Stock-Based Compensation - Stock option activity (Details) - Stock options - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Shares
Options Outstanding at beginning of the period (in shares)	866,132
Granted (in shares)	863,290
Canceled (in shares)	(39,348)
Options Outstanding at the end of the period (in shares)	1,690,074	866,132
Options Vested and Expected to Vest at the end of the period (in shares)	1,538,046
Options Exercisable at the end of the period (in shares)	529,136
Shares Available for Grant Under the 2008 Plan	433,391
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 93.70
Granted (in dollars per share)	2.78
Canceled (in dollars per share)	131.86
Options Outstanding at the end of the period (in dollars per share)	46.37	$ 93.70
Options Vested and Expected to Vest at the end of the period (in dollars per share)	50.45
Options Exercisable at the end of the period (in dollars per share)	$ 132.85
Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at the end of the period	8 years 4 months 24 days	7 years 1 month 6 days
Options Vested and Expected to Vest at the end of the period	8 years 3 months 18 days
Options Exercisable at the end of the period	5 years 8 months 12 days