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Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Available-for-sale Securities [Abstract] |
Cash equivalents and marketable securities classified as available-for-sale | Total cash, cash equivalents and marketable securities are summarized as follows:
| As of December 31, 2017
(Dollars in thousands) | Fair Value |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Weighted Rate of Return
Money market funds | $ | 10,915 |  |  | $ | 10,915 |  |  | $ | — |  |  | $ | — |  |  | 1.16 | %
U.S. Government bonds and notes | 55,220 |  |  | 55,221 |  |  | — |  |  | (1 | ) |  | 1.12 | %
Corporate notes, bonds, commercial paper and other | 195,073 |  |  | 195,204 |  |  | — |  |  | (131 | ) |  | 1.39 | %
Total cash equivalents and marketable securities | 261,208 |  |  | 261,340 |  |  | — |  |  | (132 | ) |  |
Cash | 68,168 |  |  | 68,168 |  |  | — |  |  | — |  |  |
Total cash, cash equivalents and marketable securities | $ | 329,376 |  |  | $ | 329,508 |  |  | $ | — |  |  | $ | (132 | ) |  |
| As of December 31, 2016
(Dollars in thousands) | Fair Value |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Weighted Rate of Return
Money market funds | $ | 10,681 |  |  | $ | 10,681 |  |  | $ | — |  |  | $ | — |  |  | 0.41 | %
U.S. Government bonds and notes | 48,292 |  |  | 48,291 |  |  | 1 |  |  | — |  |  | 0.39 | %
Corporate notes, bonds, commercial paper and other | 62,178 |  |  | 62,199 |  |  | — |  |  | (21 | ) |  | 0.66 | %
Total cash equivalents and marketable securities | 121,151 |  |  | 121,171 |  |  | 1 |  |  | (21 | ) |  |
Cash | 51,031 |  |  | 51,031 |  |  | — |  |  | — |  |  |
Total cash, cash equivalents and marketable securities | $ | 172,182 |  |  | $ | 172,202 |  |  | $ | 1 |  |  | $ | (21 | ) |  |
Available-for-sale securities reported at fair value | Available-for-sale securities are reported at fair value on the balance sheets and classified as follows:
| As of
| December 31, 2017 |  | December 31, 2016
| (Dollars in thousands)
Cash equivalents | $ | 157,676 |  |  | $ | 84,263 |
Short term marketable securities | 103,532 |  |  | 36,888 |
Total cash equivalents and marketable securities | 261,208 |  |  | 121,151 |
Cash | 68,168 |  |  | 51,031 |
Total cash, cash equivalents and marketable securities | $ | 329,376 |  |  | $ | 172,182 |
Estimated fair value of cash equivalents and marketable securities classified by date of contractual maturity and the length of time that the securities have been in a continuous unrealized loss position | The estimated fair value of cash equivalents and marketable securities classified by the length of time that the securities have been in a continuous unrealized loss position at December 31, 2017 and 2016 are as follows:
| Fair Value |  | Gross Unrealized Loss
| December 31, 2017 |  | December 31, 2016 |  | December 31, 2017 |  | December 31, 2016
| (In thousands)
Less than one year |  |  |  |  |  |  |
U.S. Government bonds and notes | $ | 42,581 |  |  | $ | 18,395 |  |  | $ | (1 | ) |  | $ | — |
Corporate notes, bonds and commercial paper | 194,015 |  |  | 54,377 |  |  | (131 | ) |  | (21 | )
Total Corporate notes, bonds, and commercial paper and U.S. Government bonds and notes | $ | 236,596 |  |  | $ | 72,772 |  |  | $ | (132 | ) |  | $ | (21 | )

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- Definition Tabular disclosure of cash and the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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- Definition Tabular disclosure of the components of cash, cash equivalents, and investments. + References No definition available. + Details
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- Definition For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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