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Investment Securities - Additional Information (Detail) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 USD ($) security | Dec. 31, 2020 USD ($) security | Dec. 31, 2019 USD ($) security | Dec. 31, 2018 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |
Reclassification of securities | $ 382,000,000.0 |  | $ 382,000,000.0 |
Increase in capital due to reclassification of held-to-maturity portfolio to available-for-sale portfolio | 19,400,000 |  |  |
Unrealized holding gains, net of tax | 13,000,000.0 |  |  |
Unrealized holding gains (losses) on available-for-sale debt securities |  | $ 89,204,000 | 93,479,000 | $ 5,792,000
Securities pledged to secure public and other funds, carrying value | 2,104,000,000 | 2,427,000,000 | 2,104,000,000 |
Accrued interest receivable on available-for-sale debt securities |  | $ 27,000,000.0 |  |
OTTI losses on securities |  |  | 0 | 0
Number of securities in security portfolio | security |  | 1,918 |  |
Number of securities in unrealized loss position | security |  | 69 |  |
Total investment securities - available-for-sale | 5,385,091,000 | $ 5,970,115,000 | 5,385,091,000 |
Unrealized losses | 21,660,000 | 11,385,000 | 21,660,000 |
Equity securities, at fair value | 6,842,000 | 2,547,000 | 6,842,000 |
Gains on equity securities |  | 1,400,000 | 700,000 | 100,000
Impairments on equity securities without readily determinable fair value |  | 117,000 | 0 | 0
Downward adjustments on equity securities without readily determinable fair value |  | 117,000 | 0 | $ 0
Other assets |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |
Equity securities without readily determinable fair value | 91,400,000 | 105,800,000 | 91,400,000 |
Pooled trust preferred securities |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |
OTTI losses on securities |  | $ 0 | 0 |
Number of pooled trust preferred securities | security |  | 2 |  |
Total investment securities - available-for-sale | 8,222,000 | $ 7,913,000 | 8,222,000 |
Unrealized losses | $ 5,589,000 | 5,850,000 | $ 5,589,000 |
Indiana |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |
Investment securities issued by states and political subdivisions, market value |  | $ 515,600,000 |  |
State and political subdivision investment, equity percentage |  | 17.30% |  |
Percentage of municipal bonds rated A or better |  | 99.00% |  |
Percentage of non rated local interest bonds |  | 1.00% |  |
Texas |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |
Investment securities issued by states and political subdivisions, market value |  | $ 194,300,000 |  |
State and political subdivision investment, equity percentage |  | 6.50% |  |
Municipal Bond |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |
Number of securities sold | security | 6 |  | 6 |
Investment securities held-to-maturity sold | $ 9,700,000 |  |  |
Gain on investment securities held-to-maturity sold | $ 300,000 |  |  |

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