Fair Value Measurements - Marketable Debt Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost	$ 87,868	$ 87,016
Gross Unrealized Gains	6,178	3,695
Gross Unrealized Losses	(4)	0
Estimated Fair Value	94,042	90,711
Other-than-temporary Impairments	0	0
Municipal bonds — available-for-sale
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost	87,868	87,016
Gross Unrealized Gains	6,178	3,695
Gross Unrealized Losses	(4)	0
Estimated Fair Value	94,042	90,711
Other-than-temporary Impairments	$ 0	$ 0

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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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).

+ References

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
-Subparagraph (a)
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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).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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