Stock-based compensation and warrants - Summary of Stock-Based Compensation Expense by Award Type (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 2,914	$ 2,516	$ 7,757	$ 4,833
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	2,303	2,495	7,115	4,757
Restricted Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	16	21	47	76
Restricted Stock Units R S U [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	567		567
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 28		$ 28