Income Taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in uncertain tax positions
Uncertain tax positions at the beginning of the period	$ 3.4	$ 12.8	$ 13.1
Gross increase - tax position in prior periods	0.2	0.2	0.2
Gross decrease - tax position in prior periods	(0.1)	(0.3)
Settlements		(7.8)
Foreign exchange		(1.5)	(0.5)
Uncertain tax positions at the end of the period	3.5	$ 3.4	$ 12.8
Interest and penalties included within uncertain tax positions	2.2
Impact of uncertain tax positions on effective tax rate, if recognized	$ 3.5