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Preferred and Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Number of Shares of common Stock Issuable	494,054	5,146,400
Issuance On August 14, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Aug. 14, 2015
Number of Shares of common Stock Issuable	81,228
Exercise Price	$ 21.78
Expiration Date	Aug. 14, 2020
Issuance On August 21, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Aug. 21, 2015
Number of Shares of common Stock Issuable	69,603
Exercise Price	$ 21.78
Expiration Date	Aug. 21, 2020
Issuance On October 25, 2016 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Nov. 25, 2016	Oct. 25, 2016
Number of Shares of common Stock Issuable	5,155	54,256
Exercise Price	$ 19.78	$ 1.88
Expiration Date	Oct. 24, 2026	Oct. 24, 2026
Issuance On November 1, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Nov. 01, 2017
Number of Shares of common Stock Issuable	130,609
Exercise Price	$ 19.78
Expiration Date	Oct. 31, 2020
Issuance On November 17, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Nov. 17, 2017
Number of Shares of common Stock Issuable	8,442
Exercise Price	$ 19.78
Expiration Date	Nov. 16, 2020
Issuance On December 4, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 04, 2017
Number of Shares of common Stock Issuable	5,661
Exercise Price	$ 19.78
Expiration Date	Dec. 03, 2020
Issuance On December 28, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 28, 2017
Number of Shares of common Stock Issuable	6,925
Exercise Price	$ 19.78
Expiration Date	Dec. 27, 2020
Issuance On December 28, 2017 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 28, 2017
Number of Shares of common Stock Issuable	115,916
Exercise Price	$ 19.78
Expiration Date	Dec. 28, 2027
Issuance On September 12, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Sep. 12, 2018
Number of Shares of common Stock Issuable	25,275
Exercise Price	$ 19.78
Expiration Date	Sep. 12, 2021
Issuance On September 12, 2018 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Sep. 12, 2018
Number of Shares of common Stock Issuable	20,220
Exercise Price	$ 19.78
Expiration Date	Sep. 12, 2028
Issuance On October 19, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Oct. 19, 2018
Number of Shares of common Stock Issuable	20,016
Exercise Price	$ 19.78
Expiration Date	Oct. 19, 2028
Issuance On December 11, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date	Dec. 11, 2018
Number of Shares of common Stock Issuable	5,004
Exercise Price	$ 19.80
Expiration Date	Oct. 19, 2021
Series A Preferred Stock Warrants [Member] | Issuance On August 14, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Aug. 14, 2015
Number of Shares of common Stock Issuable		854,785
Exercise Price		$ 2.07
Expiration Date		Aug. 14, 2020
Series A Preferred Stock Warrants [Member] | Issuance On August 21, 2015 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Aug. 21, 2015
Number of Shares of common Stock Issuable		732,453
Exercise Price		$ 2.07
Expiration Date		Aug. 21, 2020
Series B Preferred Stock Warrants [Member] | Issuance On November 1, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Nov. 01, 2017
Number of Shares of common Stock Issuable		1,374,435
Exercise Price		$ 1.88
Expiration Date		Oct. 31, 2020
Series B Preferred Stock Warrants [Member] | Issuance On November 17, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Nov. 17, 2017
Number of Shares of common Stock Issuable		88,845
Exercise Price		$ 1.88
Expiration Date		Nov. 16, 2020
Series B Preferred Stock Warrants [Member] | Issuance On December 4, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 04, 2017
Number of Shares of common Stock Issuable		59,576
Exercise Price		$ 1.88
Expiration Date		Dec. 03, 2020
Series B Preferred Stock Warrants [Member] | Issuance On December 28, 2017 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 28, 2017
Number of Shares of common Stock Issuable		72,875
Exercise Price		$ 1.88
Expiration Date		Dec. 27, 2020
Series B Preferred Stock Warrants [Member] | Issuance On December 28, 2017 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Dec. 28, 2017
Number of Shares of common Stock Issuable		1,219,815
Exercise Price		$ 1.88
Expiration Date		Dec. 28, 2027
Series B Preferred Stock Warrants [Member] | Issuance On September 12, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Sep. 12, 2018
Number of Shares of common Stock Issuable		265,957
Exercise Price		$ 1.88
Expiration Date		Sep. 12, 2021
Series B Preferred Stock Warrants [Member] | Issuance On September 12, 2018 One [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Sep. 12, 2018
Number of Shares of common Stock Issuable		212,765
Exercise Price		$ 1.88
Expiration Date		Sep. 12, 2028
Series B Preferred Stock Warrants [Member] | Issuance On October 19, 2018 [Member]
Class of Warrant or Right [Line Items]
Issuance Date		Oct. 19, 2018
Number of Shares of common Stock Issuable		210,638
Exercise Price		$ 1.88
Expiration Date		Oct. 19, 2028

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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