Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 1.7	$ 3.3	$ 3.0	$ 4.5
Foreign earnings repatriated, measurement period adjustment			0.9
Refundable alternative minimum tax credit			$ 0.8