Nature of the Business and Basis of Presentation - Stock Split & Public Offering (Details) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended	9 Months Ended
	May 11, 2017 USD ($) $ / shares shares	May 09, 2017 USD ($) $ / shares shares	Oct. 31, 2016	Feb. 29, 2016 USD ($) $ / shares shares	Jul. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	May 31, 2017 shares	Apr. 21, 2017 shares	Dec. 31, 2016 $ / shares shares
Stock Split
Stock Split			500
Public Offering
Issuance of common shares | shares				429,000	1,090,500
Fair value of common share | $ / shares				$ 7.00	$ 7.70
Proceeds from issuance of common shares upon completion of initial public offering, net of underwriting commissions and discounts						$ 179,996
Other offering expenses				$ 23	$ 97	5,039	$ 238
Aggregate net proceeds from the IPO and underwriters issuance	$ 176,128
Common shares value issued						$ 176,128
Common stock, shares authorized | shares	200,000,000					200,000,000		200,000,000	50,000,000	38,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0					$ 0				$ 0
Preferred stock, shares authorized | shares	10,000,000					0		10,000,000		11,242,172
Preferred stock, par value (in dollars per share) | $ / shares	$ 0					$ 0				$ 0
IPO
Public Offering
Issuance of common shares | shares		9,900,000
Fair value of common share | $ / shares		$ 17.00
Proceeds from issuance of common shares upon completion of initial public offering, net of underwriting commissions and discounts		$ 152,651
Underwriting discounts and commissions		11,781
Other offering expenses		$ 3,868
Convertible preferred stock converted into shares of common stock | shares		9,358,560
Over allotment option
Public Offering
Issuance of common shares | shares	1,485,000
Proceeds from issuance of common shares upon completion of initial public offering, net of underwriting commissions and discounts	$ 23,478
Underwriting discounts and commissions	1,767
Aggregate net proceeds from the IPO and underwriters issuance	$ 176,128
Over allotment option | BMS and AstraZeneca
Public Offering
Issuance of common shares | shares	1,883,523
Common shares value issued	$ 0