Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.9	$ 2.5
Interest cost on projected benefit obligation	9.0	11.1
Return on assets – expected	(14.1)	(15.3)
Amortization of losses	8.1	6.0
Settlement loss	0.4	0.3
Net periodic pension cost	6.3	4.6
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	4.2	5.9
Return on assets – expected	(3.3)	(3.3)
Amortization of losses	6.0	6.2
Amortization of prior service credit	(1.2)	(1.2)
Net periodic pension cost	5.7	7.6
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	6.6	8.5
Return on assets – expected	(11.5)	(12.7)
Amortization of losses	6.9	5.0
Settlement loss	0.0	0.0
Net periodic pension cost	2.0	0.8
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.9	2.5
Interest cost on projected benefit obligation	2.4	2.6
Return on assets – expected	(2.6)	(2.6)
Amortization of losses	1.2	1.0
Settlement loss	0.4	0.3
Net periodic pension cost	4.3	3.8
UMWA Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	3.3	5.0
Return on assets – expected	(3.3)	(3.3)
Amortization of losses	4.0	5.1
Amortization of prior service credit	(1.2)	(1.1)
Net periodic pension cost	2.8	5.7
Black Lung and Other Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	0.9	0.9
Return on assets – expected	0.0	0.0
Amortization of losses	2.0	1.1
Amortization of prior service credit	0.0	(0.1)
Net periodic pension cost	$ 2.9	$ 1.9