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Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Assets:
Money market funds	$ 375,515	$ 311,732	$ 171,600
Marketable securities:	65,025	74,799
Recurring
Assets:
Total assets at fair value on a recurring basis	101,340	77,090
Money market funds \| Recurring
Assets:
Money market funds	36,315	1,791
Corporate debt securities
Assets:
Marketable securities:	44,026	47,930
Corporate debt securities \| Recurring
Assets:
Marketable securities:	44,026	48,430
Tax-exempt municipal securities
Assets:
Marketable securities:	15,399	22,869
Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	15,399	22,869
Variable rate demand notes
Assets:
Marketable securities:	5,600	4,000
Variable rate demand notes \| Recurring
Assets:
Marketable securities:	5,600	4,000
Deferred payments \| Recurring
Liabilities:
Deferred payments	3,664	4,120
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Recurring
Assets:
Total assets at fair value on a recurring basis	36,315	1,791
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Money market funds \| Recurring
Assets:
Money market funds	36,315	1,791
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Corporate debt securities \| Recurring
Assets:
Marketable securities:	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Variable rate demand notes \| Recurring
Assets:
Marketable securities:	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Deferred payments \| Recurring
Liabilities:
Deferred payments	0	0
Significant Other Observable Inputs (Level 2) \| Recurring
Assets:
Total assets at fair value on a recurring basis	65,025	75,299
Significant Other Observable Inputs (Level 2) \| Money market funds \| Recurring
Assets:
Money market funds	0	0
Significant Other Observable Inputs (Level 2) \| Corporate debt securities \| Recurring
Assets:
Marketable securities:	44,026	48,430
Significant Other Observable Inputs (Level 2) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	15,399	22,869
Significant Other Observable Inputs (Level 2) \| Variable rate demand notes \| Recurring
Assets:
Marketable securities:	5,600	4,000
Significant Other Observable Inputs (Level 2) \| Deferred payments \| Recurring
Liabilities:
Deferred payments	0	0
Significant Unobservable Inputs (Level 3) \| Recurring
Assets:
Total assets at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) \| Money market funds \| Recurring
Assets:
Money market funds	0	0
Significant Unobservable Inputs (Level 3) \| Corporate debt securities \| Recurring
Assets:
Marketable securities:	0	0
Significant Unobservable Inputs (Level 3) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	0	0
Significant Unobservable Inputs (Level 3) \| Variable rate demand notes \| Recurring
Assets:
Marketable securities:	0	0
Significant Unobservable Inputs (Level 3) \| Deferred payments \| Recurring
Liabilities:
Deferred payments	$ 3,664	$ 4,120