Atlantic Power 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 06, 2021
Cover [Abstract]
Entity Registrant Name	ATLANTIC POWER CORPORATION
Entity Central Index Key	0001419242
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-34691
Entity Incorporation, State or Country Code	A1
Entity Tax Identification Number	55-0886410
Entity Address, Address Line One	3 Allied Drive, Suite 155
Entity Address, City or Town	Dedham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02026
City Area Code	617
Local Phone Number	977-2400
Title of 12(b) Security	Common Shares
Trading Symbol	AT
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		89,797,798
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 21.6	$ 38.8
Restricted cash	6.9	7.1
Accounts receivable	33.9	31.3
Current portion of derivative instruments asset (Notes 7 and 8)		0.4
Inventory	17	18.3
Prepayments	6.7	7
Income taxes receivable (Note 9)	2	3.2
Current assets held for sale (Note 2)	4.6
Other current assets	0.3	0.3
Total current assets	93	106.4
Property, plant, and equipment, net	481.3	491.8
Equity investments in unconsolidated affiliates (Note 5)	91.8	85
Power purchase agreements and intangible assets, net	114.8	120.3
Goodwill	21.3	21.3
Operating lease right-of-use assets (Note 15)	4.1	4.6
Deferred income taxes (Note 9)	17	17.2
Other assets	0.6	0.6
Total assets	823.9	847.2
Current liabilities:
Accounts payable	5.6	6.3
Accrued interest	5.9	2.5
Other accrued liabilities	14.7	19.3
Current portion of long-term debt (Note 6)	90.8	95.7
Current portion of derivative instruments liability (Notes 7 and 8)	9.9	11
Operating lease liabilities (Note 15)	1.8	1.9
Current liabilities held for sale (Note 2)	3.2
Other current liabilities	0.4	0.2
Total current liabilities	132.3	136.9
Long-term debt, net of unamortized discount and deferred financing costs (Note 6)	368.3	384.1
Convertible debentures, net of discount and unamortized deferred financing costs	85.5	84.1
Derivative instruments liability (Notes 7 and 8)	5.4	8.1
Power purchase agreements and intangible liabilities, net	17.6	18
Asset retirement obligations, net	45	48.1
Operating lease liabilities (Note 15)	2.7	3.1
Other long-term liabilities	6.7	6.2
Total liabilities	663.5	688.6
Equity
Common shares, no par value, unlimited authorized shares; 89,797,798 and 89,222,568 issued and outstanding at March 31, 2021 and December 31, 2020 (Note 12)	1,219.8	1,219.7
Accumulated other comprehensive loss (Note 4)	(138.1)	(139.9)
Retained deficit	(1,090.1)	(1,090)
Total Atlantic Power Corporation shareholders' deficit	(8.4)	(10.2)
Preferred shares issued by a subsidiary company (Note 12)	168.8	168.8
Total equity	160.4	158.6
Total liabilities and equity	$ 823.9	$ 847.2

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
CONDENSED CONSOLIDATED BALANCE SHEETS
Common shares, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized, unlimited	Unlimited	Unlimited
Common shares, issued shares (in shares)	89,797,798	89,222,568
Common shares, outstanding shares (in shares)	89,797,798	89,222,568

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Project revenue:
Project revenues	$ 72	$ 72.8
Project expenses:
Fuel	20.7	19.6
Operations and maintenance	21.1	20.7
Depreciation and amortization	14.3	15.6
Total project expenses	56.1	55.9
Project other income (loss):
Change in fair value of derivative instruments (Notes 7 and 8)	3.2	(5.6)
Equity in earnings of unconsolidated affiliates (Note 5)	13.4	13.7
Interest, net	(0.3)	(0.3)
Other income, net	0.2	0
Total project other income (loss)	16.5	7.8
Project income	32.4	24.7
Administrative and other expenses:
Administration	14.1	6.7
Interest expense, net	11.4	10.8
Foreign exchange loss (gain)	3.2	(20.6)
Other (income) expense, net (Note 7)	(0.1)	2.6
Total administrative and other expenses	28.6	(0.5)
Income from operations before income taxes	3.8	25.2
Income tax expense (Note 9)	2.2	1.5
Net income	1.6	23.7
Net income (loss) attributable to preferred shares of a subsidiary company (Note 12)	1.7	(5.8)
Net (loss) income attributable to Atlantic Power Corporation	$ (0.1)	$ 29.5
Net (loss) earnings per share attributable to Atlantic Power Corporation shareholders: (Note 11)
Basic (in dollars per share)	$ 0	$ 0.28
Diluted (in dollars per share)	$ 0	$ 0.23
Weighted average number of common shares outstanding: (Note 11)
Basic (in shares)	89.4	107.2
Diluted (in shares)	89.4	134.8
Energy sales
Project revenue:
Project revenues	$ 36	$ 40.7
Energy capacity revenue
Project revenue:
Project revenues	29.8	28
Other
Project revenue:
Project revenues	$ 6.2	$ 4.1

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 1.6	$ 23.7
Other comprehensive income, net of tax:
Unrealized gain (loss) on hedging activities	0.1	(0.4)
Net amount reclassified to earnings	0	0.1
Net realized and unrealized gain (loss) on derivatives	0.1	(0.3)
Foreign currency translation adjustments	1.7	(11.2)
Other comprehensive income (loss), net of tax	1.8	(11.5)
Comprehensive income	3.4	12.2
Less: Comprehensive income (loss) attributable to preferred shares of a subsidiary company	1.7	(5.8)
Comprehensive income attributable to Atlantic Power Corporation	$ 1.7	$ 18

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash provided by operating activities:
Net income	$ 1.6	$ 23.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14.3	15.6
Share-based compensation	0.1	0.4
Other gain	(0.2)
Equity in earnings from unconsolidated affiliates	(13.4)	(13.7)
Distributions from unconsolidated affiliates	6.9	6
Unrealized foreign exchange loss (gain)	3.1	(20.9)
Change in fair value of derivative instruments	(3.3)	8.2
Amortization of debt discount and deferred financing costs	1.3	2
Non-cash operating lease expense	0.5	0.5
Deferred income taxes	0.3	0.3
Change in other operating balances:
Accounts receivable	(3.1)	(2.1)
Inventory	0.2	2.8
Prepayments and other assets	1.6	(1.7)
Accounts payable	(0.9)	(5.7)
Accruals and other liabilities	(0.6)	(7)
Cash provided by operating activities	8.4	8.4
Cash provided by (used in) investing activities:
Insurance proceeds		7.4
Proceeds from sale of equity investment	0.5
Purchase of property, plant and equipment	(0.3)	(10)
Cash provided by (used in) investing activities	0.2	(2.6)
Cash used in financing activities:
Common share repurchases		(8.2)
Preferred share repurchases		(6.4)
Repayment of corporate and project-level debt	(23.7)	(21.6)
Cash payments for vested LTIP withheld for taxes	(0.6)	(0.7)
Deferred financing costs		(1.5)
Dividends paid to preferred shareholders	(1.7)	(1.7)
Cash used in financing activities	(26)	(40.1)
Net decrease in cash, restricted cash and cash equivalents	(17.4)	(34.3)
Cash, restricted cash and cash equivalents at beginning of period	45.9	82.6
Cash, restricted cash and cash equivalents at end of period	28.5	48.3
Supplemental cash flow information
Interest paid	6.8	8.3
Income taxes paid, net	$ 0.7	0.7
Accruals for construction in progress		$ 0.3

Nature of business

Nature of business	3 Months Ended
	Mar. 31, 2021
Nature of business
Nature of business	1. Nature of business ​ General ​ Atlantic Power is an independent power producer that owns power generation assets in eleven states in the United States and two provinces in Canada. Our power generation projects, which are diversified by geography, fuel type, dispatch profile and offtaker, sell electricity to utilities and other large customers predominantly under long-term power purchase agreements (“PPAs”), which seek to minimize exposure to changes in commodity prices. As of March 31, 2021, our portfolio consisted of twenty-one operating projects with an aggregate electric generating capacity of approximately 1,723 megawatts (“MW”) on a gross ownership basis and approximately 1,327 MW on a net ownership basis. Sixteen of the projects are majority-owned by the Company. ​ Atlantic Power is a corporation established under the laws of the Province of Ontario on June 18, 2004 and continued to the Province of British Columbia on July 8, 2005. Our shares trade on the Toronto Stock Exchange (“TSX”) under the symbol “ATP” and on the New York Stock Exchange (“NYSE”) under the symbol “AT.” Our registered office is located at 1066 West Hastings Street, Suite 2600, Vancouver, British Columbia V6E 3X1, Canada and our headquarters is located at 3 Allied Drive, Suite 155, Dedham, Massachusetts 02026, USA. Our telephone number in Dedham is (617) 977-2400 and the address of our website is www.atlanticpower.com. Information contained on Atlantic Power’s website or that can be accessed through its website is not incorporated into and does not constitute a part of this Quarterly Report on Form 10-Q. We have included our website address only as an inactive textual reference and do not intend it to be an active link to our website. ​ Basis of presentation ​ The interim condensed consolidated financial statements included in this Quarterly Report on Form 10-Q have been prepared in accordance with the SEC regulations for interim financial information and with the instructions to Form 10-Q. The following notes should be read in conjunction with the accounting policies and other disclosures as set forth in the notes to our financial statements in our Annual Report on Form 10-K for the year ended December 31, 2020. Interim results are not necessarily indicative of results for the full year. ​ In our opinion, the accompanying unaudited interim condensed consolidated financial statements present fairly our consolidated financial position as of March 31, 2021, the results of operations and comprehensive income for the three months ended March 31, 2021 and 2020, and our cash flows for the three months ended March 31, 2021 and 2020, in accordance with U.S. generally accepted accounting principles (“GAAP”). In the opinion of management, all adjustments (consisting of normal recurring accruals and other adjustments) considered necessary for a fair presentation have been included. ​ Use of estimates ​ The preparation of financial statements requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the year. Actual results could differ from those estimates. During the periods presented, we have made a number of estimates and valuation assumptions, including the fair value of assets acquired and liabilities assumed in purchase accounting, the useful lives and recoverability of property, plant and equipment, valuation of goodwill, intangible assets and liabilities related to PPAs and fuel supply agreements, the recoverability of equity method investments, the recoverability of deferred tax assets, tax provisions, recovery of expected insurance proceeds, the fair value of financial instruments and derivatives, pension obligations, asset retirement obligations and equity-based compensation. In addition, estimates are used to test long-lived assets and goodwill for impairment and to determine the fair value of impaired assets. These estimates and valuation assumptions are based on present conditions and our planned course of action, as well as assumptions about future business and economic conditions. See “Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations—Critical Accounting Policies and Estimates ​ COVID-19 Pandemic ​ There are many uncertainties regarding the ongoing coronavirus (“COVID-19”) pandemic, and we are closely monitoring the impact of COVID-19 on all aspects of our business, including how it will impact our customers, employees, suppliers, vendors and business partners. We have taken extra precautions for our employees who continue to work at our facilities and have implemented work-from-home policies where appropriate. Currently, we do not anticipate any employee layoffs and are continuing to maintain the high level of reliability and availability of our plants. We continue to implement strong physical and cybersecurity measures to ensure that our systems remain functional in order to serve our operational needs with a remote workforce and to keep our operations running to ensure uninterrupted service to our offtakers. ​ Proposed Transaction with I Squared Capital ​ On January 14, 2021, Atlantic Power announced that it had entered into a definitive agreement (as amended on April 1, 2021 and April 29, 2021, and as may be further amended from time to time, the “Arrangement Agreement”) , under which the Company's outstanding common shares and 6.00% Series E convertible unsecured subordinated debentures due January 31, 2025 (“Convertible Debentures”), and the outstanding preferred shares and medium term notes of certain of its subsidiaries, would be acquired (the “Transaction”). The total enterprise value of the deal is approximately $961 million and the Transaction was unanimously approved by Atlantic Power's board of directors. The parties are currently targeting May 14, 2021 as the closing date for the Transaction. ​ In connection with the Transaction, and subject to and conditional to the closing of the Transaction: ​ ● Holders of common shares of Atlantic Power will receive $3.03 per common share in cash. ● Atlantic Power Preferred Equity Ltd.’s (“APPEL’s”) cumulative redeemable preferred shares, Series 1, cumulative rate reset preferred shares, Series 2, and cumulative floating rate preferred shares, Series 3, will be purchased by APPEL for Cdn $22.00 per preferred share in cash. ● Atlantic Power Limited Partnership’s (“APLP’s”) 5.95% medium term notes due June 23, 2036 (the “MTNs”) will be redeemed for consideration equal to 106.071% of the principal amount of MTNs held as of the closing of the Transaction, plus accrued and unpaid interest on the MTNs up to, but excluding, the closing date of the Transaction. Holders of MTNs that have delivered a valid consent to the proposed amendments to the trust indenture governing the MTNs (as described below) prior to 5:00 p.m. (Toronto time) on March 16, 2021 will also be entitled to a consent fee equal to 0.25% of the principal amount of MTNs held by such holders in respect of which a consent was delivered, conditional on closing of the Transaction. ● Holders of Atlantic Power’s Convertible Debentures who deliver a conversion notice prior to 4:00 p.m. (Toronto time) on May 11, 2021 (the “Conversion Deadline”) will receive $3.03 per common share issuable on conversion of the Convertible Debentures in respect of which such conversion notice has been delivered (including common shares issuable on account of the Make Whole Premium (as defined in the Arrangement Agreement), plus accrued and unpaid interest up to, but excluding, the date of conversion, and all other Convertible Debentures will be defeased in accordance with the terms of the trust indenture governing the Convertible Debentures (the “Debenture Indenture”) (as described below) ​ The Arrangement Agreement was amended on April 1, 2021 to provide for certain administrative and housekeeping amendments to the Arrangement Agreement and to, among other things, clarify the mechanics surrounding the payment of consideration payable pursuant to the Transaction. The amendments also added a step to the Arrangement to, among other things, provide for the creation of a class of non-voting preferred shares in connection with the Pre-Acquisition Reorganization (as defined in the Arrangement Agreement). The creation and issuance of any such shares will not occur until immediately prior to the effective time of the Arrangement and will only occur if the Purchasers have irrevocably waived or confirmed in writing the satisfaction of all conditions to closing in their favor and shall have confirmed in writing that they are prepared, and able, to promptly and without condition proceed to effect the Arrangement (as defined in the Arrangement Agreement). ​ The Arrangement Agreement was further amended on April 29, 2021 to provide for mutual covenants in connection with the proposed defeasance of the Convertible Debentures (the “Defeasance”), clarify the mechanics surrounding the payment of the amounts required to effect the Defeasance, and effect a waiver of certain conditions precedent in the Arrangement Agreement relating to the approval of the Transaction by holders of the Convertible Debentures. Notwithstanding the Defeasance, any holder of Convertible Debentures who converts their Convertible Debentures during the period beginning 10 trading days (as defined in the Debenture Indenture) prior to the closing of the Transaction (“Closing”) and ending 30 calendar days following the delivery of the change of control notice under the Indenture (the “Make Whole Conversion Period”) will be entitled to receive the Make Whole Premium. Assuming that the Closing occurs on May 14, 2021 and the change of control notice is delivered on Closing as is currently anticipated, the Make Whole Conversion Period opened on April 30, 2021 and will close on June 14, 2021. ​ In connection with the Defeasance, and if Closing occurs (i) Atlantic Power expects to de-list its Convertible Debentures from the TSX and its common shares from the TSX and the NYSE; (ii) Atlantic Power intends to apply to Canadian securities regulators to cease being a reporting issuer, or to be exempt from its reporting obligations as a Canadian reporting issuer, and also intends to file to deregister under the U.S. Securities Exchange Act of 1934; (iii) the Convertible Debentures will no longer be convertible into common shares and, if converted after the Conversion Deadline, holders will be entitled to receive a cash amount equal to Cdn $3.72 in lieu of each Common Share previously issuable on a conversion (including any common shares otherwise issuable on account of the Make Whole Premium if converted within the Make Whole Conversion Period), plus accrued and unpaid interest paid in Canadian dollars up to, but excluding, the date of conversion; and (iv) except as otherwise provided in the Debenture Indenture, any Convertible Debentures which remain outstanding following the expiry of the Make Whole Conversion Period will continue to receive interest at a rate of 6.00% per annum, payable semi-annually in arrears until, and the repayment of principal upon, the redemption of the Convertible Debentures. Following the Defeasance, the Convertible Debentures will be redeemed at par on January 31, 2023. ​ The Transaction has received approval from the holders of common shares of the Company and the holders of preferred shares and medium term notes of certain of the Company's subsidiaries. The Transaction also has received certain required regulatory approvals, including an advance ruling certificate from the Canadian Commissioner of Competition under the Competition Act (Canada) on February 5, 2021, the expiration of the required waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 on March 9, 2021, the approval of the Federal Energy Regulatory Commission on April 2, 2021 and the approval in April 2021 from the Federal Communications Commission (“FCC”) for the transfer of control of certain FCC licenses held by the Company or its subsidiaries. ​ The Transaction remains subject to the satisfaction or waiver of certain remaining third-party consents and other customary closing conditions. ​ Recently adopted and issued accounting standards ​ Accounting standards adopted in 2021 ​ In December 2019, the FASB issued amendments to the guidance for income taxes through ASU 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes.” The amendments in this update simplify the accounting for income taxes by removing certain exceptions such as: 1) the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items, 2) the requirement to recognize a deferred tax liability for equity method investments when a foreign subsidiary becomes an equity method investment, 3) the ability not to recognize a deferred tax liability for a foreign subsidiary when a foreign equity method investment becomes a subsidiary, and 4) the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. Adoption of this guidance did not have a material impact on the condensed consolidated financial statements. ​ Accounting standards not yet adopted ​ In March 2020, the FASB issued amendments to the guidance for reference rate reform through ASU 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the effects of reference rate reform on financial reporting.” The amendments in this update provide optional expedients and exceptions for applying GAAP to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments apply only to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the amendments do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The amendments are elective and are effective upon issuance for all entities. We ​

Assets and liabilities held for

Assets and liabilities held for sale	3 Months Ended
	Mar. 31, 2021
Assets and liabilities held for sale.
Assets and liabilities held for sale	2. Assets and liabilities held for sale ​ North Bay and Kapuskasing projects ​ ​ We determined the North Bay Property and asset groups met the criteria for held for sale accounting, as specified by Accounting Standards Codification (“ASC”) 360, “Property, Plant, and Equipment,” as of March 31, 2021. In accordance with accounting guidance for Property, Plant and Equipment, assets held for sale are measured at the lower of the carrying value or fair value less costs to sell. We determined the estimated sales value of the disposal group, less costs to sell, exceeded the carrying value as of March 31, 2021. As a result, no long-lived asset impairment charge was recorded during the three months ended March 31, 2021. ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ ​ ​ 2021 ​ Assets (unaudited) Inventory ​ $ 1.1 ​ Property, plant, and equipment, net ​ 3.5 ​ Total current assets held for sale ​ $ 4.6 ​ Liabilities ​ ​ ​ ​ Asset retirement obligations ​ $ 3.2 ​ Total current liabilities held for sale ​ $ 3.2 ​ ​

Revenue from contracts

Revenue from contracts	3 Months Ended
	Mar. 31, 2021
Revenue from contracts
Revenue from contracts	3. Revenue from contracts ​ Revenue, receivables and contract liabilities by segment consists of following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, 2021 ​ ​ ​ ​ ​ ​ ​ Consolidated ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Total Project revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Energy sales ​ $ 18.2 ​ $ 5.6 ​ $ 12.2 $ ​ — ​ $ 36.0 Energy capacity revenue ​ 8.1 ​ ​ 21.7 ​ ​ — ​ ​ — ​ 29.8 Steam energy and capacity revenue ​ ​ — ​ ​ 3.2 ​ ​ — ​ ​ — ​ ​ 3.2 Waste heat revenue ​ ​ 0.6 ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.6 Ancillary and transmission services ​ ​ — ​ ​ 1.0 ​ ​ 1.0 ​ ​ — ​ ​ 2.0 Asset management and operation ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.3 ​ ​ 0.3 Miscellaneous revenue ​ ​ ​ ​ ​ 0.2 ​ ​ (0.1) ​ ​ — ​ ​ 0.1 ​ ​ 26.9 ​ 31.7 ​ 13.1 ​ 0.3 ​ 72.0 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, 2020 ​ ​ ​ ​ ​ ​ ​ Consolidated ​ ​ ​ Solid Fuel ​ ​ Natural Gas ​ ​ Hydroelectric ​ ​ Corporate ​ ​ Total Project revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Energy sales ​ $ 16.8 ​ $ 6.2 ​ $ 17.7 ​ $ — ​ $ 40.7 Energy capacity revenue ​ 6.3 ​ 21.7 ​ — ​ — ​ 28.0 Steam energy and capacity revenue ​ ​ — ​ ​ 2.7 ​ ​ — ​ ​ — ​ ​ 2.7 Waste heat revenue ​ ​ 0.3 ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.3 Ancillary and transmission services ​ ​ — ​ ​ 0.7 ​ ​ 0.7 ​ ​ — ​ ​ 1.4 Asset management and operation ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.2 ​ ​ 0.2 Miscellaneous revenue ​ ​ — ​ ​ (0.5) ​ ​ — ​ ​ — ​ ​ (0.5) ​ ​ 23.4 ​ 30.8 ​ 18.4 ​ 0.2 ​ 72.8 ​ Contract balances ​ Contract liabilities as of March 31, 2021 include a $0.2 million fuel reserve fund at Dorchester (Solid Fuel segment), a $0.1 million steam sale credit at the San Diego plants (Natural Gas segment), and $0.3 million water license fee at Mamquam (Hydroelectric segment). Contract liabilities as of December 31, 2020 include a $0.2 million fuel reserve fund at Dorchester and a $0.1 million steam sale credit at the San Diego plants. We had no contract assets at March 31, 2021.

Changes in accumulated other co

Changes in accumulated other comprehensive loss by component	3 Months Ended
	Mar. 31, 2021
Changes in accumulated other comprehensive loss by component
Changes in accumulated other comprehensive loss by component	4. Changes in accumulated other comprehensive loss by component ​ The changes in accumulated other comprehensive (loss) income by component were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ 2021 2020 Foreign currency translation ​ ​ ​ Balance at beginning of period ​ $ (138.4) ​ $ (140.6) Other comprehensive income: ​ ​ ​ ​ ​ ​ Foreign currency translation adjustments (1) ​ 1.7 ​ (11.2) Balance at end of period ​ $ (136.7) ​ $ (151.8) Pension ​ ​ ​ ​ ​ ​ Balance at beginning and end of period (2) ​ $ (3.1) ​ $ (1.7) Cash flow hedges ​ ​ ​ ​ ​ ​ Balance at beginning of period ​ $ 1.6 ​ $ 1.6 Other comprehensive income (loss): ​ ​ ​ ​ ​ ​ Net change from periodic revaluations ​ 0.2 ​ (0.6) Tax (expense) benefit ​ (0.1) ​ 0.2 Total other comprehensive income (loss) before reclassifications, net of tax ​ 0.1 ​ (0.4) Net amount reclassified to earnings: ​ ​ ​ ​ ​ ​ Interest rate swaps (3) ​ ​ — ​ ​ 0.1 Tax expense ​ ​ — ​ ​ — Total amount reclassified from accumulated other comprehensive income, net of tax ​ — ​ 0.1 Total other comprehensive income (loss) ​ 0.1 ​ (0.3) Balance at end of period ​ $ 1.7 ​ $ 1.3 (1) In all periods presented, there were no tax impacts related to rate changes and no amounts were reclassified to earnings. (2) Quarterly activity was immaterial. (3) This amount was included in interest expense, net on the accompanying condensed consolidated statements of operations .

Equity method investments in un

Equity method investments in unconsolidated affiliates	3 Months Ended
	Mar. 31, 2021
Equity method investments in unconsolidated affiliates
Equity method investments in unconsolidated affiliates	5. Equity method investments in unconsolidated affiliates ​ The following summarizes the operating results for the three months ended March 31, 2021 and 2020, respectively, for our proportional ownership interest in equity method investments: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, Operating results 2021 2020 Revenue ​ ​ ​ ​ ​ ​ Frederickson ​ $ 7.3 ​ $ 8.0 Orlando Cogen, LP ​ 15.5 ​ 14.6 Chambers Cogen, LP ​ ​ 10.8 ​ ​ 10.6 Craven County Wood Energy, LP ​ ​ 3.1 ​ ​ 3.1 Grayling Generating Station, LP ​ ​ 1.6 ​ ​ 1.5 ​ ​ 38.3 ​ 37.8 Project expenses ​ ​ ​ ​ ​ ​ Frederickson ​ 4.8 ​ ​ 5.4 Orlando Cogen, LP ​ ​ 7.2 ​ ​ 6.3 Chambers Cogen, LP ​ 8.2 ​ ​ 8.1 Craven County Wood Energy, LP ​ ​ 3.0 ​ ​ 2.6 Grayling Generating Station, LP ​ ​ 1.2 ​ ​ 1.4 ​ ​ 24.4 ​ 23.8 Project other income ​ ​ ​ ​ ​ ​ Frederickson ​ — ​ — Orlando Cogen, LP ​ ​ — ​ ​ — Chambers Cogen, LP ​ (0.2) ​ ​ (0.3) Craven County Wood Energy, LP ​ ​ — ​ ​ — Grayling Generating Station, LP ​ ​ — ​ ​ — ​ ​ (0.2) ​ (0.3) Net income ​ ​ ​ ​ ​ ​ Frederickson ​ ​ 2.5 ​ ​ 2.6 Orlando Cogen, LP ​ 8.3 ​ 8.3 Chambers Cogen, LP ​ ​ 2.4 ​ ​ 2.2 Craven County Wood Energy, LP ​ ​ 0.1 ​ ​ 0.5 Grayling Generating Station, LP ​ ​ 0.4 ​ ​ 0.1 Equity in earnings of unconsolidated affiliates ​ $ 13.7 ​ $ 13.7 ​ ​ ​ ​ ​ ​ ​ Distributions from equity method investments ​ (6.9) ​ (6.0) Earnings of equity method investments, net of distributions ​ $ 6.8 ​ $ 7.7 ​ ​

Long-term debt

Long-term debt	3 Months Ended
	Mar. 31, 2021
Long-term debt
Long-term debt	6. Long-term debt ​ Long-term debt consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ ​ ​ 2021 ​ 2020 ​ Interest Rate Recourse Debt: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Senior secured term loan facility, due 2025 (1) ​ $ 284.5 ​ $ 307.5 ​ LIBOR (2) plus 2.50 % Senior unsecured notes, due June 2036 (Cdn$210.0) ​ 167.0 ​ 164.9 ​ ​ 5.95 % Non-Recourse Debt: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cadillac term loan, due 2025 (3) ​ 14.1 ​ 14.8 ​ LIBOR plus 1.61 % Less: unamortized discount ​ ​ (3.0) ​ ​ (3.5) ​ ​ ​ ​ ​ Less: unamortized deferred financing costs ​ ​ (3.5) ​ ​ (3.9) ​ ​ ​ ​ ​ Less: current maturities ​ (90.8) ​ (95.7) ​ ​ ​ ​ ​ Total long-term debt ​ $ 368.3 ​ $ 384.1 ​ ​ ​ ​ ​ ​ Current maturities consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ ​ ​ ​ 2021 ​ 2020 ​ Interest Rate Current Maturities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Senior secured term loan facility, due 2025 (1) ​ $ 88.0 ​ $ 93.0 ​ LIBOR (2) plus 2.50 % Cadillac term loan, due 2025 (3) ​ 2.8 ​ 2.7 LIBOR plus 1.61 % Total current maturities ​ $ 90.8 ​ $ 95.7 ​ ​ ​ ​ ​ (1) On a quarterly basis, we make a cash sweep payment to fund the principal balance, based on terms as defined in the term loan credit agreement. The portion of the senior secured term loan facility classified as current is based on principal payments required to reduce the aggregate principal amount of senior secured term loan outstanding to achieve a target principal amount that declines quarterly based on a pre-determined specified schedule. (2) London Interbank Offered Rate (“LIBOR”) cannot be less than 1.00%. We have entered into interest rate swap agreements to mitigate the exposure to changes in LIBOR of the $284.5 million outstanding aggregate borrowings under our senior secured term loan facility at March 31, 2021. See Note 8, Accounting for derivative instruments and hedging activities for further details. (3) We have entered into interest rate swap agreements to economically fix our exposure to changes in interest rates for this non-recourse debt. See Note 8, Accounting for derivative instruments and hedging activities , for further details. ​ ​ Term Loan Amendment and Repricing ​ ​ Extension of Revolving Credit Facility ​ On March 18, 2020, we executed an amendment to our Revolver. The amendment provides for an extension of the Revolver maturity date to April 2025, to coincide with the maturity date of the senior Term Loan. Both the Revolver and the Term Loan are at our APLP Holdings subsidiary. In conjunction with the extension, the Revolver capacity was reduced to $180 million from $200 million previously. The amendment allows an upsizing of the Revolver capacity by up to $30 million, to a maximum aggregate amount of $210 million, subject to approval of the two letter of credit issuer banks and increased commitments by existing or new lenders. Such an upsizing would not require a further amendment. There were no other significant changes to the terms of the Revolver. At March 31, 2021, we had no borrowings under the Revolver and utilized $81.2 million of borrowing capacity for letters of credit. Renewal of Shelf Registration Statement ​ On August 24, 2020, we filed a shelf registration statement on Form S-3, which was declared effective by the SEC on August 25, 2020 (the “Shelf Registration Statement”), and is available for use for three years in the United States. The Shelf Registration Statement allows the Company to sell from time to time up to $250 million of common shares, debt securities, warrants, subscription receipts or units comprised of any combination of these securities, for its own account in one or more offerings. We also filed a base short-form prospectus dated August 24, 2020 qualifying the distribution of such securities concurrently with Canadian securities regulators. ​

Fair value of financial instrum

Fair value of financial instruments	3 Months Ended
	Mar. 31, 2021
Fair value of financial instruments
Fair value of financial instruments	7. Fair value of financial instruments ​ The following represents the recurring measurements of fair value hierarchy of our financial assets and liabilities that were recognized at fair value as of March 31, 2021 and December 31, 2020. Financial assets and liabilities are classified based on the lowest level of input that is significant to the fair value measurement. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, 2021 ​ ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Assets: ​ ​ ​ ​ ​ Cash and cash equivalents ​ $ 21.6 ​ $ — ​ $ — ​ $ 21.6 ​ Restricted cash ​ 6.9 ​ — ​ — ​ 6.9 ​ Derivative instruments asset ​ — ​ — ​ — ​ — ​ Total ​ $ 28.5 ​ $ — ​ $ — ​ $ 28.5 ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative instruments liability ​ $ — ​ $ 13.9 ​ $ 1.4 ​ $ 15.3 ​ Total ​ $ — ​ $ 13.9 ​ $ 1.4 ​ $ 15.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Assets: ​ ​ ​ ​ ​ Cash and cash equivalents ​ $ 38.8 ​ $ — ​ $ — ​ $ 38.8 ​ Restricted cash ​ 7.1 ​ — ​ — ​ 7.1 ​ Derivative instruments asset ​ — ​ 0.4 ​ — ​ 0.4 ​ Total ​ $ 45.9 ​ $ 0.4 ​ $ — ​ $ 46.3 ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative instruments liability ​ $ — ​ $ 17.6 ​ $ 1.5 ​ $ 19.1 ​ Total ​ $ — ​ $ 17.6 ​ $ 1.5 ​ $ 19.1 ​ ​ The fair values of our interest rate swaps, foreign exchange forward contracts, natural gas swaps and gas purchase agreements are based upon trades in liquid markets. Valuation model inputs can generally be verified and valuation techniques do not involve significant judgment. The fair values of such financial instruments are classified within Level 2 of the fair value hierarchy. We use our best estimates to determine the fair value of commodity and derivative contracts we hold. These estimates consider various factors including closing exchange prices, time value, volatility factors and credit exposure. The fair value of each contract is discounted using a risk-free interest rate. ​ We also adjust the fair value of financial assets and liabilities to reflect credit risk, which is calculated based on our credit rating and the credit rating of our counterparties. As of March 31, 2021, the credit valuation adjustments resulted in a $0.3 million net increase in fair value, which consists of a $0.1 million pre-tax gain in other comprehensive income and a $0.2 million gain in change in fair value of derivative instruments. As of December 31, 2020, the credit valuation adjustments resulted in a $0.5 million net increase in fair value, which consists of a $0.1 million pre-tax gain in other comprehensive income and a $0.4 million gain in change in fair value of derivative instruments. ​ The conversion option derivative for the Series E Debentures is classified within Level 3 of the fair value hierarchy. The significant unobservable inputs used in developing fair value include the volatility of our common shares and the fair value of the host contract, which is derived from recent similar convertible debenture offerings from peer companies. A discounted cash flow valuation technique is utilized to calculate the fair value of the conversion option derivative. The following table reconciles, for the three months ended March 31, 2021 and 2020, the beginning and ending balances for the conversion option derivative that is recognized at fair value in the condensed consolidated financial statements, using significant unobservable inputs: ​ ​ ​ ​ ​ ​ Fair value ​ ​ Three Months Ended ​ ​ March 31, 2021 ​ ​ ​ ​ Beginning balance of liability at January 1, 2021 ​ $ 1.5 Total unrealized gain ​ (0.1) Currency translation (gain) / loss ​ — Ending balance of liability at March 31, 2021 ​ $ 1.4 ​ ​ ​ ​ ​ ​ Fair value ​ ​ Three Months Ended ​ ​ March 31, 2020 ​ ​ ​ ​ Beginning balance of liability at January 1, 2020 ​ $ 3.2 Total unrealized loss ​ ​ 2.6 Currency translation gain ​ (0.2) Ending balance of liability at March 31, 2020 ​ $ 5.6 ​ For cash and cash equivalents, accounts and other receivables, accounts payable and restricted cash, the carrying amount approximates fair value because of the short-term maturity of those instruments and is classified as Level 1 within the fair value hierarchy.

Accounting for derivative instr

Accounting for derivative instruments and hedging activities	3 Months Ended
	Mar. 31, 2021
Accounting for derivative instruments and hedging activities
Accounting for derivative instruments and hedging activities	8. Accounting for derivative instruments and hedging activities ​ We recognize all derivative instruments on the balance sheet as either assets or liabilities and measure them at fair value in each reporting period. We have one contract designated as a cash flow hedge, and we defer the effective portion of the change in fair value of the derivatives in accumulated other comprehensive income (loss), until the hedged transactions occur and are recognized in earnings (loss). The ineffective portion of a cash flow hedge is immediately recognized in earnings (loss). For our other derivatives that are not designated as cash flow hedges, the changes in the fair value are immediately recognized in earnings (loss). These guidelines apply to our natural gas swaps, interest rate swaps, and foreign exchange contracts. ​ Gas purchase and sale agreements ​ We have a gas purchase agreement at our Nipigon project that expires on December 31, 2022 under which we purchase a minimum of 6,500 Gigajoules (“Gj”) of natural gas per day at a price of Cdn$4.57 per Gj. This agreement does not qualify for the normal purchase normal sales (“NPNS”) exemption and is accounted for as a derivative financial instrument because we could not conclude that it is probable that this contract will not settle net and will result in physical delivery. This derivative financial instrument is recorded in the condensed consolidated balance sheets at fair value and the changes in its fair market value are recorded in the condensed consolidated statements of operations. We also have a corresponding gas sales agreement at Nipigon, whereby ​ Natural gas swaps ​ Our strategy to mitigate future exposure to changes in natural gas prices at our projects consists of periodically entering into financial swaps that effectively fix the price of natural gas expected to be purchased at these projects. These natural gas swaps are derivative financial instruments and are recorded in the condensed consolidated balance sheets at fair value and the changes in their fair market value are recorded in the condensed consolidated statements of operations. ​ We have entered into various natural gas swaps to effectively fix the price of 11.3 million MMBtu of future natural gas purchases at our Orlando project, which is approximately 100% of our share of the expected natural gas purchases in 2021 through 2023. These contracts are accounted for as derivative financial instruments and are recorded in the condensed consolidated balance sheet at fair value at March 31, 2021. Changes in the fair market value of these contracts are recorded in the condensed consolidated statement of operations. ​ Interest rate swaps ​ APLP Holdings has entered into several interest rate swap agreements to mitigate its exposure to changes in interest at the Adjusted Eurodollar Rate. At March 31, 2021, these agreements totaled $284.5 million notional amount of the remaining $284.5 million aggregate principal amount of borrowings under the senior secured term loan facility (“Term Loan Facility”). These interest rate swap agreements expire at various dates through March 31, 2022. Borrowings under the $700.0 million Term Loan Facility bear interest at a rate equal to the Adjusted Eurodollar Rate plus an applicable margin of 2.50%. Based on the terms of the Term Loan Facility, the Adjusted Eurodollar Rate cannot be less than 1.00%, resulting in a minimum of a 3.50% all-in rate on the Term Loan Facility for the remaining principal amount. The weighted average rate of these swap agreements is 1.92%, resulting in an all-in rate of approximately 4.42% for $284.5 million of the Term Loan Facility. ​ The Cadillac project has an interest rate swap agreement that effectively fixes the interest rate at 6.3% through February 15, 2023, and 6.4% thereafter. The notional amount of the interest rate swap agreement matches the outstanding principal balance over the remaining life of the Cadillac Term Loan. This swap agreement, which qualifies for and is designated as a cash flow hedge, is effective through June 2025 and the effective portion of the changes in the fair market value is recorded in accumulated other comprehensive income (loss). ​ Foreign currency forward contracts ​ We use foreign currency forward contracts to manage our exposure to changes in foreign exchange rates as we generate cash flow in U.S. dollars and Canadian dollars. We currently have Canadian dollar payment obligations for preferred dividends, interest on our Canadian dollar-denominated Convertible Debentures and our Medium Term Notes due June 23, 2036 (“MTNs”). Principal and interest payments for our Term Loan are made in U.S. dollars. We have a hedging strategy for the purpose of mitigating the currency risk impact on the future interest and principal payments, preferred dividends and other working capital requirements. Foreign currency forward contracts are not designated as hedges, and changes in their market value are recorded in foreign exchange on the condensed consolidated statements of operations. As of March 31, 2021, we have no foreign currency forward contracts. ​ Volume of forecasted transactions ​ We have entered into derivative instruments in order to economically hedge the following notional volumes of forecasted transactions as summarized below, by type, excluding those derivatives that qualified for NPNS exemption at March 31, 2021 and December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ Units ​ 2021 ​ 2020 Natural gas swaps Natural Gas (MMBtu) 11.3 12.4 ​ Gas purchase agreements Natural Gas (Gigajoules) 4.2 4.0 ​ Interest rate swaps Interest (US$) 298.7 122.3 ​ ​ Fair value of derivative instruments ​ We disclose derivative instrument assets and liabilities on a trade-by-trade basis and do not offset amounts at the counterparty master agreement level. The following table summarizes the fair value of our derivative assets and liabilities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, 2021 ​ ​ Derivative ​ Derivative ​ ​ Assets ​ Liabilities Derivative instruments designated as cash flow hedges: ​ ​ ​ Interest rate swaps current ​ $ — ​ $ 0.6 ​ Interest rate swaps long-term ​ — ​ 0.7 ​ Total derivative instruments designated as cash flow hedges ​ — ​ 1.3 ​ Derivative instruments not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ Interest rate swaps current ​ — ​ 4.3 ​ Interest rate swaps long-term ​ — ​ — ​ Natural gas swaps current ​ — ​ — ​ Natural gas swaps long-term ​ — ​ 1.5 ​ Gas purchase agreements current ​ — ​ 3.6 ​ Gas purchase agreements long-term ​ — ​ 3.2 ​ Convertible debenture conversion option ​ ​ — ​ ​ 1.4 ​ Total derivative instruments not designated as cash flow hedges ​ — ​ 14.0 ​ Total derivative instruments ​ $ — ​ $ 15.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ Derivative ​ Derivative ​ ​ Assets ​ Liabilities Derivative instruments designated as cash flow hedges: ​ ​ ​ Interest rate swaps current ​ $ — ​ $ 0.6 ​ Interest rate swaps long-term ​ — ​ 1.0 ​ Total derivative instruments designated as cash flow hedges ​ — ​ 1.6 ​ Derivative instruments not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ Interest rate swaps current ​ — ​ 4.1 ​ Interest rate swaps long-term ​ — ​ 0.9 ​ Natural gas swaps current ​ — ​ 0.8 ​ Natural gas swaps long-term ​ — ​ 1.9 ​ Gas purchase agreements current ​ 0.4 ​ 4.0 ​ Gas purchase agreements long-term ​ — ​ 4.3 ​ Convertible debenture conversion option ​ ​ — ​ ​ 1.5 ​ Total derivative instruments not designated as cash flow hedges ​ 0.4 ​ 17.5 ​ Total derivative instruments ​ $ 0.4 ​ $ 19.1 ​ ​ Accumulated other comprehensive income ​ The following table summarizes the changes in the accumulated other comprehensive income (loss) (“OCI”) balance attributable to derivative financial instruments designated as a hedge, net of tax: ​ ​ ​ ​ ​ ​ ​ Interest Rate Three Months Ended March 31, 2021 Swaps Accumulated OCI balance at January 1, 2021 ​ $ 1.6 Change in fair value of cash flow hedges ​ 0.1 Realized from OCI during the period ​ — Accumulated OCI balance at March 31, 2021 ​ $ 1.7 ​ ​ ​ ​ ​ ​ Interest Rate Three Months Ended March 31, 2020 Swaps Accumulated OCI balance at January 1, 2020 ​ $ 1.6 Change in fair value of cash flow hedges ​ (0.4) Realized from OCI during the period ​ 0.1 Accumulated OCI balance at March 31, 2020 ​ $ 1.3 ​ Impact of derivative instruments on the condensed consolidated statements of operations ​ The following table summarizes realized loss (gain) for derivative instruments not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Classification of loss Three Months Ended March 31, ​ ​ recognized in income 2021 ​ 2020 Gas purchase agreements ​ Fuel $ 2.2 $ 2.2 ​ Natural gas swaps ​ Fuel ​ 0.1 ​ 0.7 ​ Interest rate swaps ​ Interest, net ​ 0.5 ​ 0.3 ​ ​ The following table summarizes the unrealized (loss) gain resulting from changes in the fair value of derivative financial instruments that are not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Classification of gain (loss) ​ Three Months Ended March 31, ​ ​ recognized in income ​ 2021 2020 Natural gas swaps ​ Change in fair value of derivative instruments $ 1.2 ​ $ (0.5) ​ Gas purchase agreements ​ Change in fair value of derivative instruments ​ 1.3 ​ 1.0 ​ Interest rate swaps ​ Change in fair value of derivative instruments ​ 0.7 ​ (6.1) ​ ​ ​ ​ ​ $ 3.2 ​ $ (5.6) ​ Convertible debenture conversion option ​ Other (income) expense, net ​ $ (0.1) ​ $ 2.6 ​ ​ X

Income taxes

Income taxes	3 Months Ended
	Mar. 31, 2021
Income taxes
Income taxes	9. Income taxes ​ The following table summarizes the current and deferred portions of the net income tax expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ Current income tax expense $ 1.9 $ 1.2 ​ Deferred income tax expense ​ 0.3 ​ 0.3 ​ Total income tax expense, net ​ $ 2.2 ​ $ 1.5 ​ ​ For the three months ended March 31, 2021 and 2020 ​ Income tax expense for the three months ended March 31, 2021 was $2.2 million. Expected income tax expense for the same period, based on the Canadian enacted statutory rate of 27%, was $1.0 million. The primary items impacting the tax rate for the three months ended March 31, 2021 were $0.5 million relating to withholding and state taxes and $0.7 million of other permanent differences. ​ Income tax expense for the three months ended March 31, 2020 was $1.5 million. Expected income tax expense for the same period, based on the Canadian enacted statutory rate of 27% , was $6.8 million. The primary item impacting the tax rate for the three months ended March 31, 2020 was a net decrease to our valuation allowances of $8.7 million, consisting of $5.9 million decreases in Canada and $2.8 million decreases in the United States due to the utilization of net operating losses. These items were partially offset by $2.0 million relating to foreign exchange and $1.4 million of other permanent differences. ​ As of March 31, 2021, we have recorded a valuation allowance of $105.5 million. The amount is comprised primarily of provisions against Canadian and U.S. net operating loss carryforwards. In assessing the recoverability of our deferred tax assets, we consider whether it is more likely than not that some portion or all of the deferred tax assets will be realized. The ultimate realization of deferred tax assets is dependent upon projected future taxable income in the U.S. and in Canada and available tax planning strategies.

Equity compensation plans

Equity compensation plans	3 Months Ended
	Mar. 31, 2021
Equity compensation plans
Equity compensation plans	10. Equity compensation plans ​ Long-term incentive plan (“LTIP”) ​ The following table summarizes the changes in outstanding LTIP notional shares during the three months ended March 31, 2021: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ ​ ​ Weighted-Average ​ Notional Shares ​ ​ ​ ​ ​ Fair Value per Notional Share Outstanding at December 31, 2020 3,710,340 ​ ​ ​ ​ $ 2.45 ​ Vested and redeemed (1,836,821) ​ ​ ​ ​ ​ 2.36 ​ Forfeitures (224,678) ​ ​ ​ ​ ​ 2.46 ​ Outstanding at March 31, 2021 1,648,841 ​ ​ ​ ​ $ 2.56 ​ ​ Cash payments made for vested notional shares for the three months ended March 31, 2021 and 2020 were $3.5 million and $3.3 million, respectively. Total compensation expense for LTIP and Transition Equity Participation Agreement notional shares was $1.4 million and $1.0 million for the three months ended March 31, 2021 and 2020, respectively. ​ Transition Equity Participation Agreement ​ We also have 269,952 transition notional shares outstanding at March 31, 2021 under the Transition Equity Participation Agreement with James J. Moore, Jr. These notional shares will vest if the weighted average Canadian dollar closing price of our common shares on the Toronto Stock Exchange exceeds Cdn$4.77 for at least three consecutive calendar months. These notional shares will also vest in the event of a change in control or if Mr. Moore is terminated without cause, resigns for good reason, or dies.

Basic and diluted earnings per

Basic and diluted earnings per share	3 Months Ended
	Mar. 31, 2021
Basic and diluted earnings per share
Basic and diluted earnings per share	11. Basic and diluted earnings per share ​ Basic earnings per share is calculated by dividing net income attributable to Atlantic Power Corporation by weighted average common shares outstanding during their respective periods. Shares issued and shares repurchased during the year are weighted for the portion of the year that they were outstanding. computed in a manner consistent with that of basic earnings per share while giving effect to all potentially dilutive common shares that were outstanding during the period . The outstanding equity compensation for non-vested LTIP and Transition Equity Participation Agreement notional shares are not considered outstanding for purposes of computing basic earnings per share. However, these instruments are included in the denominator, when dilutive, for purposes of computing diluted earnings per share under the treasury stock method ​ The following table sets forth the calculation of basic and diluted earnings per share for the three months ended March 31, 2021 and 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, Basic 2021 2020 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income attributable to Atlantic Power Corporation ​ $ (0.1) ​ $ 29.5 Denominator: ​ ​ ​ ​ ​ ​ Weighted average basic shares outstanding ​ 89.4 ​ 107.2 Basic (loss) earnings per share attributable to Atlantic Power Corporation ​ $ (0.00) ​ $ 0.28 Diluted ​ ​ ​ ​ ​ ​ Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income attributable to Atlantic Power Corporation ​ $ (0.1) ​ $ 29.5 Add: convertible debenture interest expense ​ ​ — ​ ​ 0.9 ​ ​ ​ (0.1) ​ ​ 30.4 Denominator: ​ ​ ​ ​ ​ ​ Weighted average basic shares outstanding ​ 89.4 ​ 107.2 Share-based compensation ​ — ​ 0.2 Convertible debentures ​ — ​ 27.4 ​ ​ 89.4 ​ 134.8 Diluted (loss) earnings per share attributable to Atlantic Power Corporation ​ $ (0.00) ​ $ 0.23 ​ The following table summarizes our outstanding instruments that are anti-dilutive and were not included in the computation of our diluted earnings (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ 2021 2020 Share-based compensation ​ ​ 0.6 ​ ​ — Convertible debentures ​ ​ 27.4 ​ ​ — Total ​ ​ 28.0 ​ ​ — ​

