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Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Available-For-Sale Securities |  |
Amortized Cost | $ 68,272 | $ 75,060
Gross Unrealized Gain | 178 | 57
Gross Unrealized (Losses) | (98) | (318)
Estimated Fair Value | 68,352 | 74,799
Corporate debt securities |  |
Available-For-Sale Securities |  |
Amortized Cost | 46,685 | 48,202
Gross Unrealized Gain | 95 | 12
Gross Unrealized (Losses) | (89) | (284)
Estimated Fair Value | 46,691 | 47,930
Tax-exempt municipal securities |  |
Available-For-Sale Securities |  |
Amortized Cost | 18,969 | 22,858
Gross Unrealized Gain | 83 | 45
Gross Unrealized (Losses) | (9) | (34)
Estimated Fair Value | 19,043 | 22,869
Variable rate demand notes |  |
Available-For-Sale Securities |  |
Amortized Cost | 1,600 | 4,000
Gross Unrealized Gain | 0 | 0
Gross Unrealized (Losses) | 0 | 0
Estimated Fair Value | 1,600 | $ 4,000
Commercial paper |  |
Available-For-Sale Securities |  |
Amortized Cost | 1,018 |
Gross Unrealized Gain | 0 |
Gross Unrealized (Losses) | 0 |
Estimated Fair Value | $ 1,018 |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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