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Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |
Gross Unrealized Gains (Losses) | $ 3,109 | $ 17,805 |
Securities available for sale | 902,292 | 909,247 |
Amortized Cost | 896,495 | 924,054 |
Held-to-maturity Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment | 306,800 | 250,300 |
Available-for-sale Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment | 584,800 | 636,300 |
Sales [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Available-for-sale Securities, Gross Realized Losses | 0 | 4,719 | $ 359
Available-for-sale Securities, Gross Realized Losses | 7 | 0 | 316
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments | (7) | 4,719 | 43
Calls And Maturities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Gross Gains | 29 | 3,383 | 5,057
Available-for-sale Securities, Gross Realized Losses | 0 | 0 | 60
Gain Losses Maturities And Impairment | $ 29 | 3,383 | $ 4,997
Pre Tsl Xiii [Member] | Calls And Maturities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Gross Gains |  | 4,300 |
Pre TSL VII [Member] | Calls And Maturities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Gross Gains |  | $ 700 |

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