Income taxes - Total Income Before Income Taxes and Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income (loss) before income taxes
United States									$ (88.3)	$ (130.6)	$ (19.9)
Foreign									108.8	71.8	165.4
Income (loss) before income taxes	$ 24.6	$ (15.2)	$ 44.3	$ (33.2)	$ 12.4	$ 7.3	$ (50.8)	$ (27.7)	20.5	(58.8)	145.5
Current
United States									6.7	1.0	2.1
Foreign									47.7	42.2	59.4
State and local									0.0	0.3	1.0
Total									54.4	43.5	62.5
Deferred
United States									0.0	0.0	0.0
Foreign									2.8	0.9	23.7
Provision for income taxes									$ 57.2	$ 44.4	$ 86.2