Leases - PPA Leases (Details) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2019 USD ($)	Dec. 31, 2020 USD ($) project agreement	Dec. 31, 2019 USD ($)
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		$ 149.6	$ 172.3
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		$ 65.9	79.1
Natural Gas
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		25.4	24.4
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		$ 58.3	$ 68.8
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