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Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 987,495 | $ 946,751
Gross Unrealized Gains | 25,318 | 10,721
Gross Unrealized Losses | (16,533) | (7,334)
Allowance for Credit Losses | 0 |
Estimated Fair Value | 996,280 | 950,138
Amortized Cost | 337,165 | 375,606
Gross Unrealized Gains | 17,014 | 6,399
Gross Unrealized Losses | 0 | (480)
Estimated Fair Value | 354,179 | 381,525
Allowance for Credit Losses | 0 |
Obligations of U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 414,494 | 466,139
Gross Unrealized Gains | 20,320 | 7,261
Gross Unrealized Losses | 0 | (420)
Allowance for Credit Losses | 0 |
Estimated Fair Value | 434,814 | 472,980
Amortized Cost | 324,976 | 361,785
Gross Unrealized Gains | 16,596 | 6,072
Gross Unrealized Losses | 0 | (480)
Estimated Fair Value | 341,572 | 367,377
Allowance for Credit Losses | 0 |
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 104,811 | 106,373
Gross Unrealized Gains | 4,835 | 3,229
Gross Unrealized Losses | 0 | (1)
Allowance for Credit Losses | 0 |
Estimated Fair Value | 109,646 | 109,601
Amortized Cost | 12,189 | 13,821
Gross Unrealized Gains | 418 | 327
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 12,607 | 14,148
Allowance for Credit Losses | 0 |
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,444 | 2,430
Gross Unrealized Gains | 126 | 102
Gross Unrealized Losses | 0 | 0
Allowance for Credit Losses | 0 |
Estimated Fair Value | 2,570 | 2,532
Asset backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 465,746 | 371,809
Gross Unrealized Gains | 37 | 129
Gross Unrealized Losses | (16,533) | (6,913)
Allowance for Credit Losses | 0 |
Estimated Fair Value | $ 449,250 | $ 365,025

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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=118939692&loc=d3e27161-111563 + Details
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