Retirement benefits (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
US Plans [Member]
Net periodic pension cost (credit) [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	11.6	11.6	34.7	34.9
Return on assets - expected	(16.2)	(16.7)	(48.7)	(50.1)
Amortization of losses	7.0	4.9	21.0	14.6
Settlement loss	0	0	0	0
Net periodic pension cost (credit)	2.4	(0.2)	7.0	(0.6)
Non-US Plans [Member]
Net periodic pension cost (credit) [Line Items]
Service cost	2.5	1.5	7.8	4.6
Interest cost	4.3	3.2	12.9	9.8
Return on assets - expected	(3.0)	(2.6)	(9.1)	(7.9)
Amortization of losses	0.9	0.8	3.2	2.5
Settlement loss	0.7	0	1.7	0
Net periodic pension cost (credit)	5.4	2.9	16.5	9.0
UMWA Plans Postretirement Medical [Member]
Net periodic pension cost (credit) [Line Items]
Interest cost	5.9	6.8	17.9	20.2
Return on assets - expected	(6.4)	(6.3)	(19.2)	(19.0)
Amortization of losses	3.8	4.1	11.3	12.1
Net periodic pension cost (credit)	3.3	4.6	10.0	13.3
Black Lung And Other Plans Postretirement Medical [Member]
Net periodic pension cost (credit) [Line Items]
Interest cost	0.7	0.8	2.1	2.2
Return on assets - expected	0	0	0	0
Amortization of losses	0.6	0.6	1.8	1.3
Net periodic pension cost (credit)	1.3	1.4	3.9	3.5
Pension plan [Member]
Net periodic pension cost (credit) [Line Items]
Service cost	2.5	1.5	7.8	4.6
Interest cost	15.9	14.8	47.6	44.7
Return on assets - expected	(19.2)	(19.3)	(57.8)	(58.0)
Amortization of losses	7.9	5.7	24.2	17.1
Settlement loss	0.7	0	1.7	0
Net periodic pension cost (credit)	7.8	2.7	23.5	8.4
Retirement benefits other than pension [Member]
Net periodic pension cost (credit) [Line Items]
Interest cost	6.6	7.6	20.0	22.4
Return on assets - expected	(6.4)	(6.3)	(19.2)	(19.0)
Amortization of losses	4.4	4.7	13.1	13.4
Net periodic pension cost (credit)	$ 4.6	$ 6.0	$ 13.9	$ 16.8