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Summary of Portfolio of Securities Available for Sale and Equity Investments with Readily Determinable Fair Values (Parenthetical) (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Available-for-sale Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Non-credit portion of OTTI recorded in AOCL, pre-tax | $ 8.6 | $ 8.6

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- Definition Accumulated Other Comprehensive Income Loss Noncredit Portion of Other Than Temporarily Impaired Securities Before Tax + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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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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Balance Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |