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Securities (Tables) | 3 Months Ended
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Mar. 31, 2018
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 March 31, 2018 and December 31, 2017 are summarized as follows (in thousands):
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
March 31, 2018: |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S. Treasury securities | $ | 31,002 |  |  | $ | — |  |  | $ | 56 |  |  | $ | 30,946 |
U.S. government agency securities | 176,305 |  |  | 43 |  |  | 2,905 |  |  | 173,443 |
Mortgage-backed securities | 1,302,274 |  |  | 4,393 |  |  | 29,394 |  |  | 1,277,273 |
State and municipal securities | 1,206,777 |  |  | 3,890 |  |  | 18,202 |  |  | 1,192,465 |
Asset-backed securities | 204,800 |  |  | 377 |  |  | 505 |  |  | 204,672 |
Corporate notes and other | 81,860 |  |  | 868 |  |  | 903 |  |  | 81,825 |
| $ | 3,003,018 |  |  | $ | 9,571 |  |  | $ | 51,965 |  |  | $ | 2,960,624 |
Securities held-to-maturity: |  |  |  |  |  |  |  |  |  |  |
State and municipal securities | $ | 20,677 |  |  | $ | 46 |  |  | $ | 120 |  |  | $ | 20,603 |
| $ | 20,677 |  |  | $ | 46 |  |  | $ | 120 |  |  | $ | 20,603 |
December 31, 2017: |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S. Treasury securities | $ | 30,505 |  |  | $ | — |  |  | $ | 60 |  |  | $ | 30,445 |
U.S. government agency securities | 182,500 |  |  | 67 |  |  | 1,766 |  |  | 180,801 |
Mortgage-backed securities | 1,270,625 |  |  | 5,318 |  |  | 12,124 |  |  | 1,263,819 |
State and municipal securities | 774,949 |  |  | 12,251 |  |  | 2,588 |  |  | 784,612 |
Asset-backed securities | 173,346 |  |  | 262 |  |  | 316 |  |  | 173,292 |
Corporate notes and other | 81,615 |  |  | 1,346 |  |  | 647 |  |  | 82,314 |
| $ | 2,513,540 |  |  | $ | 19,244 |  |  | 17,501 |  |  | $ | 2,515,283 |
Securities held-to-maturity: |  |  |  |  |  |  |  |  |  |  |
State and municipal securities | $ | 20,762 |  |  | $ | 114 |  |  | $ | 46 |  |  | $ | 20,830 |
| $ | 20,762 |  |  | $ | 114 |  |  | $ | 46 |  |  | $ | 20,830 |
Amortized Cost and Fair Value of Debt Securities by Contractual Maturity | The amortized cost and fair value of debt securities as of March 31, 2018 by contractual maturity are shown below. Actual maturities may differ from contractual maturities of mortgage- and asset-backed securities since the mortgages and assets 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
March 31, 2018: | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Due in one year or less | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Due in one year to five years | 96,941 |  |  | 97,087 |  |  | 7,496 |  |  | 7,489 |
Due in five years to ten years | 177,898 |  |  | 177,231 |  |  | 10,385 |  |  | 10,316 |
Due after ten years | 1,221,105 |  |  | 1,204,361 |  |  | 2,796 |  |  | 2,798 |
Mortgage-backed securities | 1,302,274 |  |  | 1,277,273 |  |  | — |  |  | — |
Asset-backed securities | 204,800 |  |  | 204,672 |  |  | — |  |  | — |
| $ | 3,003,018 |  |  | $ | 2,960,624 |  |  | $ | 20,677 |  |  | $ | 20,603 |
Classification of Investments According to Term of Unrealized Losses of Less than Twelve Months or Twelve Months or Longer | At March 31, 2018 and December 31, 2017, the following investments had unrealized losses. The table below classifies these investments according to the term of the unrealized losses 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
At March 31, 2018 |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | 30,697 |  |  | $ | 56 |  |  | $ | — |  |  | $ | — |  |  | $ | 30,697 |  |  | $ | 56 |
U.S. government agency securities | 145,967 |  |  | 2,904 |  |  | 248 |  |  | 1 |  |  | 146,215 |  |  | 2,905 |
Mortgage-backed securities | 779,194 |  |  | 17,392 |  |  | 297,735 |  |  | 12,002 |  |  | 1,076,929 |  |  | 29,394 |
State and municipal securities | 863,010 |  |  | 15,987 |  |  | 47,894 |  |  | 2,335 |  |  | 910,904 |  |  | 18,322 |
Asset-backed securities | 79,428 |  |  | 449 |  |  | 9,482 |  |  | 56 |  |  | 88,910 |  |  | 505 |
Corporate notes | 31,880 |  |  | 754 |  |  | 11,697 |  |  | 149 |  |  | 43,577 |  |  | 903 |
Total temporarily-impaired securities | $ | 1,930,176 |  |  | $ | 37,542 |  |  | $ | 367,056 |  |  | $ | 14,543 |  |  | $ | 2,297,232 |  |  | $ | 52,085 |
At December 31, 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | 29,948 |  |  | $ | 60 |  |  | $ | — |  |  | $ | — |  |  | $ | 29,948 |  |  | $ | 60 |
U.S. government agency securities | 173,677 |  |  | 1,766 |  |  | — |  |  | — |  |  | 173,677 |  |  | 1,766 |
Mortgage-backed securities | 607,408 |  |  | 5,042 |  |  | 285,561 |  |  | 7,082 |  |  | 892,969 |  |  | 12,124 |
State and municipal securities | 115,403 |  |  | 1,408 |  |  | 50,083 |  |  | 1,226 |  |  | 165,486 |  |  | 2,634 |
Asset-backed securities | 68,742 |  |  | 198 |  |  | 14,136 |  |  | 118 |  |  | 82,878 |  |  | 316 |
Corporate notes | 22,168 |  |  | 547 |  |  | 11,944 |  |  | 100 |  |  | 34,112 |  |  | 647 |
Total temporarily-impaired securities | $ | 1,017,346 |  |  | $ | 9,021 |  |  | $ | 361,724 |  |  | $ | 8,526 |  |  | $ | 1,379,070 |  |  | $ | 17,547 |

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