Fair Value of Financial Assets and Liabilities - Valuation of Contingent Equity Liability (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 28, 2017 shares	Oct. 31, 2016 USD ($) tranche $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Probability of occurrence of event that was most probable of triggering the issuance of shares			85.00%	75.00%
Value of the contingently issuable equity			$ 18,750	$ 18,750
Discount assessed			0.00%	0.00%
Fair value of the contingent equity liability			$ 15,938	$ 14,063
Maximum
Expected timing of event that was most probable of triggering the issuance of shares			1 year	1 year
AstraZeneca
Number of tranches in which contingently issuable equity is issuable | tranche		2
Fixed dollar amount per tranche		$ 5,000
Total amount of tranches		$ 10,000
AstraZeneca | Second tranche
Probability of occurrence of event that was most probable of triggering the issuance of shares			85.00%	65.00%
Value of the contingently issuable equity			$ 7,500	$ 7,500
Discount assessed			0.00%	0.00%
Fair value of the contingent equity liability			$ 6,375	$ 4,875
AstraZeneca | Second tranche | Maximum
Expected timing of event that was most probable of triggering the issuance of shares			3 months	1 year
Series A Convertible Preferred Shares
Preferred stock issued and sold | shares	4,305,182	4,305,209
Series A Convertible Preferred Shares | AstraZeneca
Preferred stock issued and sold | shares		538,150
Series A Convertible Preferred Shares | AstraZeneca | First tranche
Preferred stock issued and sold | shares		538,150
Aggregate fair value		$ 5,000
Issuance price per share | $ / shares		$ 9.2911