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Share-based compensation - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Mar. 15, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ (3.3)	$ 3.0	$ 0.7	$ 3.1
Amount paid to employees for share-based awards		$ 1.9	1.9	$ 3.3	3.3
Unrecognized share-based compensation costs			$ 27.1		$ 27.1
Options granted in the period (in shares)		0		0	1,374,944
Retention program expense		$ 1.6		$ 9.2
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
Restricted stock \| Board of Directors Chairman
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted in the period (in shares)			2,424		37,039
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted in the period (in shares)			7,059		408,370
Forfeited (in shares)			43,230		57,565
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average recognition period (years)				1 year 3 months 18 days
Awards granted in the period (in shares)		0	16,471	0	1,428,185
Forfeited (in shares)		1,060	25,936	80,160	71,988
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 2020 \| Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 1.1		$ 4.2
Long term incentive plan 2020 \| RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 6.1