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Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Net Investment Income [Line Items] |  |
Total interest income from available-for-sale securities | $ 11,868 | $ 12,640
Total interest income from held-to-maturity securities | 4,765 | 5,507
Total investment income | 16,633 | 18,147
Investment securities available-for-sale [Member] |  |
Net Investment Income [Line Items] |  |
Taxable | 11,445 | 11,926
Tax-advantaged | 423 | 714
Investment securities held-to-maturity [Member] |  |
Net Investment Income [Line Items] |  |
Taxable | 2,878 | 3,277
Tax-advantaged | $ 1,887 | $ 2,230

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- Definition Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.2,3) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24721-111560 + Details
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- Definition Amount of operating interest income, including amortization and accretion of premiums and discounts, on held-to-maturity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.2) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24721-111560 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 + Details
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