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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Investment Securities [Line Items] |  |  |
Due within 1 year, Amortized Cost |  | $ 104,995 |
Due after 1 but within 5 years, Amortized Cost |  | 19,349 |
Due after 5 but within 10 years, Amortized Cost |  | 10,747 |
Due after 10 years, Amortized Cost |  | 0 |
Debt securities gross, Amortized Cost |  | 135,091 |
Amortized Cost |  | 805,515 | $ 896,495
Total debt securities, Amortized Cost |  | 805,515 |
Due within 1 year, Estimated Fair Value |  | 105,015 |
Due after 1 but within 5 years, Estimated Fair Value |  | 20,395 |
Due after 5 but within 10 years, Estimated Fair Value |  | 11,370 |
Due after 10 years, Estimated Fair Value |  | 0 |
Debt securities gross, Estimated Fair Value |  | 136,780 |
Securities available for sale |  | 831,223 | $ 902,292
Total debt securities, Estimated Fair Value |  | 831,223 |
Mortgage-Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost | [1] | 670,424 |
Securities available for sale | [1] | $ 694,443 |
[1] | Mortgage Backed Securities include an amortized cost of $189.3 million and a fair value of $198.4 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $481.1 million and a fair value of $496.0 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.

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