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Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Assets:
Money market funds	$ 238,290	$ 187,509	$ 442,845
Marketable securities:	35,735	37,827
Liabilities:
Deferred payments	97,448	104,742
Recurring
Assets:
Total assets at fair value on a recurring basis	40,676	40,668
Money market funds \| Recurring
Assets:
Money market funds	4,941	2,841
Tax-exempt municipal securities
Assets:
Marketable securities:	19,241	20,289
Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	19,241	20,289
Corporate debt securities
Assets:
Marketable securities:	16,494	17,538
Corporate debt securities \| Recurring
Assets:
Marketable securities:	16,494	17,538
Deferred payments \| Recurring
Liabilities:
Deferred payments	1,346	1,658
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Recurring
Assets:
Total assets at fair value on a recurring basis	4,941	2,841
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Money market funds \| Recurring
Assets:
Money market funds	4,941	2,841
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) \| Corporate debt securities \| 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	35,735	37,827
Significant Other Observable Inputs (Level 2) \| Money market funds \| Recurring
Assets:
Money market funds	0	0
Significant Other Observable Inputs (Level 2) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	19,241	20,289
Significant Other Observable Inputs (Level 2) \| Corporate debt securities \| Recurring
Assets:
Marketable securities:	16,494	17,538
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) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities:	0	0
Significant Unobservable Inputs (Level 3) \| Corporate debt securities \| Recurring
Assets:
Marketable securities:	0	0
Significant Unobservable Inputs (Level 3) \| Deferred payments \| Recurring
Liabilities:
Deferred payments	$ 1,346	$ 1,658