Stock incentive plans - Activities of Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Number of Options
Outstanding at the beginning of the period (in shares)	3,748
Exercised (in shares)	(365)
Canceled/forfeited (in shares)	(56)
Outstanding at the end of the period (in shares)	3,327	3,748
Vested, exercisable and expected to vest at the end of the period (in shares)	3,318
Exercisable at the end of the period (in shares)	3,156
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.89
Exercised (in dollars per share)	5.43
Canceled/forfeited (in dollars per share)	7.37
Outstanding at the end of the period (in dollars per share)	7.04	$ 6.89
Vested, exercisable and expected to vest at the end of the period (in dollars per share)	7.04
Exercisable at the end of the period (in dollars per share)	$ 7.08
Weighted-Average Remaining Contract Life (years)
Outstanding	4 years 2 months 12 days	5 years
Vested, exercisable and expected to vest at the end of the period	4 years 1 month 6 days
Exercisable at the end of the period	4 years
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 13,878	$ 6,611
Vested, exercisable and expected to vest at the end of the period	13,858
Exercisable at the end of the period	$ 13,206