Marketable Securities - Available-For-Sale Securities (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Available-For-Sale Securities
Amortized Cost	$ 71,796,000
Gross Unrealized Gain	8,000
Gross Unrealized (Losses)	(341,000)
Estimated Fair Value	71,463,000	$ 0
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	21,366,000
Gross Unrealized Gain	1,000
Gross Unrealized (Losses)	(69,000)
Estimated Fair Value	21,298,000
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	41,730,000
Gross Unrealized Gain	7,000
Gross Unrealized (Losses)	(272,000)
Estimated Fair Value	41,465,000
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost	8,700,000
Estimated Fair Value	$ 8,700,000

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).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss 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).

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount 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
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-Section 45
-Paragraph 1
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