Stockholders' equity - Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,010.9	$ 912.3	$ 596.2
Other comprehensive income (loss)	(5.0)
Amounts reclassified from accumulated other comprehensive income	0.6
Total other comprehensive income (loss), net of tax	(4.4)	(1.8)	0.6
Ending balance	1,088.5	1,010.9	912.3
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5.5)	(3.7)	(4.3)
Total other comprehensive income (loss), net of tax	(4.4)	(1.8)	0.6
Ending balance	(9.9)	(5.5)	(3.7)
Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.2)	0.0
Other comprehensive income (loss)	(3.2)
Amounts reclassified from accumulated other comprehensive income	0.0
Total other comprehensive income (loss), net of tax	(3.2)	(0.2)
Ending balance	(3.4)	(0.2)	0.0
Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0	0.0
Total other comprehensive income (loss), net of tax		0.0
Ending balance		0.0	0.0
Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss)	(2.2)
Amounts reclassified from accumulated other comprehensive income	0.6
Total other comprehensive income (loss), net of tax	(1.6)
Ending balance	(1.6)
Foreign Currency Translation Losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5.3)	(3.7)
Other comprehensive income (loss)	0.4
Amounts reclassified from accumulated other comprehensive income	0.0
Total other comprehensive income (loss), net of tax	0.4	(1.6)
Ending balance	$ (4.9)	$ (5.3)	$ (3.7)