Postretirement benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	$ 4.0	$ 20.7
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Non-current pension and other post-retirement liability	(47.6)	(59.5)
Accumulated other comprehensive loss (income)	103.4	117.1	$ 143.4	$ 140.4
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Accumulated other comprehensive loss (income)	103.4	117.1	143.4	$ 140.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0.6	0.7
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(0.9)	(1.0)
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0.6	0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost on benefit obligation	0.2	0.2	0.3
Amortization of prior service credit	0.3	0.3	0.0
Recognized actuarial net (gain) or loss	(0.8)	(0.7)	(0.3)
Net periodic benefit (income) cost	(0.9)	(0.8)	0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at the beginning of the year	4.2	5.7
Interest cost	0.2	0.2	0.3
OPEB curtailment	0.0	(1.1)
Actuarial loss	(1.8)	(0.3)
Benefit obligation at the end of the year	2.3	4.2	5.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at the beginning of the year	0.0	0.0
Employer contributions	0.3	0.3
Fair value of plan assets at the end of the year	0.0	0.0	$ 0.0
Unfunded status at year end:	2.3	4.2
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Accrued benefit liability, current	(0.3)	(0.4)
Non-current pension and other post-retirement liability	(2.0)	(3.8)
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	(2.5)	(2.2)
Accumulated other comprehensive loss (income)	(3.7)	(3.3)
Net amount related to postretirement plans	(8.5)	(9.7)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Unrecognized net actuarial losses	(5.8)	(4.7)
Unrecognized net prior service costs	(0.4)	(0.7)
Tax benefit	2.5	2.1
Accumulated other comprehensive loss (income)	(3.7)	$ (3.3)
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Net actuarial losses	1.1
Net prior service costs	0.3
Total	1.4
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	0.3
2019	0.3
2020	0.3
2021	0.2
2022	0.2
Years 2023 - 2027	0.9
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 0.3
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	6.50%	6.75%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Annual Change	0.25%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate, Next Fiscal Years	5.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Weighted-average discount rate	4.25%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Weighted-average discount rate	4.25%	4.25%	4.00%
Defined Benefit Plan, Plan Assets, Benefits Paid	$ 0.9	$ 1.0