Agreement with Eli Lilly and Company (Related Party) - Deferred revenue included in balance sheets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Deferred revenue
Deferred revenue from related party, beginning of period	$ 26,725
Up-front payment		$ 25,000
Attributed premium on the proceeds from Lilly’s investment in the Company		1,725
Research and development support billing	500
Collaboration revenue recognized	(1,357)
Total deferred revenue from related party, end of period	25,868	26,725
Deferred revenue from related party, current portion	(5,240)	(4,989)
Deferred revenue from related party, non-current portion	$ 20,628	$ 21,736