Stockholders' Equity - Summary of Company's Outstanding Common Stock Warrants (Detail) - $ / shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Number of warrants outstanding	22,216	24,381
1st Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Feb. 11, 2019
Exercise Price	$ 2.56
Number of warrants outstanding		293
2nd Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Feb. 18, 2019
Exercise Price	$ 2.75
Number of warrants outstanding		1,872
3rd Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Aug. 28, 2019
Exercise Price	$ 2.90
Number of warrants outstanding	2,700	2,700
4th Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Mar. 20, 2020
Exercise Price	$ 2.13
Number of warrants outstanding	234	234
5th Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Mar. 25, 2020
Exercise Price	$ 1.71
Number of warrants outstanding	2,920	2,920
6th Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Apr. 12, 2020
Exercise Price	$ 0.40
Number of warrants outstanding	7,437	7,437
7th Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Jun. 20, 2020
Exercise Price	$ 0.40
Number of warrants outstanding	7,437	7,437
8th Private Placement [Member]
Class of Warrant or Right [Line Items]
Expiration Date	Apr. 30, 2023
Exercise Price	$ 0.20
Number of warrants outstanding	1,488	1,488