Equity

Equity	3 Months Ended
	Mar. 31, 2021
Equity
Equity	12. Equity ​ The following tables provide reconciliations of the beginning and ending equity attributable to shareholders of Atlantic Power Corporation, preferred shares issued by a subsidiary company and total equity for the three months ended March 31, 2021 and 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Total Preferred ​ ​ ​ ​ ​ ​ ​ ​ ​ Other ​ Atlantic Power ​ Shares of a ​ ​ ​ ​ Common ​ Retained ​ Comprehensive ​ Corporation ​ Subsidiary ​ Total ​ ​ Shares ​ Deficit ​ (loss) income ​ Shareholders' Deficit ​ Company ​ Equity Balance at January 1, 2021 $ 1,219.7 ​ $ (1,090.0) ​ $ (139.9) ​ $ (10.2) ​ $ 168.8 ​ $ 158.6 ​ Net (loss) income — ​ (0.1) ​ — ​ (0.1) ​ 1.7 ​ 1.6 ​ Share-based compensation ​ 0.1 ​ ​ — ​ ​ — ​ ​ 0.1 ​ ​ — ​ ​ 0.1 ​ Dividends on preferred shares of a subsidiary company - Series 1 (Cdn$0.303125 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.8) ​ ​ (0.8) ​ Dividends on preferred shares of a subsidiary company - Series 2 (Cdn$0.358688 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.7) ​ ​ (0.7) ​ Dividends on preferred shares of a subsidiary company - Series 3 (Cdn$0.267807 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.2) ​ ​ (0.2) ​ Realized and unrealized loss on hedging activities, net of tax — ​ — ​ 0.1 ​ 0.1 ​ — ​ 0.1 ​ Foreign currency translation adjustments — ​ — ​ 1.7 ​ 1.7 ​ — ​ 1.7 ​ Balance at March 31, 2021 $ 1,219.8 ​ $ (1,090.1) ​ $ (138.1) ​ $ (8.4) ​ $ 168.8 ​ $ 160.4 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Total Preferred ​ ​ ​ ​ ​ ​ ​ ​ ​ Other ​ Atlantic Power ​ Shares of a ​ ​ ​ ​ Common ​ Retained ​ Comprehensive ​ Corporation ​ Subsidiary ​ Total ​ ​ Shares ​ Deficit ​ (loss) income ​ Shareholders' Deficit ​ Company ​ Equity Balance at January 1, 2020 $ 1,259.9 ​ $ (1,164.2) ​ $ (140.7) ​ $ (45.0) ​ $ 182.7 ​ $ 137.7 ​ Net income (loss) — ​ 29.5 ​ — ​ 29.5 ​ (5.8) ​ 23.7 ​ Share-based compensation ​ 0.4 ​ ​ — ​ ​ — ​ ​ 0.4 ​ ​ — ​ ​ 0.4 ​ Common share repurchases (8.2) ​ — ​ — ​ (8.2) ​ — ​ (8.2) ​ Preferred share repurchases ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (6.4) ​ ​ (6.4) ​ Dividends on preferred shares of a subsidiary company - Series 1 (Cdn$0.303125 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.8) ​ ​ (0.8) ​ Dividends on preferred shares of a subsidiary company - Series 2 (Cdn$0.358688 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.6) ​ ​ (0.6) ​ Dividends on preferred shares of a subsidiary company - Series 3 (Cdn$0.363342 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.3) ​ ​ (0.3) ​ Realized and unrealized loss on hedging activities, net of tax — ​ — ​ (0.3) ​ (0.3) ​ — ​ (0.3) ​ Foreign currency translation adjustments — ​ — ​ (11.2) ​ (11.2) ​ — ​ (11.2) ​ Balance at March 31, 2020 $ 1,252.1 ​ $ (1,134.7) ​ $ (152.2) ​ $ (34.8) ​ $ 168.8 ​ $ 134.0 ​ ​ Share Repurchase Program ​ Normal Course Issuer Bid ​ On December 31, 2020, we commenced a new Normal Course Issuer Bid (“NCIB”) for our Series E Debentures, our common shares and for each series of the preferred shares of APPEL, our wholly-owned subsidiary. The NCIBs expire on December 30, 2021 or such earlier date as the Company and/or APPEL complete their respective purchases pursuant to the NCIB. Under the NCIBs, we may purchase up to a total of 8,554,391 common shares based on 10% of our public float as of December 18, 2020 and we are limited to daily purchases of 10,420 common shares per day with certain exceptions including block purchases and purchases on other approved exchanges. All purchases made under the NCIBs will be made through the facilities of the TSX or other Canadian designated exchanges and published marketplaces and in accordance with the rules of the TSX at market prices prevailing at the time of purchase. Common share purchases under the NCIBs may also be made on the NYSE in compliance with Rule 10b-18 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or other designated exchanges and published marketplaces in the U.S. in accordance with applicable regulatory requirements. The ability to make certain purchases through the facilities of the NYSE is subject to regulatory approval. ​ The Board authorization permits the Company to repurchase common and preferred shares and Convertible Debentures. Therefore, in addition to the current NCIBs, from time to time we may repurchase our securities, including our common shares, our Convertible Debentures and our APPEL preferred shares through open market purchases, including pursuant to one or more “Rule 10b5-1 plans” pursuant to such provision under the Exchange Act, NCIBs, issuer self tender or substantial issuer bids, or in privately negotiated transactions. There can be no assurances as to the amount, timing or prices of repurchases, which may vary based on market conditions, other market opportunities and other factors. Any share repurchases outside of previously authorized NCIBs would be affected after taking into account our then current cash position and then anticipated cash obligations or business opportunities. ​ Atlantic Power Corporation and affiliated purchasers did not make any repurchases of common equity securities, Convertible Debentures or preferred shares during the period of January 1, 2021 through March 31, 2021.

Segment and geographic informat

Segment and geographic information	3 Months Ended
	Mar. 31, 2021
Segment and geographic information
Segment and geographic information	​ 13. Segment and geographic information ​ We have four reportable segments: Solid Fuel, Natural Gas, Hydroelectric and Corporate. We analyze the performance of our operating segments based on Project Adjusted EBITDA, which is defined as project income (loss) plus interest, taxes, depreciation and amortization, impairment charges, insurance loss (gain), other (income) expenses and changes in fair value of derivative instruments. Project Adjusted EBITDA is not a measure recognized under GAAP and does not have a standardized meaning prescribed by GAAP and is therefore unlikely to be comparable to similar measures presented by other companies. We use Project Adjusted EBITDA to provide comparative information about segment performance without considering how projects are capitalized or whether they contain derivative contracts that are required to be recorded at fair value. Our equity method investments in unconsolidated affiliates are presented as proportionately consolidated based on our ownership percentage in the reconciliation of Project Adjusted EBITDA to project income. ​ A reconciliation of Project Adjusted EBITDA to net income (loss) for the three months ended March 31, 2021 and 2020 is included in the tables below: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Consolidated Three Months Ended March 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenues ​ $ 26.9 ​ $ 31.7 ​ $ 13.1 ​ $ 0.3 ​ $ 72.0 ​ Segment assets ​ 205.6 ​ ​ 190.3 ​ ​ 301.4 ​ ​ 126.6 ​ 823.9 ​ Project Adjusted EBITDA ​ $ 9.7 ​ $ 28.7 ​ $ 10.3 ​ $ (0.5) ​ $ 48.2 ​ Change in fair value of derivative instruments ​ — ​ ​ (2.5) ​ ​ — ​ ​ (0.7) ​ ​ (3.2) ​ Depreciation and amortization ​ 6.1 ​ ​ 7.6 ​ ​ 4.9 ​ ​ — ​ ​ 18.6 ​ Interest, net ​ 0.6 ​ ​ — ​ ​ — ​ ​ — ​ 0.6 ​ Other project income ​ — ​ — ​ — ​ (0.2) ​ (0.2) ​ Project income ​ ​ 3.0 ​ ​ 23.6 ​ ​ 5.4 ​ ​ 0.4 ​ 32.4 ​ Administration ​ — ​ — ​ — ​ 14.1 ​ 14.1 ​ Interest expense, net ​ — ​ — ​ — ​ 11.4 ​ 11.4 ​ Foreign exchange loss ​ — ​ — ​ — ​ 3.2 ​ ​ 3.2 ​ Other income, net ​ — ​ — ​ — ​ (0.1) ​ ​ (0.1) ​ Net income (loss) before income taxes ​ 3.0 ​ 23.6 ​ 5.4 ​ (28.2) ​ 3.8 ​ Income tax expense ​ — ​ — ​ — ​ 2.2 ​ 2.2 ​ Net income (loss) ​ $ 3.0 ​ $ 23.6 ​ $ 5.4 ​ $ (30.4) ​ $ 1.6 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Consolidated Three Months Ended March 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenues ​ $ 23.4 ​ $ 30.8 ​ $ 18.4 ​ $ 0.2 ​ $ 72.8 ​ Segment assets ​ 225.0 ​ ​ 230.6 ​ ​ 359.5 ​ ​ 66.9 ​ 882.0 ​ Project Adjusted EBITDA ​ $ 7.8 ​ $ 28.2 ​ $ 15.3 ​ $ (0.5) ​ $ 50.8 ​ Change in fair value of derivative instruments ​ — ​ ​ (0.5) ​ ​ — ​ 6.1 ​ 5.6 ​ Depreciation and amortization ​ 5.6 ​ ​ 9.2 ​ ​ 4.9 ​ 0.1 ​ 19.8 ​ Interest, net ​ 0.7 ​ ​ — ​ ​ — ​ ​ — ​ 0.7 ​ Other project expense ​ — ​ — ​ — ​ — ​ — ​ Project income (loss) ​ 1.5 ​ ​ 19.5 ​ ​ 10.4 ​ ​ (6.7) ​ 24.7 ​ Administration ​ — ​ — ​ — ​ 6.7 ​ 6.7 ​ Interest expense, net ​ — ​ — ​ — ​ 10.8 ​ 10.8 ​ Foreign exchange gain ​ — ​ — ​ — ​ (20.6) ​ (20.6) ​ Other expense, net ​ — ​ — ​ — ​ 2.6 ​ 2.6 ​ Net income (loss) before income taxes ​ 1.5 ​ 19.5 ​ 10.4 ​ (6.2) ​ ​ 25.2 ​ Income tax expense ​ — ​ — ​ — ​ 1.5 ​ ​ 1.5 ​ Net income (loss) ​ $ 1.5 ​ $ 19.5 ​ $ 10.4 ​ $ (7.7) ​ $ 23.7 ​ ​ The tables below provide information, by country, about our consolidated operations for the three months ended March 31, 2021 and 2020 and for Property, Plant and Equipment, PPAs and other intangible assets and total assets as of March 31, 2021 and December 31, 2020, respectively. Revenue is recorded in the country in which it is earned and assets are recorded in the country in which they are located. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenue ​ ​ Three Months Ended March 31, ​ ​ 2021 ​ 2020 United States $ 46.1 $ 48.2 Canada ​ 25.9 ​ 24.6 Total ​ $ 72.0 ​ $ 72.8 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Property, Plant and ​ PPAs and ​ ​ ​ ​ Equipment, net of ​ other intangible assets, net of ​ ​ ​ ​ accumulated depreciation ​ accumulated amortization ​ Total assets ​ March 31, 2021 December 31, 2020 March 31, 2021 December 31, 2020 March 31, 2021 December 31, 2020 United States $ 345.0 ​ $ 351.2 ​ $ 114.0 ​ $ 119.4 ​ $ 644.0 ​ $ 666.4 Canada ​ 136.3 ​ 140.6 ​ 0.8 ​ 0.9 ​ 179.9 ​ 180.8 Total ​ $ 481.3 ​ $ 491.8 ​ $ 114.8 ​ $ 120.3 ​ $ 823.9 ​ $ 847.2 ​ Concentration risk ​ Independent Electricity System Operator (“IESO”), BC Hydro, Niagara Mohawk and Equistar Chemicals, LP (“Equistar”) provided 14.8%, 14.8%, 13.4%, and 12.1%, respectively, of total consolidated revenues for the three months ended March 31, 2021. Niagara Mohawk, IESO, BC Hydro and Equistar provided 20.9%, 14.7%, 13.5% and 10.3%, respectively, of total consolidated revenues for the three months ended March 31, 2020. ​ IESO purchases electricity from the Calstock, Tunis, and Nipigon projects in the Solid Fuel and Natural Gas segments, BC Hydro purchases electricity from the Mamquam and Moresby Lake projects in the Hydroelectric segment and the Williams Lake project in the Solid Fuel segment, Niagara Mohawk purchases electricity from the Curtis Palmer project in the Hydroelectric segment, and Equistar purchases electricity from the Morris project in the Natural Gas segment.

Guarantees and Contingencies

Guarantees and Contingencies	3 Months Ended
	Mar. 31, 2021
Guarantees and Contingencies
Guarantees and Contingencies	14. Guarantees and Contingencies ​ Guarantees ​ We and our subsidiaries enter into various contracts that include indemnification and guarantee provisions as a routine part of our business activities. Examples of these contracts include asset purchases and sale agreements, joint venture agreements, operation and maintenance agreements, and other types of contractual agreements with vendors and other third parties, as well as affiliates. These contracts generally indemnify the counterparty for tax, environmental liability, litigation and other matters, as well as breaches of representations, warranties and covenants set forth in these agreements. ​ Contingencies ​ From time to time, Atlantic Power, its subsidiaries and the projects are parties to disputes and litigation that arise in the normal course of business. We assess our exposure to these matters and record estimated loss contingencies when a loss is likely and can be reasonably estimated. There are no matters pending which are expected to have a material adverse impact on our financial position or results of operations or have been reserved for as of March 31, 2021. ​

Leases

Leases	3 Months Ended
	Mar. 31, 2021
Leases
Leases	15. Leases ​ Real estate leases and equipment leases ​ We lease our office properties and equipment under operating leases expiring on various dates through 2024. Certain operating lease agreements include provisions for scheduled rent increases over their lease terms. We recognize the effects of these scheduled rent increases on a straight-line basis over the lease term. One of our leased office properties is sub-leased to third parties. The sub-lease is an operating lease and the rental income received is recorded net of rental expense in the condensed consolidated statements of operations. ​ On January 1, 2019, we implemented FASB ASU No. 2016-02, Leases (Topic 842). To calculate lease liabilities on the implementation date, we utilized an incremental borrowing rate of 3.75%, which was our minimum all-in rate on the Term Loan Facility for the non-swapped portion of the remaining principal amount. ​ The following table presents the components of lease expense. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ ​ 2021 ​ ​ 2020 Lease cost: (1) ​ ​ ​ ​ ​ Operating lease cost ​ $ 0.5 ​ $ 0.5 Sublease income ​ ​ (0.3) ​ ​ (0.3) Total lease cost ​ $ 0.2 ​ $ 0.2 (1) ​ The following table presents operating and finance lease maturities and a reconciliation of the undiscounted cash flows to operating lease liabilities. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Lease ​ Income from ​ Net lease ​ ​ Payments subleasing payments ​ 2021 ​ $ 1.5 ​ $ (0.8) ​ $ 0.7 ​ 2022 ​ ​ 1.8 ​ ​ (1.1) ​ ​ 0.7 ​ 2023 ​ ​ 1.3 ​ ​ (0.7) ​ ​ 0.6 ​ 2024 ​ ​ 0.2 ​ ​ — ​ ​ 0.2 ​ 2025 ​ ​ — ​ ​ — ​ ​ — ​ Thereafter ​ ​ — ​ ​ — ​ ​ — ​ Total operating lease payments ​ $ 4.8 ​ $ (2.6) ​ $ 2.2 ​ Less: present value discount ​ ​ (0.3) ​ ​ ​ ​ ​ ​ ​ Total operating lease liabilities ​ $ 4.5 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Lease ​ ​ ​ ​ ​ ​ ​ ​ Payments ​ ​ ​ ​ ​ 2021 ​ $ — ​ ​ ​ ​ ​ ​ ​ 2022 ​ ​ 0.1 ​ ​ ​ ​ ​ ​ 2023 ​ ​ — ​ ​ ​ ​ ​ ​ Thereafter ​ ​ — ​ ​ ​ ​ ​ ​ ​ Total finance lease payments ​ $ 0.1 ​ ​ ​ ​ ​ ​ ​ Less: amount representing interest ​ ​ — ​ ​ ​ ​ ​ ​ ​ Total finance lease liabilities ​ $ 0.1 ​ ​ ​ ​ ​ ​ ​ (1) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ Other Information: ​ ​ ​ ​ ​ ​ ​ Cash paid for amounts included in the measurement of lease liabilities (1) ​ ​ ​ ​ ​ ​ ​ Operating cash flows from operating leases ​ $ 0.3 ​ $ 0.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average remaining lease term (in years): ​ ​ ​ ​ ​ ​ ​ Operating leases ​ ​ 2.5 ​ ​ 2.5 ​ Finance leases ​ ​ 1.1 ​ ​ 1.4 ​ Weighted average discount rate - operating leases ​ ​ 3.9 % ​ 3.9 % Weighted average discount rate - finance leases ​ ​ 4.1 % ​ 4.1 % ​ We have no lease transactions with related parties. ​ PPA Leases ​ We have entered into PPAs to sell power at predetermined rates. PPAs were assessed as to whether they contain leases, which convey to the counterparty the right to control the use of the project’s property, plant and equipment in return for future payments. Such arrangements are classified as either operating or finance leases. We recognize lease income consistent with the recognition of energy sales and capacity revenue. When energy is delivered and capacity is provided, we recognize lease income as a component of energy sales and capacity revenue. Finance income related to leases or arrangements accounted for as finance leases is recognized in a manner that produces a constant rate of return on the net investment in the lease. The net investment is comprised of net minimum lease payments and unearned finance income. Unearned finance income is the difference between the total minimum lease payments and the carrying value of the leased property. Unearned finance income is deferred and recognized in net income (loss) over the lease term. We elected the practical expedient that permits us to retain our existing lease assessment and classification. ​ As of March 31, 2021, we have ten PPAs accounted for as operating leases among our twenty-one projects in operation. No extension terms exist for our PPAs accounted for as leases and the remaining lease term varies from one year to twenty-three years . The following table provides lease income recorded as energy and capacity sales by segment from PPAs accounted for as operating leases: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Rental Income from operating leases ​ ​ ​ Three Months Ended ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ Solid Fuel ​ $ 14.8 ​ $ 16.6 ​ ​ ​ ​ ​ ​ ​ ​ ​ Natural Gas ​ ​ 5.6 ​ ​ 5.9 ​ ​ ​ ​ ​ ​ ​ ​ ​ Hydroelectric ​ ​ 13.1 ​ ​ 18.4 ​ ​ ​ $ 33.5 ​ $ 40.9 ​ ​ For certain of our PPAs accounted for as leases, the lessee has the option to purchase the plant. In May 2019, we entered into an agreement to sell Manchief to Public Service Company of Colorado (“PSCo”) following the expiration of the PPA in April 2022 for $45.2 million subject to working capital and other customary adjustments. BC Hydro has an option to purchase Mamquam that is exercisable in November 2021 and every five-year anniversary thereafter.

