Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Assets:
Money market funds	$ 717,804	$ 419,693	$ 397,094
Marketable securities:	51,120	71,507
Liabilities:
Deferred payments	115,829	90,828
Recurring
Assets:
Total assets at fair value on a recurring basis	61,217	102,200
Money market funds | Recurring
Assets:
Money market funds	10,097	30,693
Corporate debt securities
Assets:
Marketable securities:	23,350	42,709
Corporate debt securities | Recurring
Assets:
Marketable securities:	23,350	42,709
Tax-exempt municipal securities
Assets:
Marketable securities:	22,170	23,198
Tax-exempt municipal securities | Recurring
Assets:
Marketable securities:	22,170	23,198
Variable rate demand notes
Assets:
Marketable securities:	5,600	5,600
Variable rate demand notes | Recurring
Assets:
Marketable securities:	5,600	5,600
Deferred payments | Recurring
Liabilities:
Deferred payments	2,003	3,257
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Recurring
Assets:
Total assets at fair value on a recurring basis	10,097	30,693
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Money market funds | Recurring
Assets:
Money market funds	10,097	30,693
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	51,120	71,507
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:	23,350	42,709
Significant Other Observable Inputs (Level 2) | Tax-exempt municipal securities | Recurring
Assets:
Marketable securities:	22,170	23,198
Significant Other Observable Inputs (Level 2) | Variable rate demand notes | Recurring
Assets:
Marketable securities:	5,600	5,600
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	$ 2,003	$ 3,257