Pension plans 2 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation and Fair Value of Plan Assets and Reconciliation of Funded Status [Abstract]
Accumulated benefit obligation at the end of the year	$ 686.3	$ 751.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	2.0	4.0
Plan settlements		41.6	$ 26.3
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Non-current pension and other post-retirement liability	(75.7)	(47.6)
Deferred tax asset	47.3	61.6
Accumulated other comprehensive loss (income)	$ 140.1	$ 103.4	117.1	$ 143.4
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets Percentage	92.00%	92.00%
Defined Benefit Plan, Projected Benefit Obligation Percentage	92.00%	91.00%
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation and Fair Value of Plan Assets and Reconciliation of Funded Status [Abstract]
Service cost	$ 7.6	$ 7.4	7.6
Interest cost	26.8	30.2	32.7
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	2.0
Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets for Plan Benefits, Defined Benefit Plan	0.0	0.0
Defined Benefit Plan, Change in Benefit Obligation and Fair Value of Plan Assets and Reconciliation of Funded Status [Abstract]
Benefit obligation at the beginning of the year	685.7		693.7
Service cost	4.0	3.8
Interest cost	25.0	28.4
Plan amendments	1.2	0.3
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0.0	0.0
Plan curtailments	0.0	0.0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(37.7)	(79.6)
Actuarial loss	(40.7)	39.1
Foreign currency exchange rates	0.0	0.0
Benefit obligation at the end of the year	637.5	685.7
Accumulated benefit obligation at the end of the year	637.5	685.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at the beginning of the year	647.3		641.4
Actual return on plan assets	(43.9)	82.5
Employer contributions	0.9	3.0
Plan settlements	0.0	0.0
Foreign currency exchange rates	0.0	0.0
Fair value of plan assets at the end of the year	566.6	647.3
Defined Benefit Plan, Plan Assets, Benefits Paid	(37.7)	(79.6)
Unfunded status at year end:	(70.9)	(38.4)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Accrued benefit liability, current	(0.9)	(1.0)
Non-current pension and other post-retirement liability	(70.0)	(37.4)
Subtotal	(70.9)	(38.4)
Deferred tax asset	47.3	61.8
Accumulated other comprehensive loss (income)	138.3	97.9
Net amount related to pension plans	114.7	121.3
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Unrecognized net actuarial losses	182.0	156.5
Unrecognized net prior service costs	3.6	3.2
Unrecognized net transition costs	0.0	0.0
Tax benefit	(47.3)	(61.8)
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	10.4
Net prior service costs	1.0
Total	11.4
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	637.5	685.7
Accumulated benefit obligation	637.5	685.7
Fair value of plan assets	566.6	647.3
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	637.5	685.7
Accumulated benefit obligation	637.5	685.7
Fair value of plan assets	566.6	647.3
Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets for Plan Benefits, Defined Benefit Plan	0.3	0.0
Defined Benefit Plan, Change in Benefit Obligation and Fair Value of Plan Assets and Reconciliation of Funded Status [Abstract]
Benefit obligation at the beginning of the year	65.6		56.8
Service cost	1.8	1.7
Interest cost	1.8	1.8
Plan amendments	0.5	0.0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0.6	0.0
Plan curtailments	(0.3)	(0.3)
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(1.5)	(1.9)
Actuarial loss	(10.8)	1.9
Foreign currency exchange rates	(3.0)	5.6
Benefit obligation at the end of the year	53.5	65.6
Accumulated benefit obligation at the end of the year	48.8	59.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at the beginning of the year	57.1		$ 50.2
Actual return on plan assets	(3.9)	3.0
Employer contributions	1.1	1.0
Plan settlements	(0.6)	0.0
Foreign currency exchange rates	(2.8)	4.8
Fair value of plan assets at the end of the year	49.4	57.1
Defined Benefit Plan, Plan Assets, Benefits Paid	(1.5)	(1.9)
Unfunded status at year end:	(4.2)	(8.5)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Accrued benefit liability, current	(0.3)	(0.2)
Non-current pension and other post-retirement liability	(4.2)	(8.3)
Subtotal	(4.2)	(8.5)
Deferred tax asset	1.4	2.3
Accumulated other comprehensive loss (income)	5.7	9.2
Net amount related to pension plans	2.9	3.0
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Unrecognized net actuarial losses	5.8	10.6
Unrecognized net prior service costs	1.1	0.7
Unrecognized net transition costs	0.2	0.2
Tax benefit	(1.4)	(2.3)
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	0.1
Net prior service costs	0.1
Total	0.2
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	9.6	65.6
Accumulated benefit obligation	7.9	59.1
Fair value of plan assets	5.2	57.1
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	9.6	8.7
Accumulated benefit obligation	7.9	6.9
Fair value of plan assets	$ 5.2	$ 4.1