Equity Incentive Plans - Stock Option Activity (Details) - Common stock options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options
Options outstanding at beginning of year (in shares)	3,805,305	2,844,970
Options granted (in shares)	2,230,000	1,145,750
Options exercised (in shares)		(76,610)
Options cancelled/forfeited (in shares)	(347,404)	(108,805)
Options outstanding at end of year (in shares)	5,687,901	3,805,305	2,844,970
Options exercisable at end of year (in shares)	3,430,512
Vested and expected to vest at end of year (in shares)	5,687,901
Weighted Average Exercise Price
Options outstanding at beginning of year (in dollars per share)	$ 5.74	$ 6.97
Options granted (in dollars per share)	1.96	2.64
Options exercised (in dollars per share)		1.98
Options cancelled/forfeited (in dollars per share)	4.19	7.62
Options outstanding at end of year (in dollars per share)	4.34	$ 5.74	$ 6.97
Options exercisable at end of year (in dollars per share)	$ 5.48
Weighted Average Remaining Contractual Life (Years)
Options outstanding	7 years 4 months 24 days	7 years 4 months 24 days	7 years 6 months
Options exercisable	6 years 4 months 24 days
Estimated stock price (in dollars per share)	$ 0.58