Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		9 Months Ended
	Jan. 01, 2019	Sep. 30, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Impairment of available-for-sale securities deemed to be other-than-temporary		$ 0
Operating lease right-of-use assets		5,905,000
Property and equipment, net		28,287,000	$ 28,702,000
Accumulated deficit		$ (151,358,000)	$ (83,016,000)
ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets	$ 7,400,000
Operating lease liabilities	8,400,000
ASU 2016-02 [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	100,000
ASU 2016-02 [Member] | Restatement Adjustment [Member]
Significant Accounting Policies [Line Items]
Current and noncurrent deferred rent liabilities	1,400,000
Prepaid expenses, other current assets and other assets	400,000
ASU 2016-02 [Member] | Restatement Adjustment [Member] | 9800 Medical Center Drive Lease [Member]
Significant Accounting Policies [Line Items]
Property and equipment, net	(5,900,000)
Financing lease obligation	(5,900,000)
ASU 2018-02 [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Reclassification of stranded tax effects of unrealized gains and losses on available-for-sale securities from accumulated other comprehensive income (loss) to accumulated deficit	$ 100,000