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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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