Subsequent Events (Details) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 27, 2018 USD ($) $ / shares shares	Jun. 20, 2018 shares	Jul. 31, 2018 USD ($) employee	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares
Subsequent Events
Reverse stock split conversion ratio						0.125
Common stock, par value | $ / shares						$ 0.001	$ 0.001
Common stock, shares authorized | shares		280,000,000				70,000,000	70,000,000
Minimum
Subsequent Events
Reverse stock split conversion ratio		0.25
Maximum
Subsequent Events
Reverse stock split conversion ratio		0.125
Subsequent Events
Subsequent Events
Common stock, par value | $ / shares	$ 0.001
Common stock, shares authorized | shares	70,000,000
Subsequent Events | Cambridge, Massachusetts facility
Subsequent Events
Number of positions eliminated | employee			18
Percentage of positions eliminated			40.00%
Restructuring charges					$ 2.9
Early termination fee	$ 0.2
Non-cash gain due to write-off of remaining deferred rent liability	$ 0.4
Subsequent Events | Cambridge, Massachusetts facility | Maximum
Subsequent Events
Non-cash fixed asset impairments				$ 1.0	$ 1.0
Subsequent Events | BioCryst
Subsequent Events
Fixed expense reimbursement			$ 6.0