Fair Value of Financial Assets and Liabilities - Narrative (Details)		1 Months Ended			6 Months Ended		12 Months Ended
	Aug. 30, 2016 director	May 31, 2017 shares	Feb. 28, 2017 USD ($) shares	Oct. 31, 2016 USD ($) tranche $ / shares shares	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)
Number of directors to whom the Company agreed to issue warrants | director	2
Shares issued in connection with contingent equity liability | shares		9,358,560
Issuance price per share | $ / shares					$ 45.00
Recognized fair value of BCF as a reduction to series A preferred shares and corresponding adjustment to additional paid in capital					$ 0	$ (12,006,000)
BMS
Shares issued in connection with contingent equity liability | shares		1,345,374
Value of the contingently issuable equity							$ 18,750,000
BMS | Maximum
Expected timing of event that was most probable of triggering the issuance of shares							1 year
AstraZeneca
Shares issued in connection with contingent equity liability | shares		538,149
Number of tranches in which contingently issuable equity is issuable | tranche				2
Fixed dollar amount per tranche				$ 5,000,000
Total amount of tranches				$ 10,000,000
AstraZeneca | Second tranche
Value of the contingently issuable equity							$ 7,500,000
Fair value of the contingent equity liability							$ 4,875,000
AstraZeneca | Second tranche | Maximum
Expected timing of event that was most probable of triggering the issuance of shares							1 year
Series A Convertible Preferred Shares
Shares issued in connection with contingent equity liability | shares		9,358,560
Preferred stock issued and sold | shares			4,305,182	4,305,209
Recognized fair value of BCF as a reduction to series A preferred shares and corresponding adjustment to additional paid in capital			$ 12,006,000
Shares issued on conversion of preferred stock | shares		9,358,560
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				$ 5,000,000
Issuance price per share | $ / shares				$ 9.2911
Expected Dividend Yield
Measurement input for warrant liability					0
Probability of Triggering Event | BMS
Measurement input for contingent equity liability							0.75
Probability of Triggering Event | AstraZeneca | Second tranche
Measurement input for contingent equity liability							0.65
Discount Rate | BMS
Measurement input for contingent equity liability							0
Discount Rate | AstraZeneca | Second tranche
Measurement input for contingent equity liability							0
Recurring
Financial assets at fair value					$ 0
Financial liabilities at fair value					$ 0			$ 4,021,000