CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Preferred Stock	Preferred Additional Paid‑In Capital	Common Stock	Additional Paid‑In Capital	Accumulated Deficit	Common Stock	Common Stock Common Stock	Common Stock Additional Paid‑In Capital	Preferred Stock	Preferred Stock Preferred Stock	Preferred Stock Preferred Additional Paid‑In Capital
BALANCE at Dec. 31, 2014	$ 114,185	$ 0	$ 0	$ 35	$ 222,060	$ (107,910)
Balance (in shares) at Dec. 31, 2014		0		34,621,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock‑based compensation cost	1,239				1,239
Net income (loss)	(45,853)					(45,853)
BALANCE at Dec. 31, 2015	69,571	$ 0	0	$ 35	223,299	(153,763)
Balance (in shares) at Dec. 31, 2015		0		34,621,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock‑based compensation cost	1,649				1,649
Stock issued, net of costs							$ 40,425	$ 18	$ 40,407	$ 162,511	$ 17	$ 162,494
Stock issued (in shares)								18,007,000			17,000,000
Net income (loss)	(53,147)					(53,147)
BALANCE at Dec. 31, 2016	221,009	$ 17	162,494	$ 53	265,355	(206,910)
Balance (in shares) at Dec. 31, 2016		17,000,000		52,628,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock‑based compensation cost	9,489				9,489
Stock issued, net of costs	170,141			$ 13	170,128
Stock issued (in shares)				13,250,000
Conversion of preferred stock to common stock at Initial Public Offering	0	$ (17)	(162,494)	$ 17	162,494
Conversion of preferred stock to common stock at Initial Public Offering (in shares)		(17,000,000)		17,000,000
Exercise of stock options—net	$ 0			$ 0	0
Exercise of stock options—net (in shares)	226,194			162,000
Net income (loss)	$ 12,613					12,613
BALANCE at Dec. 31, 2017	$ 413,252	$ 0	$ 0	$ 83	$ 607,466	$ (194,297)
Balance (in shares) at Dec. 31, 2017		0		83,040,000