Stock incentive plans (Details 4) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Number of Options
Outstanding at the beginning of the period (in shares)	5,045	6,601	5,288
Granted (in shares)	1,351	1,391	2,381
Exercised (in shares)	(461)	(1,899)	(1,015)
Canceled/forfeited (in shares)	(980)	(1,048)	(53)
Outstanding at the end of the period (in shares)	4,955	5,045	6,601	5,288
Vested, exercisable and expected to vest at the end of the period (in shares)	4,788
Exercisable at the end of the period (in shares)	2,739
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.50	$ 5.50	$ 1.25
Granted (in dollars per share)	$ 6.73	$ 8.29	$ 13.02
Exercised (in dollars per share)	$ 1.33	$ 1.36	$ 1.01
Canceled/forfeited (in dollars per share)	$ 10.20	$ 11.85	$ 6.58
Outstanding at the end of the period (in dollars per share)	$ 6.31	$ 6.50	$ 5.50	$ 1.25
Vested and expected to vest at the end of the period (in dollars per share)	$ 6.25
Exercisable at the end of the period (in dollars per share)	$ 4.52
Weighted-Average Remaining Contract Life (years)
Outstanding	6 years 7 months 6 days	6 years 4 months 24 days	7 years	6 years 3 months 18 days
Vested and expected to vest at the end of the period	6 years 6 months
Exercisable at the end of the period	4 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 14,742	$ 30,996	$ 38,279
Outstanding at the end of the period	9,535	14,742	30,996	38,279
Vested and expected to vest at the end of the period	9,520
Exercisable at the end of the period	9,329
Cash proceeds from exercise of stock option	614	2,573	1,028
Total intrinsic value of stock options exercised (in dollars)	2,662	14,901	9,754
Tax benefits for the deductions from stock option exercises	$ 55	$ 2,190	$ 246
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 6.60	$ 8.10	$ 6.68