Business and geographic segments - Net Sales by Point of Origin and Point of Destination (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Net sales	$ 427.4	$ 508.7	$ 568.4	$ 582.8	$ 456.7	$ 568.6	$ 558.9	$ 440.9	$ 2,087.3	$ 2,025.1	$ 1,533.5
Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									2,087.3	2,025.1	1,533.5
Point Of Destination [Member]
Segment Reporting Information [Line Items]
Net sales									2,087.3	2,025.1	1,533.5
United States [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									1,153.8	831.4	648.5
Germany [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									977.7	1,039.7	714.2
Canada [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									339.1	301.7	245.4
Norway [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									284.0	245.1	188.3
Belgium [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									272.9	301.8	209.1
Eliminations [Member] | Point Of Origin [Member]
Segment Reporting Information [Line Items]
Net sales									(940.2)	(694.6)	(472.0)
North America [Member] | Point Of Destination [Member]
Segment Reporting Information [Line Items]
Net sales									760.7	578.2	499.4
Europe [Member] | Point Of Destination [Member]
Segment Reporting Information [Line Items]
Net sales									1,011.4	1,141.3	822.2
Asia and other [Member] | Point Of Destination [Member]
Segment Reporting Information [Line Items]
Net sales									$ 315.2	$ 305.6	$ 211.9