Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Deficit
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2017		34,666,666
Convertible preferred stock, Beginning balance at Dec. 31, 2017		$ 34,587
Beginning balance at Dec. 31, 2017	$ (33,265)			$ 1		$ 2,270	$ (35,536)
Beginning balance, Shares at Dec. 31, 2017				972,266	8,036
Issuance of convertible preferred stock		$ 12,659
Issuance of convertible preferred stock, Shares		12,666,667
Issuance of common stock	4					4
Issuance of common stock, Shares				125,123
Repurchase of unvested restricted stock awards					12,599
Retirement of treasury shares					(20,635)
Stock-based compensation expense	429					429
Net loss	(6,940)						(6,940)
Convertible preferred stock, Ending balance, Shares at Mar. 31, 2018		47,333,333
Convertible preferred stock, Ending balance at Mar. 31, 2018		$ 47,246
Ending balance at Mar. 31, 2018	(39,772)			$ 1		2,703	(42,476)
Ending balance, Shares at Mar. 31, 2018				1,097,389
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2017		34,666,666
Convertible preferred stock, Beginning balance at Dec. 31, 2017		$ 34,587
Beginning balance at Dec. 31, 2017	(33,265)			$ 1		2,270	(35,536)
Beginning balance, Shares at Dec. 31, 2017				972,266	8,036
Net loss	(14,786)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2018		60,000,000
Convertible preferred stock, Ending balance at Jun. 30, 2018		$ 59,909
Ending balance at Jun. 30, 2018	(47,148)			$ 1		3,173	(50,322)
Ending balance, Shares at Jun. 30, 2018				1,284,810
Convertible preferred stock, Beginning balance, Shares at Mar. 31, 2018		47,333,333
Convertible preferred stock, Beginning balance at Mar. 31, 2018		$ 47,246
Beginning balance at Mar. 31, 2018	(39,772)			$ 1		2,703	(42,476)
Beginning balance, Shares at Mar. 31, 2018				1,097,389
Issuance of convertible preferred stock		$ 12,663
Issuance of convertible preferred stock, Shares		12,666,667
Issuance of common stock	8					8
Issuance of common stock, Shares				187,421
Stock-based compensation expense	462					462
Net loss	(7,846)						(7,846)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2018		60,000,000
Convertible preferred stock, Ending balance at Jun. 30, 2018		$ 59,909
Ending balance at Jun. 30, 2018	(47,148)			$ 1		3,173	(50,322)
Ending balance, Shares at Jun. 30, 2018				1,284,810
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018		60,000,000	40,000,000
Convertible preferred stock, Beginning balance at Dec. 31, 2018		$ 59,909	$ 79,761
Beginning balance at Dec. 31, 2018	(63,670)			$ 2		4,452	(68,124)
Beginning balance, Shares at Dec. 31, 2018				1,587,953	67,024
Issuance of convertible preferred stock			$ 25,466
Issuance of convertible preferred stock, Shares			12,500,000
Issuance of common stock	5					5
Issuance of common stock, Shares				134,013
Repurchase of unvested restricted stock awards					43,922
Retirement of treasury shares					(110,946)
Stock-based compensation expense	821					821
Net loss	(36,843)						(36,843)
Convertible preferred stock, Ending balance, Shares at Mar. 31, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Mar. 31, 2019		$ 59,909	$ 105,227
Ending balance at Mar. 31, 2019	(99,687)			$ 2		5,278	(104,967)
Ending balance, Shares at Mar. 31, 2019				1,721,966
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018		60,000,000	40,000,000
Convertible preferred stock, Beginning balance at Dec. 31, 2018		$ 59,909	$ 79,761
Beginning balance at Dec. 31, 2018	(63,670)			$ 2		4,452	(68,124)
Beginning balance, Shares at Dec. 31, 2018				1,587,953	67,024
Net loss	(50,012)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Jun. 30, 2019		$ 59,909	$ 105,227
Ending balance at Jun. 30, 2019	(111,681)			$ 2		6,453	(118,136)
Ending balance, Shares at Jun. 30, 2019				1,870,286	1,432
Convertible preferred stock, Beginning balance, Shares at Mar. 31, 2019		60,000,000	52,500,000
Convertible preferred stock, Beginning balance at Mar. 31, 2019		$ 59,909	$ 105,227
Beginning balance at Mar. 31, 2019	(99,687)			$ 2		5,278	(104,967)
Beginning balance, Shares at Mar. 31, 2019				1,721,966
Issuance of common stock	225					225
Issuance of common stock, Shares				148,320
Repurchase of unvested restricted stock awards					7,451
Retirement of treasury shares					(6,019)
Stock-based compensation expense	950					950
Net loss	(13,169)						(13,169)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Jun. 30, 2019		$ 59,909	$ 105,227
Ending balance at Jun. 30, 2019	$ (111,681)			$ 2		$ 6,453	$ (118,136)
Ending balance, Shares at Jun. 30, 2019				1,870,286	1,432