Stockholders' Equity - Issuances Related to the License Agreement (Details)						9 Months Ended
	Oct. 19, 2018 shares	Sep. 14, 2018 USD ($) $ / shares shares	Mar. 02, 2018 USD ($) $ / shares shares	Oct. 24, 2017 shares	Sep. 29, 2017 item shares	Sep. 30, 2018 USD ($) shares
Common Stock Offering [Abstract]
Issuance of common stock in connection with equity offering (in shares)		10,878,379	20,535,714
Share price | $ / shares		$ 18.50	$ 4.20
Net proceeds from issuance of stock | $		$ 188,900,000	$ 80,900,000			$ 269,825,618
Underwriting discounts and commissions | $		12,100,000	5,200,000
Offering expenses | $		$ 300,000	$ 100,000
Issuances Related to the License Agreement
Number of shares of common stock filed on registration statement per Registration Rights Agreement				6,000,000
License Agreement | ABX
Issuances Related to the License Agreement
Common stock issued per agreement					2,000,000
Number Of Warrants Issued Per Agreement | item					2
Number of warrants outstanding						1
Aggregate number of common stock shares that may be purchased by exercise of warrants					4,000,000
Number of shares of common stock that may be purchased per warrant						722,000
Number of shares subject to issuance under outstanding warrant	442,000
Number of warrants exercised	280,000				3,278,000
License Agreement | ABX | Maximum
Issuances Related to the License Agreement
Aggregate number of common stock shares that may be purchased by exercise of warrants					4,000,000