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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net (loss) income attributable to Atlantic Power Corporation	$ (0.6)	$ (21.9)	$ 15.2	$ (24.6)
Add: convertible debenture interest expense			2.2
Net income (loss) available to common stockholder, Diluted	$ (0.6)	$ (21.9)	$ 17.4	$ (24.6)
Denominator:
Weighted average basic shares outstanding	112.4	115.2	113.6	115.0
Basic (loss) earnings per share attributable to Atlantic Power Corporation	$ (0.01)	$ (0.19)	$ 0.13	$ (0.21)
Dilutive potential shares:
Convertible debentures (in shares)			26.5
Potentially dilutive shares	112.4	115.2	140.1	115.0
Diluted (loss) earnings per share attributable to Atlantic Power Corporation	$ (0.01)	$ (0.19)	$ 0.13	$ (0.21)