Stockholders' Equity - Company's Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 $ / shares shares
2003 Stock Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period \| shares	667,080
Options exercised \| shares	(11,000)
Outstanding at end of period \| shares	656,080	667,080
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period \| $ / shares	$ 1.24
Options exercised \| $ / shares	1.12
Outstanding at end of period \| $ / shares	$ 1.25	$ 1.24
VESTED AND EXPECTED TO VEST
Outstanding options \| shares	656,080
Weighted-Average Exercise Price \| $ / shares	$ 1.25
Aggregate Intrinsic Value (in thousands) \| $	$ 7,004
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period \| shares	656,080
Weighted-average exercise price exercisable at end of period \| $ / shares	$ 1.25
Weighted-average contractual term (years) exercisable at end of period	1 year 8 months 12 days
Aggregate intrinsic value (in thousands) exercisable at end of period \| $	$ 7,004
Weighted-average contractual term outstanding	1 year 8 months 12 days	2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period \| $	$ 7,004
2013 Equity Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period \| shares	3,265,254
Options granted \| shares	1,172,596
Options exercised \| shares	(16,606)
Options forfeited \| shares	(4,175)
Outstanding at end of period \| shares	4,417,069	3,265,254
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period \| $ / shares	$ 8.39
Options granted \| $ / shares	11.00
Options exercised \| $ / shares	5.66
Options forfeited \| $ / shares	16.24
Outstanding at end of period \| $ / shares	$ 10.45	$ 8.39
VESTED AND EXPECTED TO VEST
Outstanding options \| shares	4,417,069
Weighted-Average Exercise Price \| $ / shares	$ 10.45
Weighted-Average Remaining Contractual Term	7 years 4 months 24 days
Aggregate Intrinsic Value (in thousands) \| $	$ 12,795
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period \| shares	2,450,440
Weighted-average exercise price exercisable at end of period \| $ / shares	$ 9.00
Weighted-average contractual term (years) exercisable at end of period	6 years 1 month 6 days
Aggregate intrinsic value (in thousands) exercisable at end of period \| $	$ 9,830
Weighted-average contractual term outstanding	7 years 4 months 24 days	7 years 1 month 6 days
Aggregate intrinsic value (in thousands) outstanding at end of period \| $	$ 12,795

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