Stock-Based Compensation - Summary of Non-Vested Restricted Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining Contractual Term (Years)	8 years 11 months 26 days	9 years 3 months 10 days	9 years 10 months 2 days	8 years 9 months 14 days
Non-vested Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Beginning balance, Number of Shares	983,796	1,678,240	2,372,684
Vested during the year, Number of Shares	694,444	694,444	694,444
Ending balance, Number of Shares	289,352	983,796	1,678,240	2,372,684
Beginning balance, Weighted Average Grant Date Fair Value	$ 0.000396	$ 0.000396	$ 0.000396
Vested during the year, Weighted Average Grant Date Fair Value	0.000396	0.000396	0.000396
Ending balance, Weighted Average Grant Date Fair Value	$ 0.000396	$ 0.000396	$ 0.000396	$ 0.000396
Remaining Contractual Term (Years)	4 months 24 days	1 year 4 months 24 days	2 years 4 months 24 days	3 years 4 months 24 days