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Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value Disclosures [Abstract] |  |
Net derivative liability adjustment | $ 2.8 | $ 2.0

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- Definition Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_FairValueDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration