Debt - Narrative (Details) - A&R Loan Agreement - USD ($)		1 Months Ended	9 Months Ended
	Sep. 30, 2016	Oct. 31, 2016	Sep. 30, 2017	Sep. 29, 2016
Debt
Maximum borrowing capacity of term loan	$ 55,000,000.0
Amount outstanding	35,000,000			$ 25,000,000
Amount of borrowings during the period	$ 10,000,000
Facility fee paid (as a percent)	0.375%
Interest rate (as a percent)		9.25%
Backend fee (as a percent)		4.15%	4.15%
Facility fee paid		$ 337,500
Minimum cash liquidity requirement eliminated			$ 25,000,000.0
Limit on lender's participation rights in certain future private equity financings			$ 2,000,000.0
License Agreement with AstraZeneca
Debt
Amount borrowed during period		$ 20,000,000
Prime Rate
Debt
Basis spread (as a percent)		(4.50%)