Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.8	$ 2.5	$ 7.7	$ 7.4
Interest cost on projected benefit obligation	9.8	11.1	28.4	33.4
Return on assets – expected	(14.7)	(15.1)	(43.5)	(45.7)
Amortization of losses	8.7	6.0	25.2	17.8
Amortization of prior service credit	0.0	(0.1)
Settlement loss	0.6	0.6	1.1	1.5
Net periodic pension cost	6.2	5.0	18.9	14.4
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.1
Interest cost on projected benefit obligation	4.1	5.0	12.4	15.9
Return on assets – expected	(3.3)	(3.4)	(9.8)	(10.0)
Amortization of losses	6.3	5.1	18.5	16.3
Amortization of prior service credit	(1.2)	(1.3)	(3.7)	(3.8)
Net periodic pension cost	5.9	5.4	17.5	18.5
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost on projected benefit obligation	6.7	8.5	20.0	25.6
Return on assets – expected	(11.6)	(12.6)	(34.7)	(38.0)
Amortization of losses	7.3	4.9	21.4	14.7
Amortization of prior service credit	0.0	0.0
Settlement loss	0.0	0.0	0.0	0.0
Net periodic pension cost	2.4	0.8	6.7	2.3
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8	2.5	7.7	7.4
Interest cost on projected benefit obligation	3.1	2.6	8.4	7.8
Return on assets – expected	(3.1)	(2.5)	(8.8)	(7.7)
Amortization of losses	1.4	1.1	3.8	3.1
Amortization of prior service credit	0.0	(0.1)
Settlement loss	0.6	0.6	1.1	1.5
Net periodic pension cost	3.8	4.2	12.2	12.1
UMWA Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.0	0.0
Interest cost on projected benefit obligation	3.2	4.1	9.6	13.2
Return on assets – expected	(3.3)	(3.4)	(9.8)	(10.0)
Amortization of losses	4.2	3.9	12.3	12.8
Amortization of prior service credit	(1.2)	(1.2)	(3.5)	(3.5)
Net periodic pension cost	2.9	3.4	8.6	12.5
Black Lung and Other Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.1
Interest cost on projected benefit obligation	0.9	0.9	2.8	2.7
Return on assets – expected	0.0	0.0	0.0	0.0
Amortization of losses	2.1	1.2	6.2	3.5
Amortization of prior service credit	0.0	(0.1)	(0.2)	(0.3)
Net periodic pension cost	$ 3.0	$ 2.0	$ 8.9	$ 6.0