Equity (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2019 $ / shares	Jun. 30, 2020 USD ($)	Jun. 30, 2020 $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2019 $ / shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	$ 134.0		$ 187.8		$ 137.7		$ 192.4
Net (loss) income	(4.0)		2.9		19.7		5.3
Share-based compensation	0.4		0.2		0.8		0.8
Common share repurchases	(27.9)		(0.7)		(36.1)		(0.8)
Preferred share repurchases			(0.2)		(6.4)		(7.9)
Realized and unrealized gain on hedging activities, net of tax			(0.1)		(0.3)		(0.2)
Foreign currency translation adjustments	4.9		2.7		(6.3)		4.9
Balance at the end of the period	105.7		190.8		105.7		190.8
Series 1
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.8)		(0.9)		(1.6)		(1.8)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		$ 0.303125		$ 0.303125		$ 0.606250		$ 0.606250
Series 2
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.7)		(0.6)		(1.3)		(1.2)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		0.358688		0.348125		0.717376		0.696250
Series 3
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.2)		(0.3)		(0.5)		(0.7)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		$ 0.363146		$ 0.363697		$ 0.726488		$ 0.726590
Common Shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	1,252.1		1,261.4		1,259.9		1,260.9
Share-based compensation	0.4		0.2		0.8		0.8
Common share repurchases	(27.9)		(0.7)		(36.1)		(0.8)
Balance at the end of the period	1,224.6		1,260.9		1,224.6		1,260.9
Retained Deficit
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(1,134.7)		(1,112.7)		(1,164.2)		(1,121.6)
Net (loss) income	(5.7)		1.2		23.8		10.1
Balance at the end of the period	(1,140.4)		(1,111.5)		(1,140.4)		(1,111.5)
Accumulated Other Comprehensive (Loss) Income
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(152.2)		(144.1)		(140.7)		(146.2)
Realized and unrealized gain on hedging activities, net of tax			(0.1)		(0.3)		(0.2)
Foreign currency translation adjustments	4.9		2.7		(6.3)		4.9
Balance at the end of the period	(147.3)		(141.5)		(147.3)		(141.5)
Total Atlantic Power Corporation Shareholder's Equity
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(34.8)		4.6		(45.0)		(6.9)
Net (loss) income	(5.7)		1.2		23.8		10.1
Share-based compensation	0.4		0.2		0.8		0.8
Common share repurchases	(27.9)		(0.7)		(36.1)		(0.8)
Realized and unrealized gain on hedging activities, net of tax			(0.1)		(0.3)		(0.2)
Foreign currency translation adjustments	4.9		2.7		(6.3)		4.9
Balance at the end of the period	(63.1)		7.9		(63.1)		7.9
Preferred Shares of a Subsidiary Company
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	168.8		183.2		182.7		199.3
Net (loss) income	1.7		1.7		(4.1)		(4.8)
Preferred share repurchases			(0.2)		(6.4)		(7.9)
Balance at the end of the period	168.8		182.9		168.8		182.9
Preferred Shares of a Subsidiary Company | Series 1
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.8)		(0.9)		(1.6)		(1.8)
Preferred Shares of a Subsidiary Company | Series 2
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.7)		(0.6)		(1.3)		(1.2)
Preferred Shares of a Subsidiary Company | Series 3
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	$ (0.2)		$ (0.3)		$ (0.5)		$ (0.7)