Background (Details) - USD ($) $ / shares in Units, $ in Thousands						1 Months Ended	12 Months Ended
	Jan. 22, 2018	Jun. 26, 2017	Sep. 13, 2016	Mar. 03, 2016	Feb. 10, 2016	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liquidity and Risks
Retained Earnings (Accumulated Deficit)							$ (359,613)	$ (141,562)
Cash and cash equivalents							324,117	240,430	$ 62,252	$ 3,120
Net proceeds from issuance of common stock		$ 269,900
Public Offering
Shares issued (in shares)		4,111,250
Share price (in dollars per share)		$ 70.00
Net Proceeds from Initial Public Offering							$ 269,869	$ 246,490
Subsequent Event [Member]
Liquidity and Risks
Net proceeds from issuance of common stock	$ 431,900					$ 431,900
Public Offering
Shares issued (in shares)	4,509,840
Share price (in dollars per share)	$ 102.00
Over-Allotment Option
Public Offering
Shares issued (in shares)		536,250
Over-Allotment Option | Subsequent Event [Member]
Public Offering
Shares issued (in shares)	588,240
Class B-1 Preferred Stock
Public Offering
Shares converted (in shares)					3,278,938
Class B-2 Preferred Stock
Public Offering
Shares converted (in shares)					326,557
Class C Preferred Stock
Liquidity and Risks
Net proceeds from issuance of common stock									5,000
Public Offering
Shares converted (in shares)					2,365,020
Class D Preferred Stock
Liquidity and Risks
Net proceeds from issuance of common stock									$ 64,788
Public Offering
Shares converted (in shares)					3,105,000
Common Stock
Public Offering
Shares issued (in shares)							14,000
Shares converted (in shares)								456,000
Common Stock | IPO
Public Offering
Shares issued (in shares)					4,750,000			5,278,000
Share price (in dollars per share)					$ 20.00
Net Proceeds from Initial Public Offering					$ 88,400
Common Stock | Over-Allotment Option
Public Offering
Shares issued (in shares)			637,500	527,941
Share price (in dollars per share)				$ 20.00
Net Proceeds from Initial Public Offering				$ 9,800
Common Stock | Stock Sold by Selling Stockholder
Public Offering
Shares issued (in shares)			289,855
Common Stock | Follow-On Offering
Liquidity and Risks
Net proceeds from issuance of common stock			$ 149,100
Public Offering
Number of common shares to be sold under public offering			4,887,500
Shares issued (in shares)			4,597,645
Share price (in dollars per share)			$ 34.50