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 |