Fair Value Measurement (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of assets and liabilities measured at fair value on a recurring basis
Money market funds	$ 276,382,000	$ 155,933,000	$ 150,483,000
Marketable securities	71,463,000	0
Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Total assets at fair value on a recurring basis	74,734,000
Money market funds | Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Money market funds	3,271,000	113,000
Corporate debt securities
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	41,465,000
Corporate debt securities | Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	41,465,000
Tax-exempt municipal securities
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	21,298,000
Tax-exempt municipal securities | Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	21,298,000
Variable rate demand notes
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	8,700,000
Variable rate demand notes | Recurring
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	8,700,000
Deferred payments | Recurring
Liabilities:
Deferred payments	4,366,000	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	3,271,000
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	3,271,000	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	71,463,000
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	41,465,000
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	21,298,000
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	8,700,000
Significant Unobservable Inputs (Level 3) [Member] | Deferred payments | Recurring
Liabilities:
Deferred payments	$ 4,366,000	$ 4,514,000