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Securities (Year End Securities Held to Maturity) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | $ 1,945,833 | $ 2,030,005
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 107,063 | 28,931
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 10,261
Debt Securities, Held to Maturity, Estimated Fair Value | 2,052,896 | 2,048,675
Debt Securities, Held-to-maturity, Allowance for Credit Loss | (160) | 0
Debt Securities, Held-to-Maturity Net Carrying Amount | 1,945,673 | 2,030,005
Residential Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 528,784 | 530,861
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 41,742 | 22
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 9,365
Debt Securities, Held to Maturity, Estimated Fair Value | 570,526 | 521,518
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Debt Securities, Held-to-Maturity Net Carrying Amount | 528,784 | 530,861
States and Political Subdivisions [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 1,415,549 | 1,497,644
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 65,321 | 28,909
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 896
Debt Securities, Held to Maturity, Estimated Fair Value | 1,480,870 | 1,525,657
Debt Securities, Held-to-maturity, Allowance for Credit Loss | (160) | 0
Debt Securities, Held-to-Maturity Net Carrying Amount | 1,415,389 | 1,497,644
Other [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 1,500 | 1,500
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 0 | 0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 0
Debt Securities, Held to Maturity, Estimated Fair Value | 1,500 | 1,500
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Debt Securities, Held-to-Maturity Net Carrying Amount | $ 1,500 | $ 1,500

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