Nature of business (Policies)

Nature of business (Policies)	3 Months Ended
	Mar. 31, 2021
Nature of business
Basis of presentation	Basis of presentation ​ The interim condensed consolidated financial statements included in this Quarterly Report on Form 10-Q have been prepared in accordance with the SEC regulations for interim financial information and with the instructions to Form 10-Q. The following notes should be read in conjunction with the accounting policies and other disclosures as set forth in the notes to our financial statements in our Annual Report on Form 10-K for the year ended December 31, 2020. Interim results are not necessarily indicative of results for the full year. ​ In our opinion, the accompanying unaudited interim condensed consolidated financial statements present fairly our consolidated financial position as of March 31, 2021, the results of operations and comprehensive income for the three months ended March 31, 2021 and 2020, and our cash flows for the three months ended March 31, 2021 and 2020, in accordance with U.S. generally accepted accounting principles (“GAAP”). In the opinion of management, all adjustments (consisting of normal recurring accruals and other adjustments) considered necessary for a fair presentation have been included.
Use of estimates	Use of estimates ​ The preparation of financial statements requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the year. Actual results could differ from those estimates. During the periods presented, we have made a number of estimates and valuation assumptions, including the fair value of assets acquired and liabilities assumed in purchase accounting, the useful lives and recoverability of property, plant and equipment, valuation of goodwill, intangible assets and liabilities related to PPAs and fuel supply agreements, the recoverability of equity method investments, the recoverability of deferred tax assets, tax provisions, recovery of expected insurance proceeds, the fair value of financial instruments and derivatives, pension obligations, asset retirement obligations and equity-based compensation. In addition, estimates are used to test long-lived assets and goodwill for impairment and to determine the fair value of impaired assets. These estimates and valuation assumptions are based on present conditions and our planned course of action, as well as assumptions about future business and economic conditions. See “Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations—Critical Accounting Policies and Estimates
COVID-19 Pandemic	COVID-19 Pandemic ​ There are many uncertainties regarding the ongoing coronavirus (“COVID-19”) pandemic, and we are closely monitoring the impact of COVID-19 on all aspects of our business, including how it will impact our customers, employees, suppliers, vendors and business partners. We have taken extra precautions for our employees who continue to work at our facilities and have implemented work-from-home policies where appropriate. Currently, we do not anticipate any employee layoffs and are continuing to maintain the high level of reliability and availability of our plants. We continue to implement strong physical and cybersecurity measures to ensure that our systems remain functional in order to serve our operational needs with a remote workforce and to keep our operations running to ensure uninterrupted service to our offtakers.
Proposed Transaction with I Squared Capital	Proposed Transaction with I Squared Capital ​ On January 14, 2021, Atlantic Power announced that it had entered into a definitive agreement (as amended on April 1, 2021 and April 29, 2021, and as may be further amended from time to time, the “Arrangement Agreement”) , under which the Company's outstanding common shares and 6.00% Series E convertible unsecured subordinated debentures due January 31, 2025 (“Convertible Debentures”), and the outstanding preferred shares and medium term notes of certain of its subsidiaries, would be acquired (the “Transaction”). The total enterprise value of the deal is approximately $961 million and the Transaction was unanimously approved by Atlantic Power's board of directors. The parties are currently targeting May 14, 2021 as the closing date for the Transaction. ​ In connection with the Transaction, and subject to and conditional to the closing of the Transaction: ​ ● Holders of common shares of Atlantic Power will receive $3.03 per common share in cash. ● Atlantic Power Preferred Equity Ltd.’s (“APPEL’s”) cumulative redeemable preferred shares, Series 1, cumulative rate reset preferred shares, Series 2, and cumulative floating rate preferred shares, Series 3, will be purchased by APPEL for Cdn $22.00 per preferred share in cash. ● Atlantic Power Limited Partnership’s (“APLP’s”) 5.95% medium term notes due June 23, 2036 (the “MTNs”) will be redeemed for consideration equal to 106.071% of the principal amount of MTNs held as of the closing of the Transaction, plus accrued and unpaid interest on the MTNs up to, but excluding, the closing date of the Transaction. Holders of MTNs that have delivered a valid consent to the proposed amendments to the trust indenture governing the MTNs (as described below) prior to 5:00 p.m. (Toronto time) on March 16, 2021 will also be entitled to a consent fee equal to 0.25% of the principal amount of MTNs held by such holders in respect of which a consent was delivered, conditional on closing of the Transaction. ● Holders of Atlantic Power’s Convertible Debentures who deliver a conversion notice prior to 4:00 p.m. (Toronto time) on May 11, 2021 (the “Conversion Deadline”) will receive $3.03 per common share issuable on conversion of the Convertible Debentures in respect of which such conversion notice has been delivered (including common shares issuable on account of the Make Whole Premium (as defined in the Arrangement Agreement), plus accrued and unpaid interest up to, but excluding, the date of conversion, and all other Convertible Debentures will be defeased in accordance with the terms of the trust indenture governing the Convertible Debentures (the “Debenture Indenture”) (as described below) ​ The Arrangement Agreement was amended on April 1, 2021 to provide for certain administrative and housekeeping amendments to the Arrangement Agreement and to, among other things, clarify the mechanics surrounding the payment of consideration payable pursuant to the Transaction. The amendments also added a step to the Arrangement to, among other things, provide for the creation of a class of non-voting preferred shares in connection with the Pre-Acquisition Reorganization (as defined in the Arrangement Agreement). The creation and issuance of any such shares will not occur until immediately prior to the effective time of the Arrangement and will only occur if the Purchasers have irrevocably waived or confirmed in writing the satisfaction of all conditions to closing in their favor and shall have confirmed in writing that they are prepared, and able, to promptly and without condition proceed to effect the Arrangement (as defined in the Arrangement Agreement). ​ The Arrangement Agreement was further amended on April 29, 2021 to provide for mutual covenants in connection with the proposed defeasance of the Convertible Debentures (the “Defeasance”), clarify the mechanics surrounding the payment of the amounts required to effect the Defeasance, and effect a waiver of certain conditions precedent in the Arrangement Agreement relating to the approval of the Transaction by holders of the Convertible Debentures. Notwithstanding the Defeasance, any holder of Convertible Debentures who converts their Convertible Debentures during the period beginning 10 trading days (as defined in the Debenture Indenture) prior to the closing of the Transaction (“Closing”) and ending 30 calendar days following the delivery of the change of control notice under the Indenture (the “Make Whole Conversion Period”) will be entitled to receive the Make Whole Premium. Assuming that the Closing occurs on May 14, 2021 and the change of control notice is delivered on Closing as is currently anticipated, the Make Whole Conversion Period opened on April 30, 2021 and will close on June 14, 2021. ​ In connection with the Defeasance, and if Closing occurs (i) Atlantic Power expects to de-list its Convertible Debentures from the TSX and its common shares from the TSX and the NYSE; (ii) Atlantic Power intends to apply to Canadian securities regulators to cease being a reporting issuer, or to be exempt from its reporting obligations as a Canadian reporting issuer, and also intends to file to deregister under the U.S. Securities Exchange Act of 1934; (iii) the Convertible Debentures will no longer be convertible into common shares and, if converted after the Conversion Deadline, holders will be entitled to receive a cash amount equal to Cdn $3.72 in lieu of each Common Share previously issuable on a conversion (including any common shares otherwise issuable on account of the Make Whole Premium if converted within the Make Whole Conversion Period), plus accrued and unpaid interest paid in Canadian dollars up to, but excluding, the date of conversion; and (iv) except as otherwise provided in the Debenture Indenture, any Convertible Debentures which remain outstanding following the expiry of the Make Whole Conversion Period will continue to receive interest at a rate of 6.00% per annum, payable semi-annually in arrears until, and the repayment of principal upon, the redemption of the Convertible Debentures. Following the Defeasance, the Convertible Debentures will be redeemed at par on January 31, 2023. ​ The Transaction has received approval from the holders of common shares of the Company and the holders of preferred shares and medium term notes of certain of the Company's subsidiaries. The Transaction also has received certain required regulatory approvals, including an advance ruling certificate from the Canadian Commissioner of Competition under the Competition Act (Canada) on February 5, 2021, the expiration of the required waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 on March 9, 2021, the approval of the Federal Energy Regulatory Commission on April 2, 2021 and the approval in April 2021 from the Federal Communications Commission (“FCC”) for the transfer of control of certain FCC licenses held by the Company or its subsidiaries. ​ The Transaction remains subject to the satisfaction or waiver of certain remaining third-party consents and other customary closing conditions.
Recently adopted and issued accounting standards	Recently adopted and issued accounting standards ​ Accounting standards adopted in 2021 ​ In December 2019, the FASB issued amendments to the guidance for income taxes through ASU 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes.” The amendments in this update simplify the accounting for income taxes by removing certain exceptions such as: 1) the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items, 2) the requirement to recognize a deferred tax liability for equity method investments when a foreign subsidiary becomes an equity method investment, 3) the ability not to recognize a deferred tax liability for a foreign subsidiary when a foreign equity method investment becomes a subsidiary, and 4) the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. Adoption of this guidance did not have a material impact on the condensed consolidated financial statements. ​ Accounting standards not yet adopted ​ In March 2020, the FASB issued amendments to the guidance for reference rate reform through ASU 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the effects of reference rate reform on financial reporting.” The amendments in this update provide optional expedients and exceptions for applying GAAP to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments apply only to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the amendments do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The amendments are elective and are effective upon issuance for all entities. We

Assets and liabilities held f_2

Assets and liabilities held for sale (Tables)	3 Months Ended
	Mar. 31, 2021
Assets and liabilities held for sale.
Schedule of assets and liabilities held for sale	​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ ​ ​ 2021 ​ Assets (unaudited) Inventory ​ $ 1.1 ​ Property, plant, and equipment, net ​ 3.5 ​ Total current assets held for sale ​ $ 4.6 ​ Liabilities ​ ​ ​ ​ Asset retirement obligations ​ $ 3.2 ​ Total current liabilities held for sale ​ $ 3.2 ​

Revenue from contracts (Tables)

Revenue from contracts (Tables)	3 Months Ended
	Mar. 31, 2021
Revenue from contracts
Schedule of disaggregation of revenue	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, 2021 ​ ​ ​ ​ ​ ​ ​ Consolidated ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Total Project revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Energy sales ​ $ 18.2 ​ $ 5.6 ​ $ 12.2 $ ​ — ​ $ 36.0 Energy capacity revenue ​ 8.1 ​ ​ 21.7 ​ ​ — ​ ​ — ​ 29.8 Steam energy and capacity revenue ​ ​ — ​ ​ 3.2 ​ ​ — ​ ​ — ​ ​ 3.2 Waste heat revenue ​ ​ 0.6 ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.6 Ancillary and transmission services ​ ​ — ​ ​ 1.0 ​ ​ 1.0 ​ ​ — ​ ​ 2.0 Asset management and operation ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.3 ​ ​ 0.3 Miscellaneous revenue ​ ​ ​ ​ ​ 0.2 ​ ​ (0.1) ​ ​ — ​ ​ 0.1 ​ ​ 26.9 ​ 31.7 ​ 13.1 ​ 0.3 ​ 72.0 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, 2020 ​ ​ ​ ​ ​ ​ ​ Consolidated ​ ​ ​ Solid Fuel ​ ​ Natural Gas ​ ​ Hydroelectric ​ ​ Corporate ​ ​ Total Project revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Energy sales ​ $ 16.8 ​ $ 6.2 ​ $ 17.7 ​ $ — ​ $ 40.7 Energy capacity revenue ​ 6.3 ​ 21.7 ​ — ​ — ​ 28.0 Steam energy and capacity revenue ​ ​ — ​ ​ 2.7 ​ ​ — ​ ​ — ​ ​ 2.7 Waste heat revenue ​ ​ 0.3 ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.3 Ancillary and transmission services ​ ​ — ​ ​ 0.7 ​ ​ 0.7 ​ ​ — ​ ​ 1.4 Asset management and operation ​ ​ — ​ ​ — ​ ​ — ​ ​ 0.2 ​ ​ 0.2 Miscellaneous revenue ​ ​ — ​ ​ (0.5) ​ ​ — ​ ​ — ​ ​ (0.5) ​ ​ 23.4 ​ 30.8 ​ 18.4 ​ 0.2 ​ 72.8

Changes in accumulated other _2

Changes in accumulated other comprehensive loss by component (Tables)	3 Months Ended
	Mar. 31, 2021
Changes in accumulated other comprehensive loss by component
Schedule of changes in accumulated other comprehensive (loss) income	The changes in accumulated other comprehensive (loss) income by component were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ 2021 2020 Foreign currency translation ​ ​ ​ Balance at beginning of period ​ $ (138.4) ​ $ (140.6) Other comprehensive income: ​ ​ ​ ​ ​ ​ Foreign currency translation adjustments (1) ​ 1.7 ​ (11.2) Balance at end of period ​ $ (136.7) ​ $ (151.8) Pension ​ ​ ​ ​ ​ ​ Balance at beginning and end of period (2) ​ $ (3.1) ​ $ (1.7) Cash flow hedges ​ ​ ​ ​ ​ ​ Balance at beginning of period ​ $ 1.6 ​ $ 1.6 Other comprehensive income (loss): ​ ​ ​ ​ ​ ​ Net change from periodic revaluations ​ 0.2 ​ (0.6) Tax (expense) benefit ​ (0.1) ​ 0.2 Total other comprehensive income (loss) before reclassifications, net of tax ​ 0.1 ​ (0.4) Net amount reclassified to earnings: ​ ​ ​ ​ ​ ​ Interest rate swaps (3) ​ ​ — ​ ​ 0.1 Tax expense ​ ​ — ​ ​ — Total amount reclassified from accumulated other comprehensive income, net of tax ​ — ​ 0.1 Total other comprehensive income (loss) ​ 0.1 ​ (0.3) Balance at end of period ​ $ 1.7 ​ $ 1.3 (1) In all periods presented, there were no tax impacts related to rate changes and no amounts were reclassified to earnings. (2) Quarterly activity was immaterial. (3) This amount was included in interest expense, net on the accompanying condensed consolidated statements of operations .

Equity method investments in _2

Equity method investments in unconsolidated affiliates (Tables)	3 Months Ended
	Mar. 31, 2021
Equity method investments in unconsolidated affiliates
Summary of operating results	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, Operating results 2021 2020 Revenue ​ ​ ​ ​ ​ ​ Frederickson ​ $ 7.3 ​ $ 8.0 Orlando Cogen, LP ​ 15.5 ​ 14.6 Chambers Cogen, LP ​ ​ 10.8 ​ ​ 10.6 Craven County Wood Energy, LP ​ ​ 3.1 ​ ​ 3.1 Grayling Generating Station, LP ​ ​ 1.6 ​ ​ 1.5 ​ ​ 38.3 ​ 37.8 Project expenses ​ ​ ​ ​ ​ ​ Frederickson ​ 4.8 ​ ​ 5.4 Orlando Cogen, LP ​ ​ 7.2 ​ ​ 6.3 Chambers Cogen, LP ​ 8.2 ​ ​ 8.1 Craven County Wood Energy, LP ​ ​ 3.0 ​ ​ 2.6 Grayling Generating Station, LP ​ ​ 1.2 ​ ​ 1.4 ​ ​ 24.4 ​ 23.8 Project other income ​ ​ ​ ​ ​ ​ Frederickson ​ — ​ — Orlando Cogen, LP ​ ​ — ​ ​ — Chambers Cogen, LP ​ (0.2) ​ ​ (0.3) Craven County Wood Energy, LP ​ ​ — ​ ​ — Grayling Generating Station, LP ​ ​ — ​ ​ — ​ ​ (0.2) ​ (0.3) Net income ​ ​ ​ ​ ​ ​ Frederickson ​ ​ 2.5 ​ ​ 2.6 Orlando Cogen, LP ​ 8.3 ​ 8.3 Chambers Cogen, LP ​ ​ 2.4 ​ ​ 2.2 Craven County Wood Energy, LP ​ ​ 0.1 ​ ​ 0.5 Grayling Generating Station, LP ​ ​ 0.4 ​ ​ 0.1 Equity in earnings of unconsolidated affiliates ​ $ 13.7 ​ $ 13.7 ​ ​ ​ ​ ​ ​ ​ Distributions from equity method investments ​ (6.9) ​ (6.0) Earnings of equity method investments, net of distributions ​ $ 6.8 ​ $ 7.7 ​

Long-term debt (Tables)

Long-term debt (Tables)	3 Months Ended
	Mar. 31, 2021
Schedule of current maturities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ ​ ​ ​ 2021 ​ 2020 ​ Interest Rate Current Maturities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Senior secured term loan facility, due 2025 (1) ​ $ 88.0 ​ $ 93.0 ​ LIBOR (2) plus 2.50 % Cadillac term loan, due 2025 (3) ​ 2.8 ​ 2.7 LIBOR plus 1.61 % Total current maturities ​ $ 90.8 ​ $ 95.7 ​ ​ ​ ​ ​ (1) On a quarterly basis, we make a cash sweep payment to fund the principal balance, based on terms as defined in the term loan credit agreement. The portion of the senior secured term loan facility classified as current is based on principal payments required to reduce the aggregate principal amount of senior secured term loan outstanding to achieve a target principal amount that declines quarterly based on a pre-determined specified schedule. (2) London Interbank Offered Rate (“LIBOR”) cannot be less than 1.00%. We have entered into interest rate swap agreements to mitigate the exposure to changes in LIBOR of the $284.5 million outstanding aggregate borrowings under our senior secured term loan facility at March 31, 2021. See Note 8, Accounting for derivative instruments and hedging activities for further details. (3) We have entered into interest rate swap agreements to economically fix our exposure to changes in interest rates for this non-recourse debt. See Note 8, Accounting for derivative instruments and hedging activities , for further details.
Long-term debt excluding debentures
Schedule of long-term debt	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ ​ ​ 2021 ​ 2020 ​ Interest Rate Recourse Debt: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Senior secured term loan facility, due 2025 (1) ​ $ 284.5 ​ $ 307.5 ​ LIBOR (2) plus 2.50 % Senior unsecured notes, due June 2036 (Cdn$210.0) ​ 167.0 ​ 164.9 ​ ​ 5.95 % Non-Recourse Debt: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cadillac term loan, due 2025 (3) ​ 14.1 ​ 14.8 ​ LIBOR plus 1.61 % Less: unamortized discount ​ ​ (3.0) ​ ​ (3.5) ​ ​ ​ ​ ​ Less: unamortized deferred financing costs ​ ​ (3.5) ​ ​ (3.9) ​ ​ ​ ​ ​ Less: current maturities ​ (90.8) ​ (95.7) ​ ​ ​ ​ ​ Total long-term debt ​ $ 368.3 ​ $ 384.1 ​ ​ ​ ​ ​

