Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Share-based compensation - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Mar. 15, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 2.5	$ 3.4	$ 3.6	$ 0.0
Amount paid to employees for share-based awards		$ 0.3	3.3	$ 0.3	3.3
Unrecognized share-based compensation costs			$ 31.8		$ 31.8
Awards granted in the period (in shares)					1,374,944
Options granted in the period (in shares)		0		0	34,615
Retention program expense		$ 1.4		$ 7.7
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average recognition period (years)					2 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average recognition period (years)					3 years
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted in the period (in shares)					401,311
Options granted in the period (in shares)					14,335
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average recognition period (years)				1 year 5 months 24 days
Awards granted in the period (in shares)		0		0	1,411,714
Options granted in the period (in shares)					46,052
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted in the period (in shares)					34,615
Retention awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted under the retention program	$ 13.8
Amount paid for retention awards	$ 6.9
Long term incentive plan 2019 \| Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average recognition period (years)				1 year 6 months
Awards granted in the period (in shares)		0		0
Long term incentive plan 2020 \| Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 1.5		$ 3.1
Award vesting period				3 years