Income taxes - Total Income (Loss) Before Income Taxes and Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income (loss) before income taxes
United States									$ (48.1)	$ (53.6)	$ (152.7)
Foreign									87.8	196.8	80.6
Income (loss) before income taxes	$ (5.7)	$ 6.5	$ 41.9	$ (3.0)	$ 38.1	$ 22.2	$ 20.3	$ 62.6	39.7	143.2	(72.1)
Current
United States									7.6	4.7	(42.8)
Foreign									41.0	51.4	33.9
Total									48.6	56.1	(8.9)
Deferred
Foreign									4.4	8.2	3.4
Total provision (benefit) for income taxes									$ 53.0	$ 64.3	$ (5.5)