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Fair Value of Financial Instruments - Schedule of Inputs of the Black-Scholes Option Pricing Model (Details) - Black-Scholes option pricing model - Level 3 | Dec. 31, 2017 $ / shares | Dec. 31, 2015 $ / shares | Dec. 31, 2014 $ / shares
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Weichai warrant liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Contractual term | 1 year |  |
Private placement warranty liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Contractual term |  | 6 months 29 days | 1 year 4 months
Market value of the Common Stock | Weichai warrant liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input | 7.50 |  |
Market value of the Common Stock | Private placement warranty liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input |  | 18.25 | 51.61
Exercise price | Private placement warranty liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input |  | 13.00 | 13.00
Risk-free interest rate | Weichai warrant liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input | 0.018 |  |
Estimated price volatility | Weichai warrant liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input | 0.95 |  |
Estimated price volatility | Private placement warranty liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input |  | 0.55 | 0.55
Dividend yield | Weichai warrant liability |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Option pricing model input | 0 |  |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Period between issuance and expiration of outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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