Common stock (Early exercise of stock options) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Unvested shares subject to repurchase rights at original issuance price (in shares)	14,360	48,260	112,967
Exercise of unvested stock options included in accrued liabilities	$ 100	$ 198	$ 400