Subsequent Events - Additional Information (Detail)		1 Months Ended	3 Months Ended
	Apr. 08, 2019 USD ($) Program $ / shares shares	Jan. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)
Subsequent Event [Line Items]
Issuance of common stock, net of offering costs		$ 381,900,000	$ 381,900,000
Common stock at a price per share | $ / shares		$ 77.50
Issuance of common stock, net of offering costs (in shares) | shares		5,000,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Subsequent Event [Line Items]
Upfront fee received	$ 400,000,000
Maximum additional milestone payments to be receive upon achievement of certain criteria	$ 200,000,000
Number of targeted programs | Program	30
Discovery period of programs development	5 years
Extended additional discovery period of programs development	2 years
Issuance of common stock, net of offering costs	$ 400,000,000
Common stock at a price per share | $ / shares	$ 90.00
Issuance of common stock, net of offering costs (in shares) | shares	4,440,000
Number of trading period	15 days
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Program Initiation
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	$ 2,500,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Lead Candidate Identification
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	2,500,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Steady State
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	$ 30,000,000