License and Acquisition Agreements - Primatope Stock Purchase Option Agreement (Details) $ in Thousands	1 Months Ended	3 Months Ended		7 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Oct. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
License and Acquisition Agreements
Research and development		$ 20,644	$ 14,008		$ 50,475	$ 26,426
Primatope Agreement
License and Acquisition Agreements
Percentage of capital stock available to purchase in call option	100.00%
Upfront payment for initial option period	$ 500
Payment for extension of option period				$ 600
Upfront consideration payable on exercise of call option to acquire all outstanding equity	$ 10,000		10,000			10,000
Research and development		$ 250	500		$ 500	500
Maximum | Primatope Agreement
License and Acquisition Agreements
Number of extensions allowed for initial option period | item	3
Total aggregate extension payments	$ 800
Milestone payments payable on exercise of call option to acquire all outstanding equity	$ 10,000		$ 10,000			$ 10,000