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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
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Investment Securities [Line Items] |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost |  | $ 5,597 |
Amortized Cost |  | 947,345 | $ 896,495
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 25,496 | 8,906
Gross Unrealized Losses |  | (215) | (3,109)
Securities available for sale, at fair value |  | 972,626 | 902,292
Due within one year, Estimated Fair Value |  | 5,601 |
Due after one but within five years, Amortized Cost |  | 23,248 |
Due after one but within five years, Estimated Fair Value |  | 23,536 |
Due after five but within ten years, Amortized Cost |  | 4,970 |
Due after five but within ten years, Estimated Fair Value |  | 5,392 |
Due after ten years, Amortized Cost |  | 0 |
Due after ten years, Estimated Fair Value |  | 0 |
Due within one year, Amortized Cost |  | 33,815 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value |  | 34,529 |
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost | [1] | 913,530 |
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value | [1] | 938,097 |
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 243,300 |
Securities available for sale, at fair value |  | 248,900 |
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 689,224 | 661,947
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 1,004 | 1,000
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 10,027 | 17,909
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 23,498 | 23,962
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 947,345 |
Securities available for sale, at fair value |  | 972,626 |
US Government Agency Debt Securities Commercial [Domain] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 235,884 | 186,316
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 5,088 | 2,983
Gross Unrealized Losses |  | (215) | (166)
Securities available for sale, at fair value |  | 240,757 | 189,133
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 22,924 | 22,919
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 574 | 1,043
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 23,498 | 23,962
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 9,891 | 17,738
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 136 | 171
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 10,027 | 17,909
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 1,000 | 1,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 4 | 0
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 1,004 | 1,000
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 670,251 | 660,777
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 18,973 | 4,113
Gross Unrealized Losses |  | 0 | (2,943)
Securities available for sale, at fair value |  | 689,224 | 661,947
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 7,395 | 7,745
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 721 | 596
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | $ 8,116 | $ 8,341
[1] | Mortgage-Backed Securities include an amortized cost of $243.3 million and a fair value of $248.9 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $670.3 million and a fair value of $689.2 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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