Leases - Additional Information (Detail)		1 Months Ended							6 Months Ended	12 Months Ended
	Sep. 30, 2017	Aug. 31, 2018	Mar. 31, 2017	Jun. 30, 2016 USD ($) Draw	Apr. 30, 2016 USD ($)	Oct. 31, 2015 USD ($)	Apr. 30, 2015	Nov. 30, 2014 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2018 USD ($)	Jan. 11, 2011 USD ($)
Leases [Line Items]
Right-of-use assets									$ 4,108,000
Lease liabilities									4,737,000
Rent expenses										$ 2,000,000	$ 1,900,000
Finance Lease Obligations [Member]
Leases [Line Items]
Initial borrowing capacity						$ 10,000,000
Maximum borrowings available (up to)						$ 5,000,000
Debt term (in months)						36 months
Repurchase price as percentage of original equipment value that the equipment under lease may be repurchased by lessee						10.00%
Minimum lease period to extend lease (in years)						1 year
Amount of draw				$ 2,500,000	$ 2,100,000
Amount of monthly payment				$ 79,000	$ 67,000
Number of draws | Draw				2
Operating Lease Entered into November 2014 [Member] | License Agreement [Member] | Office Space [Member]
Leases [Line Items]
Lease expiration date	Dec. 31, 2021						Dec. 31, 2017
Security deposit								$ 50,000
Office Space, Laboratory Space, and Equipment [Member]
Leases [Line Items]
Right-of-use assets									0
Lease liabilities									$ 0
Maximum lease period to not recognize right of use assets or lease liabilities									12 months
Office and Laboratory Space [Member] | Operating Lease Entered into August 2010 [Member]
Leases [Line Items]
Lease expiration date			Dec. 31, 2021
Security deposit												$ 180,000	$ 400,000
Office, Laboratory and Manufacturing Space [Member] | Operating Lease Entered into May 2013 [Member]
Leases [Line Items]
Lease expiration date		Dec. 31, 2020
Laboratory Space [Member] | Operating Lease Entered into November 2014 [Member]
Leases [Line Items]
Security deposit								$ 281,000
Term of lease								6 years
Space build-out costs paid								$ 1,400,000
Space build-out costs to be paid by landlord								$ 2,200,000