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Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 508,986 | $ 542,289
Gross Unrealized Gains | 8,859 | 10,626
Gross Unrealized Losses | (258) | (683)
Estimated Fair Value | 517,587 | 552,232
U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 445,813 | 465,416
Gross Unrealized Gains | 8,155 | 9,820
Gross Unrealized Losses | (182) |
Estimated Fair Value | 453,786 | 475,236
U.S. Federal Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 19,166 | 19,697
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | (58) | (60)
Estimated Fair Value | 19,109 | 19,638
Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 14,877 | 15,268
Gross Unrealized Gains | 372 | 428
Estimated Fair Value | 15,249 | 15,696
States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 15,789 | 28,571
Gross Unrealized Gains | 269 | 377
Gross Unrealized Losses | (18) |
Estimated Fair Value | 16,040 | 28,948
Asset backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 13,341 | 13,337
Gross Unrealized Gains | 62 |
Gross Unrealized Losses |  | (623)
Estimated Fair Value | $ 13,403 | $ 12,714

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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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