Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Cash and cash equivalents	$