Fair Value of Financial Assets and Liabilities - Fair values of the company's liabilities (Details) - USD ($) $ in Thousands			1 Months Ended		3 Months Ended
	May 09, 2017	Feb. 17, 2017	May 31, 2017	Feb. 29, 2016	Mar. 31, 2017	Jul. 31, 2016
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues				429,000		1,090,500
Recognized fair value of BCF as a reduction to series A preferred shares and corresponding adjustment to additional paid in capital		$ 12,006			$ (12,006)
Warrants to purchase common shares
Fair values of the company's liabilities
Balance at the beginning					780
Change in fair value					454
Balance at the end					1,234
Derivative liability
Fair values of the company's liabilities
Balance at the beginning					512
Change in fair value					(289)
Balance at the end					223
Contingent equity liability
Fair values of the company's liabilities
Change in fair value					$ 3,375
Initial public offering | Subsequent Event
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues	9,900,000
Product related collaborative arrangements | BMS Agreement
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues			1,345,374
Product related collaborative arrangements | BMS Agreement | Initial public offering | Subsequent Event
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues			1,345,374
Product related collaborative arrangements | AstraZeneca
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues			538,149
Product related collaborative arrangements | AstraZeneca | Initial public offering | Subsequent Event
Fair values of the company's liabilities
Stock Issued During Period, Shares, New Issues			538,149