Summary of significant accounting policies and basis of presentation - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	6 Months Ended	9 Months Ended
	Mar. 31, 2014	Jun. 30, 2014 Employees	Sep. 30, 2014 Segment
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment			1
Tax incentives	$ 306
New employees hired to maintain additional headcount		15
State Research and Development [Member]
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]
Tax credits	276
Deferred benefit amortization period		5 years
Pregenen [Member]
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]
Total purchase consideration		30,991
Cash consideration		5,093
Common stock consideration		19,348
Contingent consideration		$ 6,550