Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0
Interest cost on projected benefit obligation	9.2	9.0
Return on assets – expected	(13.7)	(13.7)
Amortization of losses	6.1	7.8
Settlement loss	0.0	0.0
Net periodic pension cost	1.6	3.1
U.S. Plans | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	1.6	3.1
U.S. Plans | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	0.0	0.0
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.8	2.9
Interest cost on projected benefit obligation	3.4	3.2
Return on assets – expected	(2.3)	(2.4)
Amortization of losses	1.2	1.3
Settlement loss	0.8	2.3
Net periodic pension cost	5.9	7.3
Non-U.S. Plans | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	5.9	6.2
Non-U.S. Plans | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	0.0	1.1
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.8	2.9
Interest cost on projected benefit obligation	12.6	12.2
Return on assets – expected	(16.0)	(16.1)
Amortization of losses	7.3	9.1
Settlement loss	0.8	2.3
Net periodic pension cost	7.5	10.4
Pension Plan | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	7.5	9.3
Pension Plan | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	0.0	1.1
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	4.6	4.5
Return on assets – expected	(4.4)	(5.2)
Amortization of losses	4.3	4.3
Amortization of prior service cost	(1.1)	(1.2)
Net periodic pension cost	3.4	2.4
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	0.6	0.7
Return on assets – expected	0.0	0.0
Amortization of losses	0.5	0.7
Amortization of prior service cost	0.5	0.5
Net periodic pension cost	1.6	1.9
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	5.2	5.2
Return on assets – expected	(4.4)	(5.2)
Amortization of losses	4.8	5.0
Amortization of prior service cost	(0.6)	(0.7)
Net periodic pension cost	$ 5.0	$ 4.3