Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Period of stranded credit terms (not more than)			75 days
Interest income	$ 500,000	$ 900,000	$ 1,400,000	$ 1,700,000
Gross realized loss	0		0
Operating lease right-of-use assets	17,882,000		17,882,000			$ 0
Operating lease, liability	$ 18,732,000		$ 18,732,000
Warrants exercise price (in dollars per share)	$ 0.05		$ 0.05
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets					$ 17,300,000
Operating lease, liability					$ 17,900,000