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Marketable Securities - Available-For-Sale Securities (Details) - USD ($) | Dec. 31, 2018 | Dec. 31, 2017
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Available-For-Sale Securities |  |
Amortized Cost | $ 75,060,000 |
Gross Unrealized Gain | 57,000 |
Gross Unrealized (Losses) | (318,000) |
Estimated Fair Value | 74,799,000 | $ 0
Corporate debt securities |  |
Available-For-Sale Securities |  |
Amortized Cost | 48,202,000 |
Gross Unrealized Gain | 12,000 |
Gross Unrealized (Losses) | (284,000) |
Estimated Fair Value | 47,930,000 |
Tax-exempt municipal securities |  |
Available-For-Sale Securities |  |
Amortized Cost | 22,858,000 |
Gross Unrealized Gain | 45,000 |
Gross Unrealized (Losses) | (34,000) |
Estimated Fair Value | 22,869,000 |
Variable rate demand notes |  |
Available-For-Sale Securities |  |
Amortized Cost | 4,000,000 |
Estimated Fair Value | $ 4,000,000 |

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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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