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Summary of Significant Accounting Policies - Schedule of Financial Instruments Recognized at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Short-term available-for-sale investments |  | $ 415 | $ 1,361
Total current assets recorded at fair value |  | 415 | 1,361
Restricted cash | $ 3,750 | 3,732 | 3,668
Total non-current assets recorded at fair value |  | 3,732 | 3,668
Fair Value Inputs Level1 [Member] |  |  |
Short-term available-for-sale investments |  | 415 | 1,361
Total current assets recorded at fair value |  | 415 | 1,361
Restricted cash | 3,750 | 3,732 | 3,668
Total non-current assets recorded at fair value |  | 3,732 | 3,668
Fair Value Inputs Level2 [Member] |  |  |
Short-term available-for-sale investments |  |  |
Total current assets recorded at fair value |  |  |
Restricted cash |  |  |
Total non-current assets recorded at fair value |  |  |
Fair Value Inputs Level3 [Member] |  |  |
Short-term available-for-sale investments |  |  |
Total current assets recorded at fair value |  |  |
Restricted cash |  |  |
Total non-current assets recorded at fair value |  |  |

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