Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2011 Y
Buildings
Useful lives of long-lived assets
Useful life, minimum (in years)	25
Useful life, maximum (in years)	33
Plant and equipment
Useful lives of long-lived assets
Useful life, minimum (in years)	2
Useful life, maximum (in years)	10
Furniture and fixtures
Useful lives of long-lived assets
Useful life, minimum (in years)	4
Useful life, maximum (in years)	8
Instrumentation
Useful lives of long-lived assets
Useful life, minimum (in years)	3
Useful life, maximum (in years)	4