Pension plans 2 (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	$ 833.1	$ 709.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	10.8	39.0
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Non-current pension and other post-retirement liability	102.4	54.7
Deferred tax asset	85.9	55.6
Accumulated other comprehensive loss (income)	140.4	90.7	193.4
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets Percentage	92.00%	90.00%
Defined Benefit Plan, Projected Benefit Obligation Percentage	92.00%	88.00%
U.S. Pension Plans [Member]
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	641.4	750.4
Service cost	4.9	11.2
Interest cost	30.7	29.5
Participant contributions	0	0
Plan amendments	0.1	0.5
Plan settlements	0	0
Plan curtailments	(0.5)	(32.6)
Benefits paid	(36.0)	(29.5)
Actuarial loss	134.0	(88.1)
Transfer in	0	0
Foreign currency exchange rates	0	0
Benefit obligation at the end of the year	774.6	641.4
Accumulated benefit obligation at the end of the year	774.6	641.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at the beginning of the year	627.2	555.3
Actual return on plan assets	88.6	65.2
Employer contributions	5.9	36.2
Plan settlements	0	0
Defined Benefit Plan, Divestitures, Plan Assets	0	0
Foreign currency exchange rates		0
Fair value of plan assets at the end of the year	685.7	627.2
Unfunded status at year end:	(88.9)	(14.2)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Prepaid benefit cost, non-current	0	23.2
Accrued benefit liability, current	(1.6)	(6.1)
Non-current pension and other post-retirement liability	(87.3)	(31.3)
Subtotal	(88.9)	(14.2)
Deferred tax asset	85.1	56.5
Accumulated other comprehensive loss (income)	134.7	89.4
Net amount related to pension plans	130.9	131.7
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Unrecognized net actuarial losses	215.7	139.7
Unrecognized net prior service costs	4.1	6.2
Unrecognized net transition costs	0	0
Tax benefit	(85.1)	(56.5)
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	19.8
Net prior service costs	0.9
Net transition costs	0
Total	20.7
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	774.6	37.4
Accumulated benefit obligation	774.6	37.4
Fair value of plan assets	685.7	0
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	774.6	37.4
Accumulated benefit obligation	774.6	37.4
Fair value of plan assets	685.7	0
Non-U.S. Pension Plans [Member]
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	83.4	80.4
Service cost	2.6	2.8
Interest cost	3.3	3.0
Participant contributions	0.3	0.4
Plan amendments	0	0
Plan settlements	(0.7)	(4.2)
Plan curtailments	(0.4)	(0.9)
Benefits paid	(4.0)	(2.1)
Actuarial loss	8.4	1.9
Transfer in	(21.1)	0
Foreign currency exchange rates	(5.9)	2.1
Benefit obligation at the end of the year	65.9	83.4
Accumulated benefit obligation at the end of the year	58.5	68.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at the beginning of the year	67.2	63.3
Actual return on plan assets	4.0	5.5
Employer contributions	4.9	2.8
Plan settlements	(0.7)	(4.2)
Defined Benefit Plan, Divestitures, Plan Assets	(9.9)	0
Foreign currency exchange rates	(4.6)	1.5
Fair value of plan assets at the end of the year	57.2	67.2
Unfunded status at year end:	(8.7)	(16.2)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Prepaid benefit cost, non-current	0	0.4
Accrued benefit liability, current	(0.2)	(0.5)
Non-current pension and other post-retirement liability	(8.5)	(16.1)
Subtotal	(8.7)	(16.2)
Deferred tax asset	2.6	2.2
Accumulated other comprehensive loss (income)	8.4	6.1
Net amount related to pension plans	2.3	(7.9)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Unrecognized net actuarial losses	10.4	6.5
Unrecognized net prior service costs	0.1	0.4
Unrecognized net transition costs	0.5	1.4
Tax benefit	(2.6)	(2.2)
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	0.3
Net prior service costs	0
Net transition costs	0.1
Total	0.4
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	65.9	74.1
Accumulated benefit obligation	58.5	61.0
Fair value of plan assets	57.2	57.4
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	13.7	30.8
Accumulated benefit obligation	11.9	21.0
Fair value of plan assets	8.2	15.7
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	7.5	14.0	14.7
Interest cost	34.0	32.5	33.7
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 3.3