Stock incentive plans (Details 4) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Number of Options
Outstanding at the beginning of the period (in shares)	4,341
Exercised (in shares)	(147)
Canceled/forfeited (in shares)	(37)
Outstanding at the end of the period (in shares)	4,157	4,341
Vested, exercisable and expected to vest at the end of the period (in shares)	4,100
Exercisable at the end of the period (in shares)	3,067
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.60
Exercised (in dollars per share)	$ 1.97
Canceled/forfeited (in dollars per share)	$ 9.21
Outstanding at the end of the period (in dollars per share)	$ 6.74	$ 6.60
Vested and expected to vest at the end of the period (in dollars per share)	$ 6.73
Exercisable at the end of the period (in dollars per share)	$ 6.16
Weighted-Average Remaining Contract Life (years)
Outstanding	5 years 7 months 6 days	5 years 9 months 18 days
Vested and expected to vest at the end of the period	5 years 7 months 6 days
Exercisable at the end of the period	4 years 10 months 24 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 11,017
Outstanding at the end of the period	9,783	11,017
Vested and expected to vest at the end of the period	9,732
Exercisable at the end of the period	$ 9,023