Collaboration Agreements - Takeda (Details) $ / shares in Units, $ in Thousands		3 Months Ended		12 Months Ended	27 Months Ended
	Feb. 23, 2018 USD ($) program targets $ / shares shares	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales		$ 3,552	$ 4,111
Collaborative Arrangement | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment	$ 40,000
Number of programs | program	3
Preclinical milestone payment received	$ 5,000
Number of targets | targets	3
Target option period	5 years
Option fee	$ 5,000
Variable consideration relating to future milestones	26,000
Remaining preclinical milestones, cost and profit sharing income, and the development and commercial milestones	$ 44,000
Number of performance obligations | program	3
Contract liability		56,000		$ 59,000	$ 56,000
Preclinical milestone payment earned not yet received		0		0	0
Revenue recognized		$ 3,300		$ 2,400
Sales					15,000
Collaborative Arrangement | Takeda Pharmaceutical Company Limited | Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales					$ 0
Collaborative Arrangement | Share Purchase Agreement | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common stock (in shares) | shares	4,214,559
Purchase price	$ 110,000
Fair market value of common stock	$ 94,400
Closing stock price (usd per share) | $ / shares	$ 22.40
Premium on sale of common stock	$ 15,600
Collaborative Arrangement | Maximum | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preclinical milestone payments per program	25,000
Total aggregate payments due upon achievement of certain preclinical milestone events	75,000
Aggregated option exercise fee	15,000
Total aggregate payments due upon achievement of certain clinical and regulatory milestone events	407,500
Regulatory milestone payment	300,000
Milestone payments per biologic product upon achievement of a certain sales-based milestone	75,000
Milestone payments upon achievement of biologic product from each program	$ 225,000