Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.8	$ 2.6	$ 8.5	$ 8.1
Interest cost on projected benefit obligation	12.0	12.3	38.7	37.3
Return on assets – expected	(15.9)	(16.1)	(47.3)	(48.2)
Amortization of losses	7.7	7.6	22.7	22.3
Amortization of prior service cost	0.4	0.0	0.8	0.2
Settlement loss	0.6	0.4	1.4	1.8
Net periodic pension cost	7.6	6.8	24.8	21.5
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.0
Interest cost on projected benefit obligation	5.5	5.4	16.1	16.1
Return on assets – expected	(4.1)	(4.4)	(12.4)	(13.1)
Amortization of losses	6.2	5.3	17.6	15.3
Amortization of prior service cost	(0.7)	(0.8)	(2.2)	(2.2)
Net periodic pension cost	6.9	5.5	19.2	16.1
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost on projected benefit obligation	8.8	9.3	26.4	27.8
Return on assets – expected	(13.3)	(13.7)	(39.9)	(41.0)
Amortization of losses	6.3	6.4	18.7	18.7
Amortization of prior service cost	0.0	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0	0.0
Net periodic pension cost	1.8	2.0	5.2	5.5
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.8	2.6	8.5	8.1
Interest cost on projected benefit obligation	3.2	3.0	12.3	9.5
Return on assets – expected	(2.6)	(2.4)	(7.4)	(7.2)
Amortization of losses	1.4	1.2	4.0	3.6
Amortization of prior service cost	0.4	0.0	0.8	0.2
Settlement loss	0.6	0.4	1.4	1.8
Net periodic pension cost	5.8	4.8	19.6	16.0
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.0	0.0
Interest cost on projected benefit obligation	4.6	4.7	13.7	14.1
Return on assets – expected	(4.1)	(4.4)	(12.4)	(13.1)
Amortization of losses	5.2	4.7	14.6	13.5
Amortization of prior service cost	(1.2)	(1.2)	(3.5)	(3.5)
Net periodic pension cost	4.5	3.8	12.4	11.0
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.0
Interest cost on projected benefit obligation	0.9	0.7	2.4	2.0
Return on assets – expected	0.0	0.0	0.0	0.0
Amortization of losses	1.0	0.6	3.0	1.8
Amortization of prior service cost	0.5	0.4	1.3	1.3
Net periodic pension cost	$ 2.4	$ 1.7	$ 6.8	$ 5.1