Income tax expense - NOL (Details) $ in Millions	Dec. 31, 2020 USD ($)
Operating loss carryforwards
Net operating loss carryforwards	$ 477.1
2029
Operating loss carryforwards
Net operating loss carryforwards	19.9
2031
Operating loss carryforwards
Net operating loss carryforwards	25.4
2032
Operating loss carryforwards
Net operating loss carryforwards	19.4
2033
Operating loss carryforwards
Net operating loss carryforwards	44.6
2034
Operating loss carryforwards
Net operating loss carryforwards	131.6
2035
Operating loss carryforwards
Net operating loss carryforwards	154.1
2036
Operating loss carryforwards
Net operating loss carryforwards	37.7
2037
Operating loss carryforwards
Net operating loss carryforwards	25.7
2038
Operating loss carryforwards
Net operating loss carryforwards	10.3
2039
Operating loss carryforwards
Net operating loss carryforwards	7.2
2040
Operating loss carryforwards
Net operating loss carryforwards	1.2
US
Operating loss carryforwards
Net operating loss carryforwards	369.5
US | 2031
Operating loss carryforwards
Net operating loss carryforwards	25.4
US | 2032
Operating loss carryforwards
Net operating loss carryforwards	13.4
US | 2033
Operating loss carryforwards
Net operating loss carryforwards	20.6
US | 2034
Operating loss carryforwards
Net operating loss carryforwards	122.3
US | 2035
Operating loss carryforwards
Net operating loss carryforwards	154.1
US | 2036
Operating loss carryforwards
Net operating loss carryforwards	17.0
US | 2037
Operating loss carryforwards
Net operating loss carryforwards	16.7
Canada.
Operating loss carryforwards
Net operating loss carryforwards	107.6
Canada. | 2029
Operating loss carryforwards
Net operating loss carryforwards	19.9
Canada. | 2032
Operating loss carryforwards
Net operating loss carryforwards	6.0
Canada. | 2033
Operating loss carryforwards
Net operating loss carryforwards	24.0
Canada. | 2034
Operating loss carryforwards
Net operating loss carryforwards	9.3
Canada. | 2036
Operating loss carryforwards
Net operating loss carryforwards	20.7
Canada. | 2037
Operating loss carryforwards
Net operating loss carryforwards	9.0
Canada. | 2038
Operating loss carryforwards
Net operating loss carryforwards	10.3
Canada. | 2039
Operating loss carryforwards
Net operating loss carryforwards	7.2
Canada. | 2040
Operating loss carryforwards
Net operating loss carryforwards	$ 1.2