Fair value of financial instr_2

Fair value of financial instruments (Tables)	3 Months Ended
	Mar. 31, 2021
Fair value of financial instruments
Schedule of recurring measurements of fair value hierarchy of financial assets and liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, 2021 ​ ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Assets: ​ ​ ​ ​ ​ Cash and cash equivalents ​ $ 21.6 ​ $ — ​ $ — ​ $ 21.6 ​ Restricted cash ​ 6.9 ​ — ​ — ​ 6.9 ​ Derivative instruments asset ​ — ​ — ​ — ​ — ​ Total ​ $ 28.5 ​ $ — ​ $ — ​ $ 28.5 ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative instruments liability ​ $ — ​ $ 13.9 ​ $ 1.4 ​ $ 15.3 ​ Total ​ $ — ​ $ 13.9 ​ $ 1.4 ​ $ 15.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Assets: ​ ​ ​ ​ ​ Cash and cash equivalents ​ $ 38.8 ​ $ — ​ $ — ​ $ 38.8 ​ Restricted cash ​ 7.1 ​ — ​ — ​ 7.1 ​ Derivative instruments asset ​ — ​ 0.4 ​ — ​ 0.4 ​ Total ​ $ 45.9 ​ $ 0.4 ​ $ — ​ $ 46.3 ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative instruments liability ​ $ — ​ $ 17.6 ​ $ 1.5 ​ $ 19.1 ​ Total ​ $ — ​ $ 17.6 ​ $ 1.5 ​ $ 19.1 ​
Schedule of FVM of conversion option derivative	​ ​ ​ ​ ​ ​ Fair value ​ ​ Three Months Ended ​ ​ March 31, 2021 ​ ​ ​ ​ Beginning balance of liability at January 1, 2021 ​ $ 1.5 Total unrealized gain ​ (0.1) Currency translation (gain) / loss ​ — Ending balance of liability at March 31, 2021 ​ $ 1.4 ​ ​ ​ ​ ​ ​ Fair value ​ ​ Three Months Ended ​ ​ March 31, 2020 ​ ​ ​ ​ Beginning balance of liability at January 1, 2020 ​ $ 3.2 Total unrealized loss ​ ​ 2.6 Currency translation gain ​ (0.2) Ending balance of liability at March 31, 2020 ​ $ 5.6

Accounting for derivative ins_2

Accounting for derivative instruments and hedging activities (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting for derivative instruments and hedging activities
Schedule of notional volumes of forecasted transactions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ ​ Units ​ 2021 ​ 2020 Natural gas swaps Natural Gas (MMBtu) 11.3 12.4 ​ Gas purchase agreements Natural Gas (Gigajoules) 4.2 4.0 ​ Interest rate swaps Interest (US$) 298.7 122.3 ​
Schedule of fair value of derivative instruments	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, 2021 ​ ​ Derivative ​ Derivative ​ ​ Assets ​ Liabilities Derivative instruments designated as cash flow hedges: ​ ​ ​ Interest rate swaps current ​ $ — ​ $ 0.6 ​ Interest rate swaps long-term ​ — ​ 0.7 ​ Total derivative instruments designated as cash flow hedges ​ — ​ 1.3 ​ Derivative instruments not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ Interest rate swaps current ​ — ​ 4.3 ​ Interest rate swaps long-term ​ — ​ — ​ Natural gas swaps current ​ — ​ — ​ Natural gas swaps long-term ​ — ​ 1.5 ​ Gas purchase agreements current ​ — ​ 3.6 ​ Gas purchase agreements long-term ​ — ​ 3.2 ​ Convertible debenture conversion option ​ ​ — ​ ​ 1.4 ​ Total derivative instruments not designated as cash flow hedges ​ — ​ 14.0 ​ Total derivative instruments ​ $ — ​ $ 15.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ Derivative ​ Derivative ​ ​ Assets ​ Liabilities Derivative instruments designated as cash flow hedges: ​ ​ ​ Interest rate swaps current ​ $ — ​ $ 0.6 ​ Interest rate swaps long-term ​ — ​ 1.0 ​ Total derivative instruments designated as cash flow hedges ​ — ​ 1.6 ​ Derivative instruments not designated as cash flow hedges: ​ ​ ​ ​ ​ ​ ​ Interest rate swaps current ​ — ​ 4.1 ​ Interest rate swaps long-term ​ — ​ 0.9 ​ Natural gas swaps current ​ — ​ 0.8 ​ Natural gas swaps long-term ​ — ​ 1.9 ​ Gas purchase agreements current ​ 0.4 ​ 4.0 ​ Gas purchase agreements long-term ​ — ​ 4.3 ​ Convertible debenture conversion option ​ ​ — ​ ​ 1.5 ​ Total derivative instruments not designated as cash flow hedges ​ 0.4 ​ 17.5 ​ Total derivative instruments ​ $ 0.4 ​ $ 19.1 ​
Schedule of changes in OCI attributable to derivative financial instruments designated as cash flow hedges	​ ​ ​ ​ ​ ​ ​ Interest Rate Three Months Ended March 31, 2021 Swaps Accumulated OCI balance at January 1, 2021 ​ $ 1.6 Change in fair value of cash flow hedges ​ 0.1 Realized from OCI during the period ​ — Accumulated OCI balance at March 31, 2021 ​ $ 1.7 ​ ​ ​ ​ ​ ​ Interest Rate Three Months Ended March 31, 2020 Swaps Accumulated OCI balance at January 1, 2020 ​ $ 1.6 Change in fair value of cash flow hedges ​ (0.4) Realized from OCI during the period ​ 0.1 Accumulated OCI balance at March 31, 2020 ​ $ 1.3
Summary of realized loss (gain) for derivative instruments not designated as cash flow hedges	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Classification of loss Three Months Ended March 31, ​ ​ recognized in income 2021 ​ 2020 Gas purchase agreements ​ Fuel $ 2.2 $ 2.2 ​ Natural gas swaps ​ Fuel ​ 0.1 ​ 0.7 ​ Interest rate swaps ​ Interest, net ​ 0.5 ​ 0.3 ​
Summary of the unrealized (loss) gain resulting from changes in the fair value of derivative financial instruments that are not designated as cash flow hedges	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Classification of gain (loss) ​ Three Months Ended March 31, ​ ​ recognized in income ​ 2021 2020 Natural gas swaps ​ Change in fair value of derivative instruments $ 1.2 ​ $ (0.5) ​ Gas purchase agreements ​ Change in fair value of derivative instruments ​ 1.3 ​ 1.0 ​ Interest rate swaps ​ Change in fair value of derivative instruments ​ 0.7 ​ (6.1) ​ ​ ​ ​ ​ $ 3.2 ​ $ (5.6) ​ Convertible debenture conversion option ​ Other (income) expense, net ​ $ (0.1) ​ $ 2.6 ​

Income taxes (Tables)

Income taxes (Tables)	3 Months Ended
	Mar. 31, 2021
Income taxes
Schedule of components of income tax expense	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ Current income tax expense $ 1.9 $ 1.2 ​ Deferred income tax expense ​ 0.3 ​ 0.3 ​ Total income tax expense, net ​ $ 2.2 ​ $ 1.5 ​

Equity compensation plans (Tabl

Equity compensation plans (Tables)	3 Months Ended
	Mar. 31, 2021
Equity compensation plans
Schedule of changes in outstanding LTIP notional units	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ ​ ​ Weighted-Average ​ Notional Shares ​ ​ ​ ​ ​ Fair Value per Notional Share Outstanding at December 31, 2020 3,710,340 ​ ​ ​ ​ $ 2.45 ​ Vested and redeemed (1,836,821) ​ ​ ​ ​ ​ 2.36 ​ Forfeitures (224,678) ​ ​ ​ ​ ​ 2.46 ​ Outstanding at March 31, 2021 1,648,841 ​ ​ ​ ​ $ 2.56 ​

Basic and diluted earnings pe_2

Basic and diluted earnings per share (Tables)	3 Months Ended
	Mar. 31, 2021
Basic and diluted earnings per share
Schedule of diluted net income and potentially dilutive shares utilized in the per share calculation	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, Basic 2021 2020 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income attributable to Atlantic Power Corporation ​ $ (0.1) ​ $ 29.5 Denominator: ​ ​ ​ ​ ​ ​ Weighted average basic shares outstanding ​ 89.4 ​ 107.2 Basic (loss) earnings per share attributable to Atlantic Power Corporation ​ $ (0.00) ​ $ 0.28 Diluted ​ ​ ​ ​ ​ ​ Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income attributable to Atlantic Power Corporation ​ $ (0.1) ​ $ 29.5 Add: convertible debenture interest expense ​ ​ — ​ ​ 0.9 ​ ​ ​ (0.1) ​ ​ 30.4 Denominator: ​ ​ ​ ​ ​ ​ Weighted average basic shares outstanding ​ 89.4 ​ 107.2 Share-based compensation ​ — ​ 0.2 Convertible debentures ​ — ​ 27.4 ​ ​ 89.4 ​ 134.8 Diluted (loss) earnings per share attributable to Atlantic Power Corporation ​ $ (0.00) ​ $ 0.23
Summary of outstanding instruments that are anti-dilutive and not included in the computation of our diluted earnings (loss) per share	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ 2021 2020 Share-based compensation ​ ​ 0.6 ​ ​ — Convertible debentures ​ ​ 27.4 ​ ​ — Total ​ ​ 28.0 ​ ​ —

Equity (Tables)

Equity (Tables)	3 Months Ended
	Mar. 31, 2021
Equity
Schedule of reconciliation of equity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Total Preferred ​ ​ ​ ​ ​ ​ ​ ​ ​ Other ​ Atlantic Power ​ Shares of a ​ ​ ​ ​ Common ​ Retained ​ Comprehensive ​ Corporation ​ Subsidiary ​ Total ​ ​ Shares ​ Deficit ​ (loss) income ​ Shareholders' Deficit ​ Company ​ Equity Balance at January 1, 2021 $ 1,219.7 ​ $ (1,090.0) ​ $ (139.9) ​ $ (10.2) ​ $ 168.8 ​ $ 158.6 ​ Net (loss) income — ​ (0.1) ​ — ​ (0.1) ​ 1.7 ​ 1.6 ​ Share-based compensation ​ 0.1 ​ ​ — ​ ​ — ​ ​ 0.1 ​ ​ — ​ ​ 0.1 ​ Dividends on preferred shares of a subsidiary company - Series 1 (Cdn$0.303125 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.8) ​ ​ (0.8) ​ Dividends on preferred shares of a subsidiary company - Series 2 (Cdn$0.358688 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.7) ​ ​ (0.7) ​ Dividends on preferred shares of a subsidiary company - Series 3 (Cdn$0.267807 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.2) ​ ​ (0.2) ​ Realized and unrealized loss on hedging activities, net of tax — ​ — ​ 0.1 ​ 0.1 ​ — ​ 0.1 ​ Foreign currency translation adjustments — ​ — ​ 1.7 ​ 1.7 ​ — ​ 1.7 ​ Balance at March 31, 2021 $ 1,219.8 ​ $ (1,090.1) ​ $ (138.1) ​ $ (8.4) ​ $ 168.8 ​ $ 160.4 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Total Preferred ​ ​ ​ ​ ​ ​ ​ ​ ​ Other ​ Atlantic Power ​ Shares of a ​ ​ ​ ​ Common ​ Retained ​ Comprehensive ​ Corporation ​ Subsidiary ​ Total ​ ​ Shares ​ Deficit ​ (loss) income ​ Shareholders' Deficit ​ Company ​ Equity Balance at January 1, 2020 $ 1,259.9 ​ $ (1,164.2) ​ $ (140.7) ​ $ (45.0) ​ $ 182.7 ​ $ 137.7 ​ Net income (loss) — ​ 29.5 ​ — ​ 29.5 ​ (5.8) ​ 23.7 ​ Share-based compensation ​ 0.4 ​ ​ — ​ ​ — ​ ​ 0.4 ​ ​ — ​ ​ 0.4 ​ Common share repurchases (8.2) ​ — ​ — ​ (8.2) ​ — ​ (8.2) ​ Preferred share repurchases ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (6.4) ​ ​ (6.4) ​ Dividends on preferred shares of a subsidiary company - Series 1 (Cdn$0.303125 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.8) ​ ​ (0.8) ​ Dividends on preferred shares of a subsidiary company - Series 2 (Cdn$0.358688 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.6) ​ ​ (0.6) ​ Dividends on preferred shares of a subsidiary company - Series 3 (Cdn$0.363342 per share) ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ (0.3) ​ ​ (0.3) ​ Realized and unrealized loss on hedging activities, net of tax — ​ — ​ (0.3) ​ (0.3) ​ — ​ (0.3) ​ Foreign currency translation adjustments — ​ — ​ (11.2) ​ (11.2) ​ — ​ (11.2) ​ Balance at March 31, 2020 $ 1,252.1 ​ $ (1,134.7) ​ $ (152.2) ​ $ (34.8) ​ $ 168.8 ​ $ 134.0 ​ ​

Segment and geographic inform_2

Segment and geographic information (Tables)	3 Months Ended
	Mar. 31, 2021
Segment and geographic information
Reconciliation of Project Adjusted EBITDA to net income (loss) from continuing operations	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Consolidated Three Months Ended March 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenues ​ $ 26.9 ​ $ 31.7 ​ $ 13.1 ​ $ 0.3 ​ $ 72.0 ​ Segment assets ​ 205.6 ​ ​ 190.3 ​ ​ 301.4 ​ ​ 126.6 ​ 823.9 ​ Project Adjusted EBITDA ​ $ 9.7 ​ $ 28.7 ​ $ 10.3 ​ $ (0.5) ​ $ 48.2 ​ Change in fair value of derivative instruments ​ — ​ ​ (2.5) ​ ​ — ​ ​ (0.7) ​ ​ (3.2) ​ Depreciation and amortization ​ 6.1 ​ ​ 7.6 ​ ​ 4.9 ​ ​ — ​ ​ 18.6 ​ Interest, net ​ 0.6 ​ ​ — ​ ​ — ​ ​ — ​ 0.6 ​ Other project income ​ — ​ — ​ — ​ (0.2) ​ (0.2) ​ Project income ​ ​ 3.0 ​ ​ 23.6 ​ ​ 5.4 ​ ​ 0.4 ​ 32.4 ​ Administration ​ — ​ — ​ — ​ 14.1 ​ 14.1 ​ Interest expense, net ​ — ​ — ​ — ​ 11.4 ​ 11.4 ​ Foreign exchange loss ​ — ​ — ​ — ​ 3.2 ​ ​ 3.2 ​ Other income, net ​ — ​ — ​ — ​ (0.1) ​ ​ (0.1) ​ Net income (loss) before income taxes ​ 3.0 ​ 23.6 ​ 5.4 ​ (28.2) ​ 3.8 ​ Income tax expense ​ — ​ — ​ — ​ 2.2 ​ 2.2 ​ Net income (loss) ​ $ 3.0 ​ $ 23.6 ​ $ 5.4 ​ $ (30.4) ​ $ 1.6 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Solid Fuel ​ Natural Gas ​ Hydroelectric ​ Corporate ​ Consolidated Three Months Ended March 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenues ​ $ 23.4 ​ $ 30.8 ​ $ 18.4 ​ $ 0.2 ​ $ 72.8 ​ Segment assets ​ 225.0 ​ ​ 230.6 ​ ​ 359.5 ​ ​ 66.9 ​ 882.0 ​ Project Adjusted EBITDA ​ $ 7.8 ​ $ 28.2 ​ $ 15.3 ​ $ (0.5) ​ $ 50.8 ​ Change in fair value of derivative instruments ​ — ​ ​ (0.5) ​ ​ — ​ 6.1 ​ 5.6 ​ Depreciation and amortization ​ 5.6 ​ ​ 9.2 ​ ​ 4.9 ​ 0.1 ​ 19.8 ​ Interest, net ​ 0.7 ​ ​ — ​ ​ — ​ ​ — ​ 0.7 ​ Other project expense ​ — ​ — ​ — ​ — ​ — ​ Project income (loss) ​ 1.5 ​ ​ 19.5 ​ ​ 10.4 ​ ​ (6.7) ​ 24.7 ​ Administration ​ — ​ — ​ — ​ 6.7 ​ 6.7 ​ Interest expense, net ​ — ​ — ​ — ​ 10.8 ​ 10.8 ​ Foreign exchange gain ​ — ​ — ​ — ​ (20.6) ​ (20.6) ​ Other expense, net ​ — ​ — ​ — ​ 2.6 ​ 2.6 ​ Net income (loss) before income taxes ​ 1.5 ​ 19.5 ​ 10.4 ​ (6.2) ​ ​ 25.2 ​ Income tax expense ​ — ​ — ​ — ​ 1.5 ​ ​ 1.5 ​ Net income (loss) ​ $ 1.5 ​ $ 19.5 ​ $ 10.4 ​ $ (7.7) ​ $ 23.7 ​ ​
Schedule of revenue and property, plant and equipment by country	The tables below provide information, by country, about our consolidated operations for the three months ended March 31, 2021 and 2020 and for Property, Plant and Equipment, PPAs and other intangible assets and total assets as of March 31, 2021 and December 31, 2020, respectively. Revenue is recorded in the country in which it is earned and assets are recorded in the country in which they are located. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Project revenue ​ ​ Three Months Ended March 31, ​ ​ 2021 ​ 2020 United States $ 46.1 $ 48.2 Canada ​ 25.9 ​ 24.6 Total ​ $ 72.0 ​ $ 72.8 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Property, Plant and ​ PPAs and ​ ​ ​ ​ Equipment, net of ​ other intangible assets, net of ​ ​ ​ ​ accumulated depreciation ​ accumulated amortization ​ Total assets ​ March 31, 2021 December 31, 2020 March 31, 2021 December 31, 2020 March 31, 2021 December 31, 2020 United States $ 345.0 ​ $ 351.2 ​ $ 114.0 ​ $ 119.4 ​ $ 644.0 ​ $ 666.4 Canada ​ 136.3 ​ 140.6 ​ 0.8 ​ 0.9 ​ 179.9 ​ 180.8 Total ​ $ 481.3 ​ $ 491.8 ​ $ 114.8 ​ $ 120.3 ​ $ 823.9 ​ $ 847.2

