Income taxes - Total Income Before Income Taxes and Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income (loss) before income taxes
United States									$ (53.6)	$ (152.7)	$ (88.3)
Foreign									196.8	80.6	108.8
Income (loss) before income taxes	$ 38.1	$ 22.2	$ 20.3	$ 62.6	$ 27.4	$ (40.4)	$ (42.3)	$ (16.8)	143.2	(72.1)	20.5
Current
United States									4.7	(42.8)	6.7
Foreign									51.4	33.9	47.7
Total									56.1	(8.9)	54.4
Deferred
Foreign									8.2	3.4	2.8
Provision (benefit) for income taxes									$ 64.3	$ (5.5)	$ 57.2