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Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Schedule of Available-for-Sale Securities [Line Items] |  |
Available for Sale, Less than 12 Months, Estimated Fair Value | $ 1,018,332 |
Available for Sale, Less than 12 Months, Unrealized Losses | 12,828 |
Available for Sale, More than 12 Months, Estimated Fair Value | 0 |
Available for Sale, More than 12 Months, Unrealized Losses | 0 |
Available for Sale, Total Estimated Fair Value | 1,018,332 |
Available-for-sale Securities, Gross Unrealized Loss, before Tax | 12,828 | $ 4
U.S. Treasury [Member] |  |
Schedule of Available-for-Sale Securities [Line Items] |  |
Available for Sale, Less than 12 Months, Estimated Fair Value | 194,344 |
Available for Sale, Less than 12 Months, Unrealized Losses | 2,742 |
Available for Sale, More than 12 Months, Estimated Fair Value | 0 |
Available for Sale, More than 12 Months, Unrealized Losses | 0 |
Available for Sale, Total Estimated Fair Value | 194,344 |
Available-for-sale Securities, Gross Unrealized Loss, before Tax | 2,742 | 0
Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-Sale Securities [Line Items] |  |
Available for Sale, Less than 12 Months, Estimated Fair Value | 673,043 |
Available for Sale, Less than 12 Months, Unrealized Losses | 8,158 |
Available for Sale, More than 12 Months, Estimated Fair Value | 0 |
Available for Sale, More than 12 Months, Unrealized Losses | 0 |
Available for Sale, Total Estimated Fair Value | 673,043 |
Available-for-sale Securities, Gross Unrealized Loss, before Tax | 8,158 | 4
States and political subdivisions [Member] |  |
Schedule of Available-for-Sale Securities [Line Items] |  |
Available for Sale, Less than 12 Months, Estimated Fair Value | 150,945 |
Available for Sale, Less than 12 Months, Unrealized Losses | 1,928 |
Available for Sale, More than 12 Months, Estimated Fair Value | 0 |
Available for Sale, More than 12 Months, Unrealized Losses | 0 |
Available for Sale, Total Estimated Fair Value | 150,945 |
Available-for-sale Securities, Gross Unrealized Loss, before Tax | $ 1,928 | $ 0

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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
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