Summary of significant accounting policies and basis of presentation - Additional Information (Detail) $ in Millions		3 Months Ended
	Sep. 21, 2015	Mar. 31, 2017 USD ($) Segment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment		1
ASU 2016-09 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred tax assets, net operating loss		$ 76.7
Deferred tax assets, valuation allowance		76.7
ASU 2016-09 [Member] | Additional Paid-in Capital and Retained Earnings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cumulative catch up to account for forfeitures		$ 0.5
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Lease expiration year	2027
Additional lease period extendable	10 years