Stock incentive plans (Details 4) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of Options
Outstanding at the beginning of the period (in shares)	4,341,000
Granted (in shares)	0
Exercised (in shares)	(440,000)	(458,000)	(461,000)
Canceled/forfeited (in shares)	(153,000)
Outstanding at the end of the period (in shares)	3,748,000	4,341,000
Vested, exercisable and expected to vest at the end of the period (in shares)	3,728,000
Exercisable at the end of the period (in shares)	3,175,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.60
Exercised (in dollars per share)	3.12
Canceled/forfeited (in dollars per share)	9.51
Outstanding at the end of the period (in dollars per share)	6.89	$ 6.60
Vested and expected to vest at the end of the period (in dollars per share)	6.89
Exercisable at the end of the period (in dollars per share)	$ 6.73
Weighted-Average Remaining Contract Life (years)
Outstanding	5 years	5 years 9 months 18 days
Vested and expected to vest at the end of the period	5 years
Exercisable at the end of the period	4 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 11,017
Outstanding at the end of the period	6,611	$ 11,017
Vested and expected to vest at the end of the period	6,605
Exercisable at the end of the period	6,475
Cash proceeds from exercise of stock option	1,373	1,158	$ 614
Total intrinsic value of stock options exercised (in dollars)	$ 2,214	$ 2,027	2,662
Tax benefits for the deductions from stock option exercises			$ 55
Weighted average grant-date fair value of stock options granted (in dollars per share)		$ 2.92	$ 3.03