Stock incentive plans (Details 4) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Number of Options
Outstanding at the beginning of the period (in shares)	4,955	5,045	6,601
Granted (in shares)	203	1,351	1,391
Exercised (in shares)	(458)	(461)	(1,899)
Canceled/forfeited (in shares)	(359)	(980)	(1,048)
Outstanding at the end of the period (in shares)	4,341	4,955	5,045	6,601
Vested, exercisable and expected to vest at the end of the period (in shares)	4,267
Exercisable at the end of the period (in shares)	3,053
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.31	$ 6.50	$ 5.50
Granted (in dollars per share)	$ 5.99	$ 6.73	$ 8.29
Exercised (in dollars per share)	$ 2.53	$ 1.33	$ 1.36
Canceled/forfeited (in dollars per share)	$ 7.53	$ 10.20	$ 11.85
Outstanding at the end of the period (in dollars per share)	$ 6.60	$ 6.31	$ 6.50	$ 5.50
Vested and expected to vest at the end of the period (in dollars per share)	$ 6.58
Exercisable at the end of the period (in dollars per share)	$ 5.83
Weighted-Average Remaining Contract Life (years)
Outstanding	5 years 9 months 18 days	6 years 7 months 6 days	6 years 4 months 24 days	7 years
Vested and expected to vest at the end of the period	5 years 8 months 12 days
Exercisable at the end of the period	4 years 10 months 24 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 9,535	$ 14,742	$ 30,996
Outstanding at the end of the period	11,017	9,535	14,742	30,996
Vested and expected to vest at the end of the period	10,946
Exercisable at the end of the period	10,014
Cash proceeds from exercise of stock option	1,158	614	2,573
Total intrinsic value of stock options exercised (in dollars)	2,027	2,662	14,901
Tax benefits for the deductions from stock option exercises		$ 55	$ 2,190
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 2.92	$ 3.03	$ 3.59