Stockholders' Equity - Company's Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
2003 Stock Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	667,080
Options exercised	(11,000)
Outstanding at end of period	656,080	667,080
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 1.24
Options exercised	1.12
Outstanding at end of period	$ 1.25	$ 1.24
VESTED AND EXPECTED TO VEST
Outstanding options	656,080
Aggregate Intrinsic Value (in thousands)	$ 7,358
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	656,080
Weighted-average exercise price exercisable at end of period	$ 1.25
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 7,358
Weighted-average contractual term outstanding	1 year 10 months 24 days	2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 7,358
2013 Equity Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	3,265,254
Options granted	922,596
Options exercised	(3,585)
Options forfeited	(4,175)
Outstanding at end of period	4,180,090	3,265,254
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 8.39
Options granted	10.65
Options exercised	5.09
Options forfeited	16.24
Outstanding at end of period	$ 10.33	$ 8.39
VESTED AND EXPECTED TO VEST
Outstanding options	4,180,090
Weighted-Average Exercise Price	$ 10.33
Weighted-Average Remaining Contractual Term	7 years 6 months
Aggregate Intrinsic Value (in thousands)	$ 14,618
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	2,283,631
Weighted-average exercise price exercisable at end of period	$ 8.60
Weighted-average contractual term (years) exercisable at end of period	6 years 2 months 12 days
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 10,545
Weighted-average contractual term outstanding	7 years 6 months	7 years 1 month 6 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 14,618

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