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Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Gain (Loss) on Investments [Line Items] |  |  |
Proceeds from sale | $ 143,660 | $ 55,744 | $ 88,054
Maturities, calls and impairment: |  |  |
Net gains (losses) and impairment | 5,040 | 617 | (153)
Calls And Maturities [Member] |  |  |
Sales Transactions: |  |  |
Gross losses | (60) | 0 | 0
Maturities, calls and impairment: |  |  |
Gross gains | 5,057 | 589 | 131
Other-than-temporary impairment | 0 | 0 | 0
Total gain (losses) maturities and impairment | 4,997 | 589 | 131
Sales [Member] |  |  |
Sales Transactions: |  |  |
Available-for-sale Securities, Gross Realized Losses | 359 | 305 | 0
Gross losses | (316) | (277) | (284)
Total gain (losses) from sales transactions | $ 43 | $ 28 | $ (284)

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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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