Equity Financings and Warrants (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 11, 2019	Jan. 11, 2017
Equity Financing And Warrants [Line Items]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares issued	1,098,813	773,558
JonesTrading
Equity Financing And Warrants [Line Items]
Percentage of selling commission per additional shares sold					3.00%
Sales Agreement
Equity Financing And Warrants [Line Items]
Common stock, shares issued	325,304	118,546	148,751
Net proceeds from issuance of common stock	$ 2,900,000	$ 3,400,000	$ 6,100,000
Sales Agreement | JonesTrading
Equity Financing And Warrants [Line Items]
Common stock, par value					$ 0.001
Aggregate offering price of common stock authorized				$ 13,300,000	$ 7,300,000