Preferred Stock - Activity (Details) $ in Thousands		3 Months Ended	8 Months Ended	12 Months Ended	36 Months Ended	72 Months Ended
	Apr. 17, 2015 USD ($) shares	Apr. 24, 2015 USD ($) tranche shares	Dec. 19, 2014 USD ($) tranche shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2013 USD ($) shares	Dec. 31, 2016 tranche shares	Jan. 03, 2011 shares
Preferred Stock
Consideration for conversion of notes payable-related party and accrued interest | $				$ 31,145
Series A Redeemable Convertible Preferred Stock
Preferred Stock
Preferred stock authorized for issuance (in shares)				0		9,000,000	9,000,000
Issuance of stock to the licensor, corresponding to Series B tranche closing (in shares)	2,720,700				6,279,300	9,000,000
Number of tranches | tranche						4
Series A Redeemable Convertible Preferred Stock | Research and development
Preferred Stock
Value of shares issued to the licensor | $	$ 2,721				$ 6,279
Series B Convertible Preferred Stock
Preferred Stock
Preferred stock authorized for issuance (in shares)				0		21,500,000
Issuance of stock (in shares)	1,967,902				15,000,000
Gross proceeds | $	$ 1,968				$ 15,000
Issuance of remaining authorized shares (in shares)	6,500,000
Series B Convertible Preferred Stock | Conversion of notes payable - related party and accrued interest into Series B Convertible Preferred Stock
Preferred Stock
Shares issued on conversion of notes payable and related accrued interest	4,532,098
Consideration for conversion of notes payable-related party and accrued interest | $	$ 4,532
Series C Convertible Preferred Stock
Preferred Stock
Preferred stock authorized for issuance (in shares)				0		12,000,000
GITR | Series C Convertible Preferred Stock
Preferred Stock
Number of tranches | tranche		2	3
Issuance of stock (in shares)		2,835,040	2,687,765
Net proceeds | $		$ 1,922	$ 1,807