Description of Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Place of incorporation	Arizona
Year of incorporation	2002
Place of reincorporation	Delaware
Date of reincorporation	2015-03
Reverse split of issued and outstanding common stock	1:20
Accumulated deficit	$ (190,978)		$ (178,366)
Cash and cash equivalents	10,438	$ 20,418	11,998	$ 9,125
Short-term investments	7,355		9,825
Net losses	(12,612)	(10,615)	(13,480)
Net cash used in operations	$ (3,969)	$ (2,617)	$ (13,600)