Equity (Details) CAD / shares in Units, shares in Millions, CAD in Millions, $ in Millions	3 Months Ended		9 Months Ended			10 Months Ended			12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 CAD CAD / shares shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Dec. 29, 2016 shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period				$ 285.9	$ 435.2				$ 303.8	$ 435.2
Net (loss) income				(54.0)	(109.8)
Realized and unrealized gain (loss) on hedging activities, net of tax				0.2	(0.1)
Foreign currency translation adjustments	$ 9.2	$ (22.0)		15.9	(2.6)
Defined benefit plan, net of tax				0.1
Common share repurchases				(0.2)	(13.9)
Preferred share repurchase				(3.1)
Stock-based compensation				1.6	1.4
Dividends declared on preferred shares of a subsidiary company				(6.5)	(6.4)
Balance at the end of the period	239.9	303.8	$ 239.9	239.9	303.8		$ 239.9	$ 303.8	$ 239.9	285.9
Stock Repurchase Program
Cost of shares repurchased and cancelled			$ 0.2			CAD 3.9		$ 13.9		$ 19.5
Share repurchase plan, number of shares authorized | shares											11.3
Stock repurchase program authorized amount											10.00%
Number of shares repurchased and cancelled | shares			0.1			0.3	0.3	5.7		8.0
Number of shares repurchased and cancelled (in dollar per share) | CAD / shares						CAD 25.0
Dividend rate on preferred shares (as a percent)									4.85%
Redemption price after specified date and thereafter (in Canadian dollars per share) | CAD / shares						CAD 15.50
Gain on redemption of preferred shares			$ 3.0
Total Atlantic Power Corporation Shareholder's Equity
Increase (decrease) in shareholders' equity
Balance at the beginning of the period				64.6	213.9				$ 82.5	$ 213.9
Net (loss) income				(57.5)	(116.2)
Realized and unrealized gain (loss) on hedging activities, net of tax				0.2	(0.1)
Foreign currency translation adjustments				15.9	(2.6)
Defined benefit plan, net of tax				0.1
Common share repurchases				(0.2)	(13.9)
Stock-based compensation				1.6	1.4
Balance at the end of the period	24.7	82.5	24.7	24.7	82.5		$ 24.7	$ 82.5	24.7	64.6
Noncontrolling Interests
Increase (decrease) in shareholders' equity
Balance at the beginning of the period				221.3	221.3				221.3	221.3
Net (loss) income				3.5	6.4
Preferred share repurchase				(3.1)
Dividends declared on preferred shares of a subsidiary company				(6.5)	(6.4)
Balance at the end of the period	$ 215.2	$ 221.3	$ 215.2	$ 215.2	$ 221.3		$ 215.2	$ 221.3	$ 215.2	$ 221.3