Stock plans (Options activity) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Activities
Options outstanding, beginning balance (in shares)	65,401	1,160,057	1,572,713
Options exercised during period (in shares)	(39,265)	(1,094,656)	(412,656)
Options outstanding, ending balance (in shares)	26,136	65,401	1,160,057
Weighted Average Grant Date Fair Value
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 7.74	$ 7.12	$ 6.66
Weighted average grant date fair value, options exercised (in dollars per share)	8.37	7.09	5.36
Weighted average grant date fair value, ending balance (in dollars per share)	6.78	7.74	7.12
Weighted Average Grant Date Exercise Price
Weighted average grant date exercise price, beginning balance (in dollars per share)	20.17	17.74	18.88
Weighted average grant date exercise price, options exercised (in dollars per share)	17.96	17.60	22.07
Weighted average grant date exercise price, ending balance (in dollars per share)	$ 23.48	$ 20.17	$ 17.74
Total intrinsic value	$ 1,260	$ 26,367	$ 7,459
Company proceeds received	277	6,277	$ 9,107
Intrinsic value of options outstanding	$ 831	$ 1,740