Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2015

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	3-7 years or product life
Leasehold improvements	Lesser of the asset life or lease term

Assumptions used in valuing stock options granted

The Company determines the fair value of restricted stock units using the closing market price of the Company's common stock on the day prior to the date of grant. The Company utilizes the Black-Scholes valuation model for estimating the fair value of all stock options granted. Set forth below are the assumptions used in valuing the stock options granted and a discussion of the Company's methodology for developing each of the assumptions used:

	Year Ended December 31,
		2015	2014	2013
Expected dividend yield		0%	0%	0%
Expected volatility		34-35%	35-38%	39-49%
Risk-free interest rate		1.27-1.61%	1.14-1.65%	0.32-0.70%
Expected average life of options		4.3 years	4.5 years	4.4 years