Leases - PPA Leases (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2019 USD ($)	Jun. 30, 2020 USD ($) project agreement	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) project agreement	Jun. 30, 2019 USD ($)
Lessor, Lease, Description [Line Items]
Lease, Practical Expedients, Package [true false]				true
Number of PPAs accounted for as operating leases | agreement		10		10
Number of project which are in operation | project		21		21
Lessor, Operating Lease, Existence of Option to Extend [true false]				false
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 39.2	$ 46.9	$ 80.2	$ 91.2
Minimum
Lessor, Lease, Description [Line Items]
Remaining lease term				6 months
Maximum
Lessor, Lease, Description [Line Items]
Remaining lease term				24 years
Manchief
Operating Lease Lessor Sale Of Plant [Abstract]
Sales price of Manchief plant	$ 45.2
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
Solid Fuel
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		15.2	18.6	$ 32.0	38.3
Natural Gas
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		6.1	5.8	12.0	11.9
Hydroelectric
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 17.9	$ 22.5	$ 36.2	$ 41.0