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Employee Benefits Plans - Summary of Discretionary Contributions (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Retirement Benefits [Abstract] |  |  |
Discretionary contributions | $ 1,013 | $ 934 | $ 763

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

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
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