Accounting for derivative instruments and hedging activities (Details) CAD in Thousands, kJ in Millions, $ in Millions		1 Months Ended			9 Months Ended	12 Months Ended
	Oct. 18, 2017	Oct. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) MMBTU	Jun. 30, 2014 kJ	Sep. 30, 2017 CAD MMBTU item	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2018 CAD	Dec. 20, 2018 CAD	Sep. 30, 2018	Jun. 30, 2018 CAD	Jun. 20, 2018 CAD	Mar. 31, 2018 CAD	Mar. 20, 2018 CAD	Dec. 31, 2017 CAD	Oct. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) item	Jul. 31, 2017 CAD	Jun. 30, 2017 CAD	Mar. 31, 2017 CAD
Derivative instruments
Number of derivative contracts designated as cash flow hedges | item					1											1
New term loan facility
Derivative instruments
Minimum fixed interest rate (as a percent)					5.25%											5.25%
Piedmont term loan, due 2018
Derivative instruments
Remaining aggregate principal amount | $			$ 56.6			$ 56.6										$ 54.6
Applicable margin (as a percent)					3.75%
Senior secured term loans
Derivative instruments
Face amount of debt | $																$ 375.7
Adjusted Eurodollar Rate | New term loan facility
Derivative instruments
Minimum fixed interest rate (as a percent)					1.00%											1.00%
Forecast
Derivative instruments
Applicable margin (as a percent)	3.50%
Forecast | New term loan facility
Derivative instruments
Face amount of debt | $															$ 200.0
Forecast | Piedmont term loan, due 2018
Derivative instruments
Interest Expense, Debt | $		$ 9.4
Piedmont | London Interbank Offered Rate (LIBOR) | Piedmont term loan, due 2018
Derivative instruments
Applicable margin of derivative (as a percent)					3.75%											3.75%
Gas purchase agreements | Morris Project
Derivative instruments
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU					220,000
Natural gas swaps
Derivative instruments
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU					9,000,000	4,900,000
Natural gas swaps | Kenilworth project
Derivative instruments
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU			800,000
Natural gas swaps | Orlando project
Derivative instruments
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU					10,900,000
Percentage of the entity's share in required natural gas purchases hedge					95.00%
Natural gas swaps during 2018 | Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge					90.00%
Natural gas swaps during 2019 | Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge					100.00%
Natural gas swaps during 2020 | Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge					50.00%
Interest rate swaps
Derivative instruments
Notional amount | $			$ 506.9			$ 506.9										$ 434.6
Interest rate swaps | New term loan facility
Derivative instruments
Notional amount | $																$ 354.0
Interest rate swaps | Swaption interest rate until February 15, 2019 | Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)					6.10%											6.10%
Interest rate swaps | Derivative Swaption Interest Rate Until August 2018 [Member] | Piedmont
Derivative instruments
Applicable margin (as a percent)					4.00%
Swaption interest rate (as a percent)					4.47%											4.47%
Swaption all in one interest rate (as a percent)					8.47%											8.47%
Interest rate swaps | Swaption interest rate from February 16, 2019 to February 15, 2023 | Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)					6.30%											6.30%
Interest rate swaps | Swaption interest rate after February 15, 2023 | Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)					6.40%											6.40%
Foreign currency forward contracts
Derivative instruments
Notional amount | CAD					CAD 35,000
Foreign Exchange Forward Rate 1.3381 | Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount | CAD																			CAD 30,000
Notional amount increments | CAD					10,000									CAD 10,000				CAD 10,000
Foreign currency exchange rate																			1.3381
Foreign Exchange Forward Rate 1.2481 | Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount | CAD																	CAD 10,000
Notional amount increments | CAD								CAD 3,330			CAD 3,330		CAD 3,330
Foreign currency exchange rate																	1.2481
Foreign Exchange Forward Rate 1.2943 | Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount | CAD																	CAD 10,000
Notional amount increments | CAD							CAD 2,000			CAD 3,000		CAD 5,000
Foreign currency exchange rate																	1.2943
Foreign Exchange Forward Rate 1.2196 | Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount | CAD					CAD 5,000
Foreign currency exchange rate									1.2196
Epsilon Power Partners | New term loan facility
Derivative instruments
Remaining aggregate principal amount | $																$ 562.7
Epsilon Power Partners | Natural gas swaps | Ontario project
Derivative instruments
Notional amount, energy (in gigajoules: Ontario, in Mmbtu: Orlando and Morris) | kJ				2.9
Epsilon Power Partners | Interest rate swaps
Derivative instruments
Notional amount | $																$ 354.2
Epsilon Power Partners | Interest rate swaps | Adjusted Eurodollar Rate | New term loan facility
Derivative instruments
Applicable margin of derivative (as a percent)					4.25%											4.25%