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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract] |
Schedule of Amortized Cost and Fair Values of Investment Securities | The following tables present the amortized cost and estimated fair values of investment securities, as of December 31:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
| (in thousands)
2020 |  |  |  |  |  |  |
Available for Sale |  |  |  |  |  |  |
State and municipal securities | $ | 891,327 |  |  | $ | 61,286 |  |  | $ | — |  |  | $ | 952,613 |
Corporate debt securities | 348,391 |  |  | 19,445 |  |  | (691) |  |  | 367,145 |
Collateralized mortgage obligations | 491,321 |  |  | 12,560 |  |  | (115) |  |  | 503,766 |
Residential mortgage-backed securities | 373,779 |  |  | 4,246 |  |  | (27) |  |  | 377,998 |
Commercial mortgage-backed securities | 741,172 |  |  | 22,384 |  |  | (1,141) |  |  | 762,415 |
Auction rate securities | 101,510 |  |  | — |  |  | (3,304) |  |  | 98,206 |
Total | $ | 2,947,500 |  |  | $ | 119,921 |  |  | $ | (5,278) |  |  | $ | 3,062,143 |
Held to Maturity |  |  |  |  |  |  |
Residential mortgage-backed securities | $ | 278,281 |  |  | $ | 18,576 |  |  | $ | — |  |  | $ | 296,857 |
Total | $ | 278,281 |  |  | $ | 18,576 |  |  | $ | — |  |  | $ | 296,857 |
2019 |  |  |  |  |  |  |
Available for Sale |  |  |  |  |  |  |
State and municipal securities | $ | 638,125 |  |  | $ | 15,826 |  |  | $ | (1,024) |  |  | $ | 652,927 |
Corporate debt securities | 370,401 |  |  | 8,490 |  |  | (1,534) |  |  | 377,357 |
Collateralized mortgage obligations | 682,307 |  |  | 11,726 |  |  | (315) |  |  | 693,718 |
Residential mortgage-backed securities | 177,183 |  |  | 1,078 |  |  | (949) |  |  | 177,312 |
Commercial mortgage-backed securities | 489,603 |  |  | 6,471 |  |  | (1,777) |  |  | 494,297 |
Auction rate securities | 107,410 |  |  | — |  |  | (5,484) |  |  | 101,926 |
Total | $ | 2,465,029 |  |  | $ | 43,591 |  |  | $ | (11,083) |  |  | $ | 2,497,537 |
Held to Maturity |  |  |  |  |  |  |
Residential mortgage-backed securities | $ | 369,841 |  |  | $ | 13,864 |  |  | $ | — |  |  | $ | 383,705 |
Total | $ | 369,841 |  |  | $ | 13,864 |  |  | $ | — |  |  | $ | 383,705 |
Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities | The amortized cost and estimated fair values of debt securities as of December 31, 2020, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.
| Available for Sale |  | Held to Maturity
| Amortized Cost |  | Estimated Fair Value |  | Amortized Cost |  | Estimated Fair Value
| (in thousands)
Due in one year or less | $ | 11,250 |  |  | $ | 11,350 |  |  | $ | — |  |  | $ | — |
Due from one year to five years | 39,069 |  |  | 40,717 |  |  | — |  |  | — |
Due from five years to ten years | 327,456 |  |  | 345,612 |  |  | — |  |  | — |
Due after ten years | 963,453 |  |  | 1,020,285 |  |  | — |  |  | — |
| 1,341,228 |  |  | 1,417,964 |  |  | — |  |  | — |
Residential mortgage-backed securities (1) | 373,779 |  |  | 377,998 |  |  | 278,281 |  |  | 296,857 |
Commercial mortgage-backed securities (1) | 741,172 |  |  | 762,415 |  |  | — |  |  | — |
Collateralized mortgage obligations (1) | 491,321 |  |  | 503,766 |  |  | — |  |  | — |
Total | $ | 2,947,500 |  |  | $ | 3,062,143 |  |  | $ | 278,281 |  |  | $ | 296,857 |
(1) Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.
Summary of Gains and Losses from Equity and Debt Securities, and Losses Recognized from Other-than-Temporary Impairment | The following table presents information related to gross gains and losses on the sales of securities:
| Gross Realized Gains |  | Gross Realized Losses |  | Net Gains
| (in thousands)
2020 | $ | 6,545 |  |  | $ | (3,492) |  |  | $ | 3,053 |
2019 | 11,554 |  |  | (6,821) |  |  | 4,733 |
2018 | 1,665 |  |  | (1,628) |  |  | 37 |
Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in Continuous Unrealized Loss Position | The following tables present the gross unrealized losses and estimated fair values of investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, as of December 31:
| Less Than 12 months |  | 12 Months or Longer |  | Total
| Number of Securities |  | Estimated Fair Value |  | Unrealized Losses |  | Number of Securities |  | Estimated Fair Value |  | Unrealized Losses |  | Estimated Fair Value |  | Unrealized Losses
2020 | (dollars in thousands)
Available for Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities | 9 |  |  | $ | 44,528 |  |  | $ | (377) |  |  | 1 |  |  | $ | 6,871 |  |  | $ | (314) |  |  | $ | 51,399 |  |  | $ | (691) |
Collateralized mortgage obligations | 3 |  |  | 57,601 |  |  | (115) |  |  | — |  |  | — |  |  | — |  |  | 57,601 |  |  | (115) |
Residential mortgage-backed securities | 1 |  |  | 20,124 |  |  | (27) |  |  | — |  |  | — |  |  | — |  |  | 20,124 |  |  | (27) |
Commercial mortgage-backed securities | 9 |  |  | 144,383 |  |  | (1,141) |  |  | — |  |  | — |  |  | — |  |  | 144,383 |  |  | (1,141) |
Auction rate securities | — |  |  | — |  |  | — |  |  | 162 |  |  | 98,206 |  |  | (3,304) |  |  | 98,206 |  |  | (3,304) |
Total available for sale | 22 |  |  | $ | 266,636 |  |  | $ | (1,660) |  |  | 163 |  |  | $ | 105,077 |  |  | $ | (3,618) |  |  | $ | 371,713 |  |  | $ | (5,278) |
2019 |
Available for Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
State and municipal securities | 44 |  |  | $ | 136,344 |  |  | $ | (1,024) |  |  | — |  |  | $ | — |  |  | $ | — |  |  | $ | 136,344 |  |  | $ | (1,024) |
Corporate debt securities | 5 |  |  | 30,719 |  |  | (346) |  |  | 8 |  |  | 18,759 |  |  | (1,188) |  |  | 49,478 |  |  | (1,534) |
Collateralized mortgage obligations | 5 |  |  | 33,865 |  |  | (190) |  |  | 1 |  |  | 5,330 |  |  | (125) |  |  | 39,195 |  |  | (315) |
Residential mortgage-backed securities | 5 |  |  | 12,247 |  |  | (40) |  |  | 26 |  |  | 127,373 |  |  | (909) |  |  | 139,620 |  |  | (949) |
Commercial mortgage-backed securities | 7 |  |  | 121,340 |  |  | (1,777) |  |  | — |  |  | — |  |  | — |  |  | 121,340 |  |  | (1,777) |
Auction rate securities | — |  |  | — |  |  | — |  |  | 177 |  |  | 101,926 |  |  | (5,484) |  |  | 101,926 |  |  | (5,484) |
Total available for sale | 66 |  |  | $ | 334,515 |  |  | $ | (3,377) |  |  | 212 |  |  | $ | 253,388 |  |  | $ | (7,706) |  |  | $ | 587,903 |  |  | $ | (11,083) |

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