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Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 542,289 | $ 485,082
Gross Unrealized Gains | 10,626 | 5,407
Gross Unrealized Losses | (683) | (766)
Estimated Fair Value | 552,232 | 489,723
U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 465,416 | 409,488
Gross Unrealized Gains | 9,820 | 4,974
Gross Unrealized Losses |  | (13)
Estimated Fair Value | 475,236 | 414,449
U.S. Federal Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 19,697 | 23,039
Gross Unrealized Gains | 1 | 23
Gross Unrealized Losses | (60) | (38)
Estimated Fair Value | 19,638 | 23,024
Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 15,268 | 16,941
Gross Unrealized Gains | 428 | 128
Gross Unrealized Losses |  | (64)
Estimated Fair Value | 15,696 | 17,005
States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 28,571 | 22,294
Gross Unrealized Gains | 377 | 282
Gross Unrealized Losses |  | (45)
Estimated Fair Value | 28,948 | 22,531
Asset backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 13,337 | 13,320
Gross Unrealized Losses | (623) | (606)
Estimated Fair Value | $ 12,714 | $ 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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