Basic and diluted earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income (loss) attributable to Atlantic Power Corporation	$ 74.2	$ (42.6)
Add: convertible debenture interest expense	3.8
Net income (loss) available to common stockholder, Diluted	$ 78.0	$ (42.6)
Denominator:
Weighted average basic shares outstanding	95.8	109.3
Basic earnings per share attributable to Atlantic Power Corporation	$ 0.77	$ (0.39)
Share-based compensation (in shares)	1.7
Convertible debentures (in shares)	27.4
Weighted Average Number of Potentially Diluted Shares	124.9	109.3
Diluted earnings per share attributable to Atlantic Power Corporation	$ 0.62	$ (0.39)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS		$ 29.3
Share-based compensation.
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS		1.5
Convertible Debentures
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS		$ 27.8