Business and geographic segments - Net Property and Equipment by Segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net property and equipment:
Net property and equipment	$ 763.1	$ 723.8	$ 646.1
United States [Member]
Net property and equipment:
Net property and equipment	211.9	189.0	120.1
Germany [Member]
Net property and equipment:
Net property and equipment	271.2	259.6	267.8
Canada [Member]
Net property and equipment:
Net property and equipment	73.0	80.0	80.4
Norway [Member]
Net property and equipment:
Net property and equipment	109.5	101.5	100.5
Belgium [Member]
Net property and equipment:
Net property and equipment	97.5	86.0	69.4
Taiwan [Member]
Net property and equipment:
Net property and equipment		$ 7.7	$ 7.9