Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Increase (decrease) in shareholders' equity
Balance at the beginning of the period			$ 196.8	$ 285.9
Net income (loss)			15.1	(20.3)
Realized and unrealized gain on hedging activities, net of tax			0.4	0.1
Foreign currency translation adjustments	$ (2.9)	$ 4.7	(7.5)	6.7
Defined benefit plan, net of tax				(0.1)
Common share repurchases			(9.2)
Preferred share repurchases			(4.5)
Share-based compensation			1.1	1.1
Dividends declared on preferred shares of a subsidiary company			(4.3)	(4.3)
Balance at the end of the period	187.9	269.3	187.9	269.3
Total Atlantic Power Corporation Shareholder's Equity
Increase (decrease) in shareholders' equity
Balance at the beginning of the period			(18.4)	64.6
Net income (loss)			15.2	(24.6)
Realized and unrealized gain on hedging activities, net of tax			0.4	0.1
Foreign currency translation adjustments			(7.5)	6.7
Defined benefit plan, net of tax				(0.1)
Common share repurchases			(9.2)
Share-based compensation			1.1	1.1
Balance at the end of the period	(18.4)	48.0	(18.4)	48.0
Noncontrolling Interests
Increase (decrease) in shareholders' equity
Balance at the beginning of the period			215.2	221.3
Net income (loss)			(0.1)	4.3
Preferred share repurchases			(4.5)
Dividends declared on preferred shares of a subsidiary company			(4.3)	(4.3)
Balance at the end of the period	$ 206.3	$ 221.3	$ 206.3	$ 221.3