Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jul. 05, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		$ 88		$ 88			$ 25
Restricted cash, non-current							63
Deferred offering costs		0		0			0
General and administrative expense		2,601	$ 1,627	6,039	$ 6,537
2016 Equity Financing [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs								$ 1,500
IPO [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs						$ 2,304
Write-off of Deferred Financing Costs [Member]
Summary Of Significant Accounting Policies [Line Items]
General and administrative expense	$ 1,500
Security Deposit Related to Credit Card Accounts [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		25		25			25
Security Deposit Related to Lease Facilities [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		$ 63		$ 63
Restricted cash, non-current							$ 63