Summary of Significant Accounting Policies - Summary of Impact on Condensed Consolidated Balance Sheets for Opening Balances Related to Lease (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Leases [Line Items]
Operating lease assets	$ 11,899
Accrued expenses and other current liabilities	10,522		$ 15,207
Operating lease liabilities	4,335
Lease incentive obligation, net of current portion			6,776
Deferred rent			$ 2,216
Operating lease liabilities, net of current portion	$ 16,844
As reported under Topic 842 [Member]
Leases [Line Items]
Operating lease assets		$ 13,737
Accrued expenses and other current liabilities		13,439
Operating lease liabilities		4,285
Operating lease liabilities, net of current portion		20,212
Impact of adoption of ASC 842 [Member] | As reported under Topic 842 [Member]
Leases [Line Items]
Operating lease assets		13,737
Accrued expenses and other current liabilities		(1,768)
Operating lease liabilities		4,285
Lease incentive obligation, net of current portion		(6,776)
Deferred rent		(2,216)
Operating lease liabilities, net of current portion		$ 20,212