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Retirement Plans - Aggregate Amounts of Contributions to the 401(k) and ESOP Plans (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items] |  |  |
Total contributions | $ 4,874 | $ 4,426 | $ 3,959
401(k) contributions [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Aggregate amounts of contributions | 2,229 | 2,074 | 1,966
ESOP contributions [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Aggregate amounts of contributions | $ 2,645 | $ 2,352 | $ 1,993

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- Definition Aggregate contribution by employer. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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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 Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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- Details
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- Details
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