$ 14.7	$ (33.5)	$ 32.2
Denominator:
Weighted average basic shares outstanding	121.6	121.9	121.8	121.7
Dilutive potential shares:
Convertible debentures (in shares)	14.8	22.6	18.3	23.0
LTIP notional units (in shares)	0.1	0.2	0.1	0.2
Potentially dilutive shares	136.5	144.7	140.2	144.9
Diluted loss per share from continuing operations attributable to Atlantic Power Corporation	$ (0.15)	$ (0.18)	$ (0.28)
Diluted earnings per share from discontinued operations		0.30		$ 0.26
Diluted (loss) earnings per share attributable to Atlantic Power Corporation	$ (0.15)	$ 0.12	$ (0.28)	$ 0.26
Potentially dilutive shares from convertible debentures excluded from fully diluted shares	$ 14.8	$ 22.6	$ 18.3	$ 23.0