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Fair Value Measurement (Tables) | 12 Months Ended
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Dec. 31, 2019
Fair Value Disclosures [Abstract] |
Schedule of Assets and Liabilities Measured at Fair Value On A Recurring Basis | Assets and liabilities measured at fair value on a recurring basis consist of the following as of December 31, 2019 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Fair Value Measurements at Reporting Date Using
​ |  | ​ | ​ |  | Quoted Prices in |  | Significant |  | ​ | ​
​ | ​ | ​ | ​ | ​ | Active Markets | ​ | Other | ​ | Significant
​ | ​ | ​ | ​ | ​ | for Identical | ​ | Observable | ​ | Unobservable
​ | ​ | December 31, | ​ | Assets/Liabilities | ​ | Inputs | ​ | Inputs
​ | ​ | 2019 | ​ | (Level 1) | ​ | (Level 2) | ​ | (Level 3)
Assets: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Money market funds | ​ | $ | 30,693 | ​ | $ | 30,693 | ​ | $ | — | ​ | $ | —
Marketable securities: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate debt securities | ​ |  | 42,709 | ​ |  | — | ​ |  | 42,709 | ​ |  | —
Tax-exempt municipal securities | ​ |  | 23,198 | ​ |  | — | ​ |  | 23,198 | ​ |  | —
Variable rate demand notes | ​ |  | 5,600 | ​ |  | — | ​ |  | 5,600 | ​ |  | —
Total assets at fair value on a recurring basis | ​ | $ | 102,200 | ​ | $ | 30,693 | ​ | $ | 71,507 | ​ | $ | —
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Liabilities: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Deferred payments | ​ | $ | 3,257 | ​ | $ | — | ​ | $ | — | ​ | $ | 3,257
​ Assets and liabilities measured at fair value on a recurring basis consist of the following as of December 31, 2018 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Fair Value Measurements at Reporting Date Using
​ |  | ​ | ​ |  | Quoted Prices in |  | Significant |  | ​ | ​
​ | ​ | ​ | ​ | ​ | Active Markets | ​ | Other | ​ | Significant
​ | ​ | ​ | ​ | ​ | for Identical | ​ | Observable | ​ | Unobservable
​ | ​ | December 31, | ​ | Assets/Liabilities | ​ | Inputs | ​ | Inputs
​ | ​ | 2018 | ​ | (Level 1) | ​ | (Level 2) | ​ | (Level 3)
Assets: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Money market funds | ​ | $ | 1,791 | ​ | $ | 1,791 | ​ | $ | — | ​ | $ | —
Marketable securities: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate debt securities | ​ |  | 48,430 | ​ |  | — | ​ |  | 48,430 | ​ |  | —
Tax-exempt municipal securities | ​ |  | 22,869 | ​ |  | — | ​ |  | 22,869 | ​ |  | —
Variable rate demand notes | ​ |  | 4,000 | ​ |  | — | ​ |  | 4,000 | ​ |  | —
Total assets at fair value on a recurring basis | ​ | $ | 77,090 | ​ | $ | 1,791 | ​ | $ | 75,299 | ​ | $ | —
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Liabilities: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Deferred payments | ​ | $ | 4,120 | ​ | $ | — | ​ | $ | — | ​ | $ | 4,120
Schedule of Changes in Fair Value of Level 3 Liability | Changes in the fair value of the Company’s Level 3 liabilities during the years ended December 31, 2018 and 2019 are as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, |  | December 31,
​ | ​ | 2018 | ​ | 2019
Balance as of the beginning of period | ​ | $ | 4,514 | ​ | $ | 4,120
Amounts paid | ​ |  | (1,412) | ​ |  | (1,579)
Other adjustments to fair value | ​ |  | 1,018 | ​ |  | 716
Balance at end of period | ​ | $ | 4,120 | ​ | $ | 3,257

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- Definition Tabular disclosure of the fair value measurement of liabilities using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets), and gains or losses recognized in other comprehensive income (loss) and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs) by class of liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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