Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2013	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Commitments [Line Items]
Operating leases, expiration year				2021
Rent expense		$ 211,000	$ 253,000	$ 695,000	$ 786,000
Bellevue [Member] | Washington [Member]
Other Commitments [Line Items]
Expiration date of operating lease	2014-08
Extended expiration date of operating lease	2020-05