Revenue from Contract with Customer - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 19, 2014	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Up-front payment allocated to preferred stock value
Kyowa Hakko Kirin Co., Ltd. [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Milestone payment agreement terms			In accordance with the terms of the license agreement
License Agreement [Member] | Kyowa Hakko Kirin Co., Ltd. [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
License agreement upfront payment received		$ 25,000,000
Up-front payment allocated to equity purchase		7,500,000
Up-front license fee received		$ 17,500,000
Period of termination after first commercial sale of first licensed product		15 years
Up-front payment allocated to preferred stock value		$ 7,700,000
Up-front payment allocated to license fee		17,300,000
Milestone payment received	$ 5,000,000			$ 5,000,000
Upfront payment allocation to performance obligation			$ 17,300,000
Performance obligation period			2029
Deferred revenue			$ 15,800,000
License Agreement [Member] | Kyowa Hakko Kirin Co., Ltd. [Member] | Maximum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Milestone payment receivable upon achievement of development and commercial milestone		$ 75,000,000