Pension plans 5 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Settlement loss (gain)	$ (10.6)	$ (6.1)	$ (0.1)
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)	35.00%
Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)	65.00%
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement loss (gain)	$ (10.1)	$ 0.0	$ 0.0
Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	7.25%	7.50%	7.50%
2018	$ 37.8
2019	38.3
2020	38.9
2021	43.1
2022	40.6
Years 2023 - 2027	$ 203.2
Weighted-average discount rate	3.75%	4.25%
Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	5.30%	6.08%	6.00%
2018	$ 1.7
2019	1.9
2020	2.0
2021	2.4
2022	2.2
Years 2023 - 2027	$ 13.8
Weighted-average discount rate	2.80%	3.05%