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2021
Leases
Schedule of operating lease information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ ​ ​ 2021 ​ ​ 2020 Lease cost: (1) ​ ​ ​ ​ ​ Operating lease cost ​ $ 0.5 ​ $ 0.5 Sublease income ​ ​ (0.3) ​ ​ (0.3) Total lease cost ​ $ 0.2 ​ $ 0.2 (1)
Schedule of other information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Lease ​ Income from ​ Net lease ​ ​ Payments subleasing payments ​ 2021 ​ $ 1.5 ​ $ (0.8) ​ $ 0.7 ​ 2022 ​ ​ 1.8 ​ ​ (1.1) ​ ​ 0.7 ​ 2023 ​ ​ 1.3 ​ ​ (0.7) ​ ​ 0.6 ​ 2024 ​ ​ 0.2 ​ ​ — ​ ​ 0.2 ​ 2025 ​ ​ — ​ ​ — ​ ​ — ​ Thereafter ​ ​ — ​ ​ — ​ ​ — ​ Total operating lease payments ​ $ 4.8 ​ $ (2.6) ​ $ 2.2 ​ Less: present value discount ​ ​ (0.3) ​ ​ ​ ​ ​ ​ ​ Total operating lease liabilities ​ $ 4.5 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Lease ​ ​ ​ ​ ​ ​ ​ ​ Payments ​ ​ ​ ​ ​ 2021 ​ $ — ​ ​ ​ ​ ​ ​ ​ 2022 ​ ​ 0.1 ​ ​ ​ ​ ​ ​ 2023 ​ ​ — ​ ​ ​ ​ ​ ​ Thereafter ​ ​ — ​ ​ ​ ​ ​ ​ ​ Total finance lease payments ​ $ 0.1 ​ ​ ​ ​ ​ ​ ​ Less: amount representing interest ​ ​ — ​ ​ ​ ​ ​ ​ ​ Total finance lease liabilities ​ $ 0.1 ​ ​ ​ ​ ​ ​ ​ (1) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ Other Information: ​ ​ ​ ​ ​ ​ ​ Cash paid for amounts included in the measurement of lease liabilities (1) ​ ​ ​ ​ ​ ​ ​ Operating cash flows from operating leases ​ $ 0.3 ​ $ 0.3 ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average remaining lease term (in years): ​ ​ ​ ​ ​ ​ ​ Operating leases ​ ​ 2.5 ​ ​ 2.5 ​ Finance leases ​ ​ 1.1 ​ ​ 1.4 ​ Weighted average discount rate - operating leases ​ ​ 3.9 % ​ 3.9 % Weighted average discount rate - finance leases ​ ​ 4.1 % ​ 4.1 %
Schedule of rental income from operating leases	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Rental Income from operating leases ​ ​ ​ Three Months Ended ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ Solid Fuel ​ $ 14.8 ​ $ 16.6 ​ ​ ​ ​ ​ ​ ​ ​ ​ Natural Gas ​ ​ 5.6 ​ ​ 5.9 ​ ​ ​ ​ ​ ​ ​ ​ ​ Hydroelectric ​ ​ 13.1 ​ ​ 18.4 ​ ​ ​ $ 33.5 ​ $ 40.9 ​

Nature of business (Details)

Nature of business (Details) $ / shares in Units, $ in Millions	Jan. 14, 2021USD ($)$ / shares	Mar. 31, 2021stateprojectDregion$ / sharesMW
Nature of business
Number of states in which power generation projects operate | state		11
Number of provinces in which power generation projects operate | region		2
Number of power generation projects | project		21
Gross generating capacity of project (in MW) | MW		1,723
Ownership interest in power generation projects (in MW) | MW		1,327
Number of projects which are majority owned | project		16
I Squared Capital Advisors (US) LLC [Member] | Atlantic Power Corp [Member]
Nature of business
Total enterprise value | $	$ 961
Cash consideration per common stock	$ 3.03
Repurchase price per share of cumulative redeemable preferred shares	$ 22
I Squared Capital Advisors (US) LLC [Member] | Atlantic Power Corp [Member] | Senior unsecured notes, due June 2036
Nature of business
Convertible debentures stated interest rate percentage	5.95%
Redemption price percentage of principal amount	106.071%
Consent fee expressed as a percentage of principal amount of debt	0.25%
I Squared Capital Advisors (US) LLC [Member] | Atlantic Power Corp [Member] | 6.00% Debenture Due January 31, 2025
Nature of business
Convertible debentures stated interest rate percentage	6.00%	6.00%
Transaction period | D		10
Conversion period		30 days
Conversion price of shares (in dollars per share)		$ 3.72

Assets and liabilities held f_3

Assets and liabilities held for sale (Details) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)	Feb. 23, 2021CAD ($)
Assets
Total current assets	$ 4.6
Liabilities
Total current liabilities	3.2
Assets and liabilities held for sale | North Bay and Kapuskasing 40MW projects
Assets and liabilities held for sale
Cash Consideration		$ 3
Long lived asset impairment charge	0
Assets
Inventory	1.1
Property, plant, and equipment, net	3.5
Total current assets	4.6
Liabilities
Asset retirement obligations	3.2
Total current liabilities	$ 3.2

Revenue from contracts - Disagg

Revenue from contracts - Disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of revenue
Project revenue	$ 72	$ 72.8
Solid Fuel
Disaggregation of revenue
Project revenue	26.9	23.4
Natural Gas
Disaggregation of revenue
Project revenue	31.7	30.8
Hydroelectric
Disaggregation of revenue
Project revenue	13.1	18.4
Corporate
Disaggregation of revenue
Project revenue	0.3	0.2
Energy sales
Disaggregation of revenue
Project revenue	36	40.7
Energy sales | Solid Fuel
Disaggregation of revenue
Project revenue	18.2	16.8
Energy sales | Natural Gas
Disaggregation of revenue
Project revenue	5.6	6.2
Energy sales | Hydroelectric
Disaggregation of revenue
Project revenue	12.2	17.7
Energy capacity revenue
Disaggregation of revenue
Project revenue	29.8	28
Energy capacity revenue | Solid Fuel
Disaggregation of revenue
Project revenue	8.1	6.3
Energy capacity revenue | Natural Gas
Disaggregation of revenue
Project revenue	21.7	21.7
Other
Disaggregation of revenue
Project revenue	6.2	4.1
Steam energy and capacity revenue
Disaggregation of revenue
Project revenue	3.2	2.7
Steam energy and capacity revenue | Natural Gas
Disaggregation of revenue
Project revenue	3.2	2.7
Waste heat revenue
Disaggregation of revenue
Project revenue	0.6	0.3
Waste heat revenue | Solid Fuel
Disaggregation of revenue
Project revenue	0.6	0.3
Ancillary and transmission services
Disaggregation of revenue
Project revenue	2	1.4
Ancillary and transmission services | Natural Gas
Disaggregation of revenue
Project revenue	1	0.7
Ancillary and transmission services | Hydroelectric
Disaggregation of revenue
Project revenue	1	0.7
Asset management and operation
Disaggregation of revenue
Project revenue	0.3	0.2
Asset management and operation | Corporate
Disaggregation of revenue
Project revenue	0.3	0.2
Miscellaneous revenue
Disaggregation of revenue
Project revenue	0.1	(0.5)
Miscellaneous revenue | Natural Gas
Disaggregation of revenue
Project revenue	0.2	$ (0.5)
Miscellaneous revenue | Hydroelectric
Disaggregation of revenue
Project revenue	$ (0.1)

Revenue from contracts - Contra

Revenue from contracts - Contract Balances (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contract assets	$ 0
Dorchester
Disaggregation of Revenue [Line Items]
Contract liabilities	0.2	$ 0.2
San Diego Projects
Disaggregation of Revenue [Line Items]
Contract liabilities	0.1	$ 0.1
Mamquam
Disaggregation of Revenue [Line Items]
Contract liabilities	$ 0.3

Changes in accumulated other _3

Changes in accumulated other comprehensive loss by component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax
Beginning Balance	$ (10.2)
Net amount reclassified to earnings:
Ending Balance	(8.4)
Foreign currency translation
AOCI Attributable to Parent, Net of Tax
Beginning Balance	(138.4)	$ (140.6)
Other comprehensive income (loss):
Foreign currency translation adjustments	1.7	(11.2)
Net amount reclassified to earnings:
Ending Balance	(136.7)	(151.8)
Accumulated other comprehensive loss, tax impacts related to rate changes	0	0
Pension
AOCI Attributable to Parent, Net of Tax
Beginning Balance	(3.1)	(1.7)
Cash flow hedges
AOCI Attributable to Parent, Net of Tax
Beginning Balance	1.6	1.6
Other comprehensive income (loss):
Net change from periodic revaluations	0.2	(0.6)
Tax (expense) benefit	(0.1)	0.2
Total other comprehensive income (loss) before reclassifications, net of tax	0.1	(0.4)
Net amount reclassified to earnings:
Interest rate swaps		0.1
Total amount reclassified from accumulated other comprehensive income, net of tax		0.1
Total other comprehensive income (loss)	0.1	(0.3)
Ending Balance	1.7	1.3
Amount reclassified from accumulated other comprehensive income | Foreign currency translation
Net amount reclassified to earnings:
Amounts reclassified to earnings, foreign currency	$ 0	$ 0

Equity method investments in _3

Equity method investments in unconsolidated affiliates - Op Results (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating results
Project revenues	$ 72	$ 72.8
Net income	1.6	23.7
Equity in earnings of unconsolidated affiliates	13.4	13.7
Distributions from equity method investments	6.9	6
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Operating results
Project revenues	38.3	37.8
Project expenses	24.4	23.8
Project other income	(0.2)	(0.3)
Equity in earnings of unconsolidated affiliates	13.7	13.7
Distributions from equity method investments	(6.9)	(6)
Earnings of equity method investments, net of distributions	6.8	7.7
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Frederickson
Operating results
Project revenues	7.3	8
Project expenses	4.8	5.4
Net income	2.5	2.6
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Orlando Cogen, LP
Operating results
Project revenues	15.5	14.6
Project expenses	7.2	6.3
Net income	8.3	8.3
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Chambers Cogen, LP
Operating results
Project revenues	10.8	10.6
Project expenses	8.2	8.1
Project other income	(0.2)	(0.3)
Net income	2.4	2.2
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Craven County Wood Energy, LP
Operating results
Project revenues	3.1	3.1
Project expenses	3	2.6
Net income	0.1	0.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Grayling Generating Station, LP
Operating results
Project revenues	1.6	1.5
Project expenses	1.2	1.4
Net income	$ 0.4	$ 0.1

Long-term debt (Details)

Long-term debt (Details) $ in Millions, $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2020USD ($)	Mar. 31, 2021USD ($)	Mar. 31, 2021CAD ($)	Dec. 31, 2020USD ($)	Mar. 31, 2020USD ($)
Long-term debt
Less: unamortized discount		$ (3)		$ (3.5)
Less: unamortized deferred financing costs		(3.5)		(3.9)
Less: current maturities		(90.8)		(95.7)
Total long-term debt		$ 368.3		384.1
Senior secured term loan facility, due 2025 | APLP Holdings
Long-term debt
Debt repriced	$ 380
Interest rate margin reduction due to repricing, as a percent	0.25%
Interest rate margin additional reduction due to repricing, as a percent	0.25%
Leverage ratio as defined in Credit Agreement	2.75
Term loan maturity	2 years
Senior unsecured notes, due June 2036
Long-term debt
Long-term Debt, Gross			$ 210
Interest rate swaps | Senior secured term loan facility, due 2025
Long-term debt
Long-term Debt, Gross					$ 284.5
London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2025
Long-term debt
Minimum percentage of variable rate base		1.00%
London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2025 | APLP Holdings
Long-term debt
Applicable margin (as a percent)	2.50%
Minimum percentage of variable rate base	1.00%
Long-term debt excluding debentures | Senior secured term loan facility, due 2025
Long-term debt
Long-term Debt, Gross		$ 284.5		307.5
Less: current maturities		(88)		(93)
Long-term debt excluding debentures | Senior unsecured notes, due June 2036
Long-term debt
Long-term Debt, Gross		$ 167		164.9
Interest rate (as a percent)		5.95%	5.95%
Long-term debt excluding debentures | Cadillac term loan, due 2025
Long-term debt
Long-term Debt, Gross		$ 14.1		14.8
Less: current maturities		$ (2.8)		$ (2.7)
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2025
Long-term debt
Applicable margin (as a percent)		2.50%
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Cadillac term loan, due 2025
Long-term debt
Applicable margin (as a percent)		1.61%

Long-term debt - Credit Facilit

Long-term debt - Credit Facilities (Details) $ in Millions	Aug. 24, 2020USD ($)	Mar. 31, 2021USD ($)Lender	Mar. 18, 2020USD ($)	Mar. 17, 2020USD ($)
Debt Instrument [Line Items]
Availability of future shelf registration period	3 years
Shelf Registration authorized amount	$ 250
APLP Holdings | Letters of credit
Debt Instrument [Line Items]
Outstanding balance of senior credit facility		$ 81.2
APLP Holdings | Revolver
Debt Instrument [Line Items]
Outstanding balance of senior credit facility		$ 0
Borrowing capacity			$ 180	$ 200
Number Of Banks | Lender		2
APLP Holdings | Revolver | Maximum
Debt Instrument [Line Items]
Upsizing borrowing capacity			30
Borrowing capacity			$ 210

Fair value of financial instr_3

Fair value of financial instruments - Recurring (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets:
Cash and cash equivalents	$ 21.6	$ 38.8
Restricted cash	6.9	7.1
Derivative instruments asset		0.4
Total	28.5	46.3
Liabilities:
Derivative instruments liability	15.3	19.1
Total	15.3	19.1
Level 1
Assets:
Cash and cash equivalents	21.6	38.8
Restricted cash	6.9	7.1
Total	28.5	45.9
Level 2
Assets:
Derivative instruments asset		0.4
Total		0.4
Liabilities:
Derivative instruments liability	13.9	17.6
Total	13.9	17.6
Level 3
Liabilities:
Derivative instruments liability	1.4	1.5
Total	$ 1.4	$ 1.5

Fair value of financial instr_4

Fair value of financial instruments - Credit Valuation (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Asset acquisitions
Gain in change in fair value of derivative instruments	$ 3.2	$ (5.6)
Recurring | Credit valuation adjustments
Asset acquisitions
Net increase in fair value	0.3		$ 0.5
Pre-tax gain in other comprehensive income	0.1		0.1
Gain in change in fair value of derivative instruments	$ 0.2		$ 0.4

Fair value of financial instr_5

Fair value of financial instruments - Conversion Option Derivative (Details) - Derivative Financial Instruments, Assets - Level 3 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance of liability	$ 1.5	$ 3.2
Total unrealized gain (loss)	(0.1)	2.6
Currency translation (gain) / loss		(0.2)
Ending balance of liability	$ 1.4	$ 5.6

