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Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Summary of available-for-sale securities | The following is a summary of available-for-sale securities at December 31:
($ in thousands) |  |  | 2017 |  | 2016
Marketable Securities: | Fair Value Hierarchy |  | Cost |  | Estimated Fair Value |  | Cost |  | Estimated Fair Value
Certificates of deposit |  |  |  |  |  |  |  |  |
with unrecognized losses for less than 12 months |  |  | $ | 6,238 |  |  | $ | 6,222 |  |  | $ | 1,868 |  |  | $ | 1,863 |
with unrecognized losses for more than 12 months |  |  | 102 |  |  | 100 |  |  | — |  |  | — |
with unrecognized gains |  |  | 2,088 |  |  | 2,089 |  |  | 3,320 |  |  | 3,329 |
Total Certificates of deposit | Level 1 |  | 8,428 |  |  | 8,411 |  |  | 5,188 |  |  | 5,192 |
U.S. Treasury and agency notes |  |  |  |  |  |  |  |  |
with unrecognized losses for less than 12 months |  |  | 29,741 |  |  | 29,669 |  |  | 947 |  |  | 946 |
with unrecognized losses for more than 12 months |  |  | 137 |  |  | 135 |  |  | — |  |  | — |
with unrecognized gains |  |  | 152 |  |  | 153 |  |  | 857 |  |  | 859 |
Total U.S. Treasury and agency notes | Level 2 |  | 30,030 |  |  | 29,957 |  |  | 1,804 |  |  | 1,805 |
Corporate notes |  |  |  |  |  |  |  |  |
with unrecognized losses for less than 12 months |  |  | 18,230 |  |  | 18,159 |  |  | 11,658 |  |  | 11,592 |
with unrecognized losses for more than 12 months |  |  | 2,804 |  |  | 2,788 |  |  | 1,053 |  |  | 1,042 |
with unrecognized gains |  |  | — |  |  | — |  |  | 3,431 |  |  | 3,435 |
Total Corporate notes | Level 2 |  | 21,034 |  |  | 20,947 |  |  | 16,142 |  |  | 16,069 |
Municipal notes |  |  |  |  |  |  |  |  |
with unrecognized losses for less than 12 months |  |  | 10,298 |  |  | 10,288 |  |  | 2,556 |  |  | 2,526 |
with unrecognized losses for more than 12 months |  |  | 999 |  |  | 987 |  |  | 271 |  |  | 269 |
with unrecognized gains |  |  | 277 |  |  | 278 |  |  | 812 |  |  | 814 |
Total Municipal notes | Level 2 |  | 11,574 |  |  | 11,553 |  |  | 3,639 |  |  | 3,609 |
|  |  | $ | 71,066 |  |  | $ | 70,868 |  |  | $ | 26,773 |  |  | $ | 26,675 |
Summary of maturities, at par, of marketable securities by year | The following tables summarize the maturities, at par, of marketable securities by year ($ in thousands):
December 31, 2017 | 2018 |  | 2019 |  | 2020 |  | 2021 |  | Total
Certificates of deposit | $ | 4,306 |  |  | $ | 2,311 |  |  | $ | 1,799 |  |  | $ | — |  |  | $ | 8,416 |
U.S. Treasury and agency notes | 6,399 |  |  | 14,599 |  |  | 9,171 |  |  | — |  |  | 30,169 |
Corporate notes | 7,954 |  |  | 6,430 |  |  | 6,450 |  |  | — |  |  | 20,834 |
Municipal notes | 1,568 |  |  | 6,957 |  |  | 3,003 |  |  | — |  |  | 11,528 |
| $ | 20,227 |  |  | $ | 30,297 |  |  | $ | 20,423 |  |  | $ | — |  |  | $ | 70,947 |
December 31, 2016 |  | 2017 |  | 2018 |  | 2019 |  | 2020 |  | Total
Certificates of deposit |  | $ | 531 |  |  | $ | 4,306 |  |  | $ | 324 |  |  | $ | — |  |  | $ | 5,161 |
U.S. Treasury and agency notes |  | 1,234 |  |  | 444 |  |  | 142 |  |  | — |  |  | 1,820 |
Corporate notes |  | 4,316 |  |  | 7,133 |  |  | 4,232 |  |  | — |  |  | 15,681 |
Municipal notes |  | 840 |  |  | 1,688 |  |  | 1,075 |  |  | — |  |  | 3,603 |
|  | $ | 6,921 |  |  | $ | 13,571 |  |  | $ | 5,773 |  |  | $ | — |  |  | $ | 26,265 |

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