Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Aug. 31, 2013	Mar. 31, 2018	Mar. 31, 2017
Other Commitments [Line Items]
Operating leases, expiration year		2021
Rent expense		$ 264,000	$ 260,000
Bellevue [Member] | Washington [Member]
Other Commitments [Line Items]
Expiration date of operating lease	2014-08
Extended expiration date of operating lease	2020-05