Basic and diluted (loss) earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net (loss) income attributable to Atlantic Power Corporation	$ (5.7)	$ 1.2	$ 23.8	$ 10.1
Add: convertible debenture interest expense			1.9	2.2
Net income (loss) available to common stockholder, Diluted	$ (5.7)	$ 1.2	$ 25.7	$ 12.3
Denominator:
Weighted average basic shares outstanding	97.6	109.7	102.4	109.3
Basic (loss) earnings per share attributable to Atlantic Power Corporation	$ (0.06)	$ 0.01	$ 0.23	$ 0.09
Share-based compensation (in shares)		0.5	0.5	0.5
Convertible debentures (in shares)			27.4	28.2
Weighted Average Number of Potentially Diluted Shares	97.6	110.2	130.3	138.0
Diluted (loss) earnings per share attributable to Atlantic Power Corporation	$ (0.06)	$ 0.01	$ 0.20	$ 0.09