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Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Fair Value Disclosures [Abstract] |  |
Transfers from Level 2 to Level 1 | $ 0 | $ 2,416

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- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Balance Type: | na
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