PPAs and other definite-lived intangible assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Components of intangible assets
Assets, gross balance at the end of the period	$ 352.1	$ 365.6
Write-off of fully amortized balances	(13.5)
Less: accumulated amortization	(231.8)	(221.3)
Assets, net balance at the end of the period	120.3	144.3
Liabilities, gross balances at the end of the period	(41.1)	(40.7)
Less: accumulated amortization	23.1	20.9
Liabilities, net balance at the end of the period	(18.0)	(19.8)
PPAs
Components of intangible assets
Assets, gross balance at the end of the period	352.1	365.6
Write-off of fully amortized balances	(13.5)
Less: accumulated amortization	(231.8)	(221.3)
Assets, net balance at the end of the period	120.3	144.3
Liabilities, gross balances at the end of the period	(28.5)	(28.1)
Less: accumulated amortization	16.2	14.4
Liabilities, net balance at the end of the period	(12.3)	(13.7)
Fuel Supply Agreements
Components of intangible assets
Liabilities, gross balances at the end of the period	(12.6)	(12.6)
Less: accumulated amortization	6.9	6.5
Liabilities, net balance at the end of the period	$ (5.7)	$ (6.1)