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Fair Value Measurement (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Schedule of assets and liabilities measured at fair value on a recurring basis
Money market funds	$ 311,732,000	$ 155,933,000
Marketable securities	74,799,000	0
Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Total assets at fair value on a recurring basis	77,090,000
Money market funds \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Money market funds	1,791,000	113,000
Corporate debt securities
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	47,930,000
Corporate debt securities \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	48,430,000
Tax-exempt municipal securities
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	22,869,000
Tax-exempt municipal securities \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	22,869,000
Variable rate demand notes
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	4,000,000
Variable rate demand notes \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	4,000,000
Deferred payments \| Recurring
Liabilities:
Deferred payments	4,120,000	4,514,000
Quoted Prices In Active Markets For Identical Assets/Liabilities (Level 1) [Member] \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Total assets at fair value on a recurring basis	1,791,000
Quoted Prices In Active Markets For Identical Assets/Liabilities (Level 1) [Member] \| Money market funds \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Money market funds	1,791,000	113,000
Significant Other Observable Inputs (Level 2) [Member] \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Total assets at fair value on a recurring basis	75,299,000
Significant Other Observable Inputs (Level 2) [Member] \| Corporate debt securities \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	48,430,000
Significant Other Observable Inputs (Level 2) [Member] \| Tax-exempt municipal securities \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	22,869,000
Significant Other Observable Inputs (Level 2) [Member] \| Variable rate demand notes \| Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	4,000,000
Significant Unobservable Inputs (Level 3) [Member] \| Deferred payments \| Recurring
Liabilities:
Deferred payments	$ 4,120,000	$ 4,514,000