Equity (Details) CAD / shares in Units, CAD in Millions, $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended		7 Months Ended	12 Months Ended
	Jul. 31, 2017 CAD CAD / shares shares	Jul. 31, 2017 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 29, 2016 shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period		$ 269.3			$ 285.9	$ 435.2		$ 435.2
Net (loss) income					(20.3)	(29.3)
Realized and unrealized gain (loss) on hedging activities, net of tax					0.1	(0.3)
Foreign currency translation adjustments			$ 4.7	$ 1.0	6.7	19.4
Defined benefit plan, net of tax					0.1
Common share repurchases						(4.7)
Stock-based compensation					1.1	0.8
Dividends declared on preferred shares of a subsidiary company					(4.3)	(4.2)
Balance at the end of the period			269.3	416.9	269.3	416.9	$ 416.9	285.9
Stock Repurchase Program
Cost of shares repurchased and cancelled	CAD 2.7						$ 4.7	$ 19.5
Share repurchase plan, number of shares authorized | shares									11,300,000
Stock repurchase program authorized amount									10.00%
Number of shares repurchased and cancelled | shares	171,612	171,612					2,000,000	8,000,000
Dividend rate on preferred shares (as a percent)	4.85%	4.85%
Redemption price after specified date and thereafter (in Canadian dollars per share) | CAD / shares	CAD 15.50
Total Atlantic Power Corporation Shareholder's Equity
Increase (decrease) in shareholders' equity
Balance at the beginning of the period		$ 48.0			64.6	213.9		$ 213.9
Net (loss) income					(24.6)	(33.5)
Realized and unrealized gain (loss) on hedging activities, net of tax					0.1	(0.3)
Foreign currency translation adjustments					6.7	19.4
Defined benefit plan, net of tax					0.1
Common share repurchases						(4.7)
Stock-based compensation					1.1	0.8
Balance at the end of the period			48.0	195.6	48.0	195.6	$ 195.6	64.6
Preferred shares issued by a subsidiary company
Increase (decrease) in shareholders' equity
Balance at the beginning of the period		$ 221.3			221.3	221.3		221.3
Net (loss) income					4.3	4.2
Dividends declared on preferred shares of a subsidiary company					(4.3)	(4.2)
Balance at the end of the period			$ 221.3	$ 221.3	$ 221.3	$ 221.3	$ 221.3	$ 221.3