Segment Information and Geographic Data - Revenue by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 50,030	$ 29	$ 29	$ 30	$ 29	$ 230	$ 29	$ 30	$ 50,118	$ 318	$ 463
Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									50,000	0	0
China
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									118	118	118
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 0	$ 200	$ 345