Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2012

Summary of significant accounting policies [Abstract]

Estimated useful lives of property, plant and equipment

Property, plant and equipment are stated at cost. Depreciation is computed using the straight-line method over the following estimated useful lives:

Buildings	31-39 years
Building improvements	10-39 years
Furniture and equipment	3-15 years
Software	Lesser of 3-5 years or product life
Leasehold improvements	Lesser of the asset life or lease term

Assumptions used in valuing stock options granted

	Year Ended December 31,
	2012			2011			2010
Expected dividend yield		0	%		0	%		0	%
Expected volatility		41-52	%		60	%		55	%
Risk-free interest rate		0.36-0.54	%		0.35-1.04	%		0.49-1.46	%
Expected average life of options	3.4 years			3.4 years			3.4 years