Accounting for derivative ins_3

Accounting for derivative instruments and hedging activities (Details) MMBTU in Millions, $ in Millions, kJ in Billions	3 Months Ended	12 Months Ended
	Mar. 31, 2021USD ($)kJMMBTUcontractitem$ / J	Dec. 31, 2020USD ($)MMBTU
Derivative instruments designated as cash flow hedges
Derivative instruments
Number of derivative contracts designated as cash flow hedges | item	1
Adjusted Eurodollar Rate
Derivative instruments
Applicable margin (as a percent)	2.50%
Nipigon Project
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) | kJ	6,500
Gas purchase agreements | Nipigon Project
Derivative instruments
Price per Gj | $ / J	4.57
Gas purchase agreements | Nipigon Project | Minimum
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) | kJ	6,500
Natural gas swaps
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU	11.3	12.4
Natural gas swaps | Orlando project
Derivative instruments
Amount of future natural gas purchases to fix the price (in Mmbtu) | MMBTU	11.3
Natural gas swaps during 2016 | Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge	100.00%
Interest rate swaps
Derivative instruments
Notional amount	$ 298.7	$ 122.3
Derivative, Average Swaption Interest Rate	1.92%
Swaption interest rate (as a percent)	4.42%
Interest rate swaps | Adjusted Eurodollar Rate
Derivative instruments
Derivative, Floor Interest Rate	1.00%
Interest rate swaps | Minimum
Derivative instruments
Minimum fixed interest rate (as a percent)	3.50%
Interest rate swaps | Swaption interest rate until February 15, 2023 | Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)	6.30%
Interest rate swaps | Swaption interest rate after February 15, 2023 | Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)	6.40%
Foreign currency forward contracts | Derivative instruments not designated as cash flow hedges
Derivative instruments
Number of forward contracts | contract	0
APLP Holdings | Senior secured term loans
Derivative instruments
Face amount of debt	$ 700
APLP Holdings | Interest rate swaps | Adjusted Eurodollar Rate | Senior secured term loans | Long-term debt excluding debentures
Derivative instruments
Notional amount	284.5
Remaining aggregate principal amount	$ 284.5

Accounting for derivative ins_4

Accounting for derivative instruments and hedging activities - Forecasted Transactions (Details) kJ in Millions, MMBTU in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021USD ($)MMBTUkJ	Dec. 31, 2020USD ($)MMBTUkJ
Natural gas swaps
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU	11.3	12.4
Gas purchase agreements
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | kJ	4.2	4
Interest rate swaps
Derivative instruments
Volume of forecasted transactions, (in dollars) | $	$ 298.7	$ 122.3

Accounting for derivative ins_5

Accounting for derivative instruments and hedging activities - FV by Bal Sht Loc (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair value of derivative instruments
Derivative Assets		$ 0.4
Derivative Liabilities	$ 15.3	19.1
Derivative instruments designated as cash flow hedges
Fair value of derivative instruments
Derivative Liabilities	1.3	1.6
Derivative instruments designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Liabilities	0.6	0.6
Derivative instruments designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	0.7	1
Derivative instruments not designated as cash flow hedges
Fair value of derivative instruments
Derivative Assets		0.4
Derivative Liabilities	14	17.5
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Liabilities	4.3	4.1
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities		0.9
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Current
Fair value of derivative instruments
Derivative Liabilities		0.8
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	1.5	1.9
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Current
Fair value of derivative instruments
Derivative Assets		0.4
Derivative Liabilities	3.6	4
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Long-term
Fair value of derivative instruments
Derivative Liabilities	3.2	4.3
Derivative instruments not designated as cash flow hedges | Convertible Debentures | Current
Fair value of derivative instruments
Derivative Liabilities	$ 1.4	$ 1.5

Accounting for derivative ins_6

Accounting for derivative instruments and hedging activities - AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax
Change in fair value of cash flow hedges	$ 0.1	$ (0.4)
Realized from OCI during the period	0	(0.1)
Interest rate swaps
AOCI Attributable to Parent, Net of Tax
Accumulated OCI balance at beginning of period	1.6	1.6
Change in fair value of cash flow hedges	0.1	(0.4)
Realized from OCI during the period	0	0.1
Accumulated OCI balance at end of period	$ 1.7	$ 1.3

Accounting for derivative ins_7

Accounting for derivative instruments and hedging activities - Gain, Loss (Details) - Derivative instruments not designated as cash flow hedges - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	$ 3.2	$ (5.6)
Gas purchase agreements
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	2.2	2.2
Gas purchase agreements | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	1.3	1
Natural gas swaps
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	0.1	0.7
Natural gas swaps | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	1.2	(0.5)
Interest rate swaps
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	0.5	0.3
Interest rate swaps | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	0.7	(6.1)
Convertible Debentures | Other (income) expense, net
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	$ (0.1)	$ 2.6

Income taxes (Details)

Income taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Components of income tax expenses (benefit)
Current income tax expense	$ 1.9	$ 1.2
Deferred income tax expense	0.3	0.3
Total income tax expense, net	$ 2.2	$ 1.5

Income tax expense - Other (Det

Income tax expense - Other (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reconciliation of income taxes
Tax expense	$ 2.2	$ 1.5
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	1	6.8
Other permanent differences	0.7	1.4
Valuation allowance	105.5
Increase (decrease) in the valuation allowance		(8.7)
Foreign exchange		2
Dividend withholding tax and other cash taxes	$ 0.5
US
Reconciliation of income taxes
Increase (decrease) in the valuation allowance		$ (2.8)
Canada.
Reconciliation of income taxes
Enacted statutory income tax rate	27.00%	27.00%
Increase (decrease) in the valuation allowance		$ (5.9)

Equity compensation plans (Deta

Equity compensation plans (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares	Mar. 31, 2020USD ($)	Mar. 31, 2021$ / shares
Grant Date Weighted-Average Fair Value per Notional Share
Compensation expense | $	$ 1.4	$ 1
Long-term incentive plan
Notional Shares
Outstanding at the beginning of the period (in shares) | shares	3,710,340
Vested and redeemed (in shares) | shares	(1,836,821)
Forfeitures (in shares) | shares	(224,678)
Outstanding at the end of the period (in shares) | shares	1,648,841
Grant Date Weighted-Average Fair Value per Notional Share
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 2.45
Vested and redeemed (in dollars per share) | $ / shares	2.36
Forfeitures (in dollars per share) | $ / shares	2.46
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 2.56
Cash payments made for vested notional units | $	$ 3.5	$ 3.3
Transition equity participation agreement
Transition equity participation agreement
Number of transition notional shares outstanding | shares	269,952
Minimum market price required for 3 consecutive months to vest remaining shares | $ / shares			$ 4.77
Transition equity participation agreement | Minimum
Transition equity participation agreement
Number of consecutive months, weighted average Canadian dollar price must exceed market price of share by 50% for vesting	3 months

Basic and diluted earnings pe_3

Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net (loss) income attributable to Atlantic Power Corporation	$ (0.1)	$ 29.5
Add: convertible debenture interest expense		0.9
Net (loss) income available to common stockholder, Diluted	$ (0.1)	$ 30.4
Denominator:
Weighted average basic shares outstanding	89.4	107.2
Basic (loss) earnings per share attributable to Atlantic Power Corporation	$ 0	$ 0.28
Share-based compensation (in shares)		0.2
Convertible debentures (in shares)		27.4
Weighted Average Number of Potentially Diluted Shares	89.4	134.8
Diluted (loss) earnings per share attributable to Atlantic Power Corporation	$ 0	$ 0.23
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS	$ 28
Share-based compensation.
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS	0.6
Convertible Debentures
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]
Anti-dilutive instruments outstanding that were excluded from computation of diluted EPS	$ 27.4

Equity (Details)

Equity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)	Mar. 31, 2021$ / shares	Mar. 31, 2020USD ($)	Mar. 31, 2020$ / shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	$ 158.6		$ 137.7
Net (loss) income	1.6		23.7
Share-based compensation	0.1		0.4
Common share repurchases			(8.2)
Preferred share repurchases			(6.4)
Realized and unrealized loss on hedging activities, net of tax	0.1		(0.3)
Foreign currency translation adjustments	1.7		(11.2)
Balance at the end of the period	160.4		134
Series 1
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.8)		(0.8)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		$ 0.303125		$ 0.303125
Series 2
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.7)		(0.6)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		0.358688		0.358688
Series 3
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.2)		(0.3)
Dividends on preferred shares of a subsidiary company (in CAD per share) | $ / shares		$ 0.267807		$ 0.363342
Common Shares
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	1,219.7		1,259.9
Share-based compensation	0.1		0.4
Common share repurchases			(8.2)
Balance at the end of the period	1,219.8		1,252.1
Retained Deficit
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(1,090)		(1,164.2)
Net (loss) income	(0.1)		29.5
Balance at the end of the period	(1,090.1)		(1,134.7)
Accumulated Other Comprehensive (loss) income
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(139.9)		(140.7)
Realized and unrealized loss on hedging activities, net of tax	0.1		(0.3)
Foreign currency translation adjustments	1.7		(11.2)
Balance at the end of the period	(138.1)		(152.2)
Total Atlantic Power Corporation Shareholder's Deficit
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	(10.2)		(45)
Net (loss) income	(0.1)		29.5
Share-based compensation	0.1		0.4
Common share repurchases			(8.2)
Realized and unrealized loss on hedging activities, net of tax	0.1		(0.3)
Foreign currency translation adjustments	1.7		(11.2)
Balance at the end of the period	(8.4)		(34.8)
Preferred Shares of a Subsidiary Company
Increase (decrease) in shareholders' equity
Balance at the beginning of the period	168.8		182.7
Net (loss) income	1.7		(5.8)
Preferred share repurchases			(6.4)
Balance at the end of the period	168.8		168.8
Preferred Shares of a Subsidiary Company | Series 1
Increase (decrease) in shareholders' equity
Dividends on preferred shares of a subsidiary company	(0.8)		(0.8)
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)
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)

Equity - Share Repurchase Progr

Equity - Share Repurchase Program (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Stock Repurchase Program
Payments for Repurchase of Redeemable Preferred Stock		$ 6.4
Gain (loss) attributable to preferred shares of a subsidiary company	$ 1.7	$ (5.8)
Common Stock, Shares, Outstanding	89,797,798		89,222,568
NCIB
Stock Repurchase Program
Public float amount used to determine share amount, percent			10.00%
Maximum | NCIB
Stock Repurchase Program
Number of shares authorized for repurchase			8,554,391
Number of shares that may be purchased per day			10,420

Segment and geographic inform_3

Segment and geographic information - EBITDA (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)segment	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Segment and geographic information
Number of reportable segments | segment	4
Project revenues	$ 72	$ 72.8
Segment assets	823.9	882	$ 847.2
Project Adjusted EBITDA	48.2	50.8
Change in fair value of derivative instruments	(3.2)	5.6
Depreciation and amortization	18.6	19.8
Interest, net	0.6	0.7
Other project (income) expense	(0.2)
Project income	32.4	24.7
Administration	14.1	6.7
Interest expense, net	11.4	10.8
Foreign exchange (gain) loss	3.2	(20.6)
Other income (expense), net	(0.1)	2.6
Income from operations before income taxes	3.8	25.2
Income tax expense	2.2	1.5
Net income (loss)	1.6	23.7
Solid Fuel
Segment and geographic information
Project revenues	26.9	23.4
Segment assets	205.6	225
Project Adjusted EBITDA	9.7	7.8
Depreciation and amortization	6.1	5.6
Interest, net	0.6	0.7
Project income	3	1.5
Income from operations before income taxes	3	1.5
Net income (loss)	3	1.5
Natural Gas
Segment and geographic information
Project revenues	31.7	30.8
Segment assets	190.3	230.6
Project Adjusted EBITDA	28.7	28.2
Change in fair value of derivative instruments	(2.5)	(0.5)
Depreciation and amortization	7.6	9.2
Project income	23.6	19.5
Income from operations before income taxes	23.6	19.5
Net income (loss)	23.6	19.5
Hydroelectric
Segment and geographic information
Project revenues	13.1	18.4
Segment assets	301.4	359.5
Project Adjusted EBITDA	10.3	15.3
Depreciation and amortization	4.9	4.9
Project income	5.4	10.4
Income from operations before income taxes	5.4	10.4
Net income (loss)	5.4	10.4
Corporate
Segment and geographic information
Project revenues	0.3	0.2
Segment assets	126.6	66.9
Project Adjusted EBITDA	(0.5)	(0.5)
Change in fair value of derivative instruments	(0.7)	6.1
Depreciation and amortization		0.1
Other project (income) expense	(0.2)
Project income	0.4	(6.7)
Administration	14.1	6.7
Interest expense, net	11.4	10.8
Foreign exchange (gain) loss	3.2	(20.6)
Other income (expense), net	(0.1)	2.6
Income from operations before income taxes	(28.2)	(6.2)
Income tax expense	2.2	1.5
Net income (loss)	$ (30.4)	$ (7.7)

Segment and geographic inform_4

Segment and geographic information - Geographic (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Revenue and Assets
Project revenue	$ 72	$ 72.8
Property, Plant and Equipment, net of accumulated depreciation	481.3		$ 491.8
PPA'S and other intangible assets, net of accumulated amortization	114.8		120.3
Assets	823.9	882	847.2
United States
Revenue and Assets
Project revenue	46.1	48.2
Property, Plant and Equipment, net of accumulated depreciation	345		351.2
PPA'S and other intangible assets, net of accumulated amortization	114		119.4
Assets	644		666.4
Canada
Revenue and Assets
Project revenue	25.9	$ 24.6
Property, Plant and Equipment, net of accumulated depreciation	136.3		140.6
PPA'S and other intangible assets, net of accumulated amortization	0.8		0.9
Assets	$ 179.9		$ 180.8

Segment and geographic inform_5

Segment and geographic information - Risk (Details) - Revenues, net - customer concentration	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
IESO
Consolidated revenue concentration
Percentage of consolidated revenue	14.80%	14.70%
BC Hydro
Consolidated revenue concentration
Percentage of consolidated revenue	14.80%	13.50%
Niagara Mohawk
Consolidated revenue concentration
Percentage of consolidated revenue	13.40%	20.90%
Equistar Chemicals, LP
Consolidated revenue concentration
Percentage of consolidated revenue	12.10%	10.30%

Leases - Real Estate and Equipm

Leases - Real Estate and Equipment (Details) - item	3 Months Ended
	Mar. 31, 2021	Jan. 01, 2019
Operating Lease Commitments
Number of properties subleased	1
Rate used to determine lease liabilities at implementation date		3.75%

Leases - Lease Cost (Details)

Leases - Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lease cost:
Operating lease cost	$ 0.5	$ 0.5
Sublease income	(0.3)	(0.3)
Total lease cost	$ 0.2	$ 0.2

Leases - Operating Lease Maturi

Leases - Operating Lease Maturities - Topic 842 (Details) $ in Millions	Mar. 31, 2021USD ($)
Future minimum lease commitments
2021	$ 1.5
2022	1.8
2023	1.3
2024	0.2
Total operating lease payments	4.8
Less: present value discount	(0.3)
Total operating lease liabilities	4.5
Finance lease payment
2022	0.1
Total finance lease payments	0.1
Total finance lease liabilities	0.1
Income from subleasing
2021	(0.8)
2022	(1.1)
2023	(0.7)
Total income from subleasing	(2.6)
Net lease payments
2021	0.7
2022	0.7
2023	0.6
2024	0.2
Total net lease payments	$ 2.2

Leases - Operating Lease Matu_2

Leases - Operating Lease Maturities - Supplemental Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)item	Mar. 31, 2020USD ($)item
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases | $	$ 0.3	$ 0.3
Weighted average items:
Weighted average remaining lease term (in years), Operating leases	2 years 6 months	2 years 6 months
Weighted average remaining lease term (in years), Finance leases	1 year 1 month 6 days	1 year 4 months 24 days
Weighted average discount rate - operating leases	3.90%	3.90%
Weighted average discount rate - finance leases	4.10%	4.10%
Number of lease transactions with related parties | item	0	0

Leases - PPA Leases (Details)

Leases - PPA Leases (Details) $ in Millions	1 Months Ended	3 Months Ended
	May 31, 2019USD ($)	Mar. 31, 2021USD ($)projectagreement	Mar. 31, 2020USD ($)
Lessor, Lease, Description [Line Items]
Lease, Practical Expedients, Package [true false]		true
Number of PPAs accounted for as operating leases | agreement		10
Number of project which are in operation | project		21
Lessor, Operating Lease, Existence of Option to Extend [true false]		false
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 33.5	$ 40.9
Minimum
Lessor, Lease, Description [Line Items]
Remaining lease term		1 year
Maximum
Lessor, Lease, Description [Line Items]
Remaining lease term		23 years
Solid Fuel
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 14.8	16.6
Natural Gas
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		5.6	5.9
Manchief
Operating Lease Lessor Sale Of Plant [Abstract]
Sales price of Manchief plant	$ 45.2
Hydroelectric
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 13.1	$ 18.4
Mamquam | BC Hydro
Operating Lease Lessor Sale Of Plant [Abstract]
Period of time between the anniversaries the option to purchase will become exercisable after the option is first exercisable		5 years