Postretirement benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employear contributions		$ 39.0	$ 65.4
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Non-current pension and other post-retirement liability		54.7	218.9
Accumulated other comprehensive loss (income)		90.7	193.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost - benefits earned during the period		0.3	0.3	0.3
Interest cost on benefit obligation		0.4	0.4	0.4
Amortization of prior service credit		(0.6)	(0.6)	(0.6)
Recognized actuarial net (gain) or loss		(0.3)	(0.3)	(0.4)
Net periodic benefit (income) cost		(0.2)	(0.2)	(0.3)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at the beginning of the year	7.9	9.1	9.4
Service cost		0.3	0.3	0.3
Interest cost		0.4	0.4	0.4
Participant contributions		0.6	0.4
Actuarial loss		(1.5)	0.1
Benefits paid		(1.0)	(1.5)
Benefit obligation at the end of the year		7.9	9.1	9.4
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
Participant contributions		0.6	0.4
Employear contributions		0.4	1.1
Benefits paid		(1.0)	(1.5)
Fair value of plan assets at the end of the year		0	0	0
Unfunded status at year end:		7.9	9.1
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued benefit liability, current		(0.6)	(0.6)
Non-current pension and other post-retirement liability		(7.3)	(8.5)
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits		(3.1)	(2.8)
Accumulated other comprehensive loss (income)		(4.8)	(4.5)
Net amount related to postretirement plans		(15.8)	(16.4)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Unrecognized net actuarial losses		(5.5)	(3.0)
Unrecognized net prior service costs		(2.4)	(4.3)
Tax benefit		3.1	2.8
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax		(4.8)	(4.5)
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		0.4
Net prior service costs		0.6
Total		1.0
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
2014		0.5
2015		0.6
2016		0.6
2017		0.6
2018		0.6
Years 2019 - 2023		3.6
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year		$ 0.5
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year	7.50%	7.00%	7.50%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed For Next Fiscal Year As Of Last Fiscal Year	6.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	5.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Weighted-average discount rate		5.00%	4.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Weighted-average discount rate		4.00%	4.25%	5.25%
Defined Benefit Plan, Description of Direction and Pattern of Change for Assumed Health Care Cost Trend Rate	0.005