CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Series 1 Preferred Shares of a Subsidiary Company	Series 1	Series 2 Preferred Shares of a Subsidiary Company	Series 2	Series 3 Preferred Shares of a Subsidiary Company	Series 3	Common Shares	Retained Deficit	Accumulated Other Comprehensive (loss) income	Total Atlantic Power Corporation Shareholder's Deficit	Preferred Shares of a Subsidiary Company	Total
Balance at the beginning of the period at Dec. 31, 2018							$ 1,260.9	$ (1,121.6)	$ (146.2)	$ (6.9)	$ 199.3	$ 192.4
Balance (in shares) at Dec. 31, 2018							108.3
Increase (decrease) in shareholders' equity
Net income (loss)								(42.6)		(42.6)	(1.2)	(43.8)
Share-based compensation							$ 1.5			1.5		1.5
Share-based compensation (in shares)							1.4
Common share repurchases							$ (2.5)			(2.5)		(2.5)
Common share repurchases (in shares)							(1.1)
Preferred share repurchases											(8.0)	(8.0)
Dividends on preferred shares of a subsidiary company	$ (3.5)	$ (3.5)	$ (2.4)	$ (2.4)	$ (1.5)	$ (1.5)
Foreign currency translation adjustments									5.8	5.8		5.8
Defined benefit plan, net of tax									(0.3)	(0.3)		(0.3)
Balance at the end of the period at Dec. 31, 2019							$ 1,259.9	(1,164.2)	(140.7)	(45.0)	182.7	137.7
Balance (in shares) at Dec. 31, 2019							108.6
Increase (decrease) in shareholders' equity
Net income (loss)								74.2		74.2	(0.7)	73.5
Share-based compensation							$ 1.4			1.4		1.4
Share-based compensation (in shares)							0.6
Common share repurchases							$ (41.6)			(41.6)		(41.6)
Common share repurchases (in shares)							(20.0)
Preferred share repurchases											(6.4)	(6.4)
Dividends on preferred shares of a subsidiary company	$ (3.3)	$ (3.3)	$ (2.6)	$ (2.6)	$ (0.9)	$ (0.9)
Foreign currency translation adjustments									2.2	2.2		2.2
Defined benefit plan, net of tax									(1.4)	(1.4)		(1.4)
Balance at the end of the period at Dec. 31, 2020							$ 1,219.7	$ (1,090.0)	$ (139.9)	$ (10.2)	$ 168.8	$ 158.6
Balance (in shares) at Dec. 31, 2020							89.2