Stock plans (Options activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Options activities:
Options outstanding, beginning balance (in shares)	65,401	1,160,057
Options exercised during period (in shares)	(35,351)	(1,094,656)
Options outstanding, ending balance (in shares)	30,050	65,401
Weighted Average Grant Date Fair Value
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 7.74	$ 7.12
Weighted average grant date fair value, options exercised (in dollars per share)	8.16	7.09
Weighted average grant date fair value, ending balance (in dollars per share)	7.24	7.74
Weighted Average Grant Date Exercise Price
Weighted average grant date exercise price, beginning balance (in dollars per share)	20.17	17.74
Weighted average grant date exercise price, options exercised (in dollars per share)	17.82	17.60
Weighted average grant date exercise price, ending balance (in dollars per share)	$ 22.93	$ 20.17