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Securities (Tables) | 12 Months Ended
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Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract] |
Summary of Amortized Cost and Fair Value of Available-for-sale and Held-to-maturity Securities | The amortized cost and fair value of securities available-for-sale and held-to-maturity at December 31, 2020 and 2019 are summarized as follows (in thousands):
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
December 31, 2020 |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S Treasury securities | $ | 82,199 |  |  | $ | 10 |  |  | $ | — |  |  | $ | 82,209 |
U.S. Government agency securities | 74,916 |  |  | 1,547 |  |  | 60 |  |  | 76,403 |
Mortgage-backed securities | 1,623,759 |  |  | 67,759 |  |  | 2,327 |  |  | 1,689,191 |
State and municipal securities | 1,411,288 |  |  | 44,559 |  |  | 12,484 |  |  | 1,443,363 |
Asset-backed securities | 177,878 |  |  | 715 |  |  | 657 |  |  | 177,936 |
Corporate notes | 117,256 |  |  | 2,632 |  |  | 2,309 |  |  | 117,579 |
| $ | 3,487,296 |  |  | $ | 117,222 |  |  | $ | 17,837 |  |  | $ | 3,586,681 |
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
December 31, 2020 |  |  |  |  |  |  |
Securities held-to-maturity: |  |  |  |  |  |  |
State and municipal securities | 1,028,550 |  |  | 38,272 |  |  | 291 |  |  | 1,066,531 |
| $ | 1,028,550 |  |  | $ | 38,272 |  |  | $ | 291 |  |  | $ | 1,066,531 |
Allowance for credit losses - securities held-to-maturity | (191) |  |  |  |  |  |  |
Securities held-to-maturity, net of allowance for credit losses | $ | 1,028,359 |  |  |  |  |  |  |
December 31, 2019 |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S Treasury securities | $ | 72,862 |  |  | $ | 19 |  |  | $ | 14 |  |  | $ | 72,867 |
U.S. Government agency securities | 80,096 |  |  | 306 |  |  | 710 |  |  | 79,692 |
Mortgage-backed securities | 1,458,894 |  |  | 12,789 |  |  | 7,776 |  |  | 1,463,907 |
State and municipal securities | 1,669,606 |  |  | 52,096 |  |  | 7,249 |  |  | 1,714,453 |
Asset-backed securities | 153,963 |  |  | 302 |  |  | 1,293 |  |  | 152,972 |
Corporate notes | 56,212 |  |  | 635 |  |  | 743 |  |  | 56,104 |
| $ | 3,491,633 |  |  | $ | 66,147 |  |  | $ | 17,785 |  |  | $ | 3,539,995 |
Securities held-to-maturity: |  |  |  |  |  |  |
State and municipal securities | 188,996 |  |  | 12,221 |  |  | — |  |  | 201,217 |
| $ | 188,996 |  |  | $ | 12,221 |  |  | $ | — |  |  | $ | 201,217 |
Amortized Cost and Fair Value of Debt Securities by Contractual Maturity | The amortized cost and fair value of debt securities as of December 31, 2020 by contractual maturity are shown below. Actual maturities may differ from contractual maturities of mortgage-backed securities since the mortgages underlying the securities may be called or prepaid with or without penalty. Therefore, these securities are not included in the maturity categories in the following summary (in thousands):
| Available-for-sale |  | Held-to-maturity
| Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Due in one year or less | $ | 83,050 |  |  | $ | 83,066 |  |  | $ | — |  |  | $ | — |
Due in one year to five years | 12,406 |  |  | 12,802 |  |  | 1,409 |  |  | 1,494 |
Due in five years to ten years | 188,991 |  |  | 198,687 |  |  | 7,180 |  |  | 7,300 |
Due after ten years | 1,401,212 |  |  | 1,424,999 |  |  | 1,019,961 |  |  | 1,057,737 |
Mortgage-backed securities | 1,623,759 |  |  | 1,689,191 |  |  | — |  |  | — |
Asset-backed securities | 177,878 |  |  | 177,936 |  |  | — |  |  | — |
| $ | 3,487,296 |  |  | $ | 3,586,681 |  |  | $ | 1,028,550 |  |  | $ | 1,066,531 |
Classification of Investments According to Term of Unrealized Losses of Less than Twelve Months or Twelve Months or Longer | At December 31, 2020 and 2019, included in securities were the following investments with unrealized losses. The information below classifies these investments according to the term of the unrealized loss of less than twelve months or twelve months or longer (in thousands):
| Investments with an Unrealized Loss of less than 12 months |  | Investments with an Unrealized Loss of 12 months or longer |  | Total Investments with an Unrealized Loss
| Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
December 31, 2020 |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
U.S. Government agency securities | 9,962 |  |  | 38 |  |  | 6,091 |  |  | 22 |  |  | 16,053 |  |  | 60 |
Mortgage-backed securities | 165,696 |  |  | 1,772 |  |  | 35,997 |  |  | 555 |  |  | 201,693 |  |  | 2,327 |
State and municipal securities | 175,115 |  |  | 2,220 |  |  | 345,435 |  |  | 10,264 |  |  | 520,550 |  |  | 12,484 |
Asset-backed securities | 46,399 |  |  | 207 |  |  | 52,840 |  |  | 450 |  |  | 99,239 |  |  | 657 |
Corporate notes | 9,978 |  |  | 40 |  |  | 23,920 |  |  | 2,269 |  |  | 33,898 |  |  | 2,309 |
Total temporarily-impaired securities | $ | 407,150 |  |  | $ | 4,277 |  |  | $ | 464,283 |  |  | $ | 13,560 |  |  | $ | 871,433 |  |  | $ | 17,837 |
December 31, 2019 |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | 40,505 |  |  | $ | 14 |  |  | $ | — |  |  | $ | — |  |  | $ | 40,505 |  |  | $ | 14 |
U.S. Government agency securities | 1,222 |  |  | 1 |  |  | 30,892 |  |  | 709 |  |  | 32,114 |  |  | 710 |
Mortgage-backed securities | 458,881 |  |  | 5,102 |  |  | 163,767 |  |  | 2,674 |  |  | 622,648 |  |  | 7,776 |
State and municipal securities | 204,958 |  |  | 1,938 |  |  | 244,884 |  |  | 5,311 |  |  | 449,842 |  |  | 7,249 |
Asset-backed securities | 75,488 |  |  | 796 |  |  | 59,816 |  |  | 497 |  |  | 135,304 |  |  | 1,293 |
Corporate notes | — |  |  | — |  |  | 16,908 |  |  | 743 |  |  | 16,908 |  |  | 743 |
Total temporarily-impaired securities | $ | 781,054 |  |  | $ | 7,851 |  |  | $ | 516,267 |  |  | $ | 9,934 |  |  | $ | 1,297,321 |  |  | $ | 17,785 |

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- Definition Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities. + References No definition available. + Details
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