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Patents and other intangible assets - Schedule of Patents and Other Intangible Assets (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Finite And Indefinite Lived Intangible Assets [Line Items]    
Cost $ 40,916 $ 39,562
Accumulated amortization (34,059) (32,272)
Patents and other intangible assets, net 6,857 7,290
Patents [Member]
   
Finite And Indefinite Lived Intangible Assets [Line Items]    
Cost 40,289 38,905
Accumulated amortization (33,620) (31,845)
Trademarks - definite lived [Member]
   
Finite And Indefinite Lived Intangible Assets [Line Items]    
Cost 627 657
Accumulated amortization $ (439) $ (427)