Income taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 11.7	$ 12.5
Pre-tax income	$ 41.1	$ 38.5
Effective income tax rate	28.50%	32.50%
Increase (decrease) in income tax expense from changes in valuation allowances	$ (21.1)