Summary of significant accounting policies - Schedule of Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2012
Buildings [Member] | Minimum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	25 years
Buildings [Member] | Maximum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	33 years
Plant and equipment [Member] | Minimum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	2 years
Plant and equipment [Member] | Maximum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	10 years
Furniture and fixtures [Member] | Minimum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	4 years
Furniture and fixtures [Member] | Maximum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	8 years
Instrumentation [Member] | Minimum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	3 years
Instrumentation [Member] | Maximum [Member]
Foreign Currency Translation [Line Items]
Useful life, in years	4 years