v3.19.3.a.u2

Basic and diluted (loss) earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net (loss) income attributable to Atlantic Power Corporation	$ (65.3)	$ 12.6	$ 1.2	$ 8.9	$ 24.7	$ (3.2)	$ (0.6)	$ 15.9	$ (42.6)	$ 36.8	$ (98.6)
Add: convertible debenture interest expense										4.7
Net income (loss) available to common stockholder, Diluted									$ (42.6)	$ 41.5	$ (98.6)
Denominator:
Weighted average basic shares outstanding	109.3	109.4	109.7	108.9	109.6	111.1	112.4	114.8	109.3	112.0	115.1
Convertible debentures (in shares)										27.8
Share-based compensation (in shares)										2.0
Weighted average diluted shares outstanding	109.3	137.8	110.2	138.6	140.7	111.1	112.4	140.6	109.3	141.8	115.1
(Loss) income per share attributable to Atlantic Power Corporation	$ (0.60)	$ 0.12	$ 0.01	$ 0.08	$ 0.23	$ (0.03)	$ (0.01)	$ 0.14	$ (0.39)	$ 0.33	$ (0.86)
Diluted earnings (loss) per share attributable to Atlantic Power Corporation	$ (0.60)	$ 0.10	$ 0.01	$ 0.07	$ 0.18	$ (0.03)	$ (0.01)	$ 0.12	$ (0.39)	$ 0.29	$ (0.86)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS									$ 29.3		$ 9.7
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		1.6
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		$ 8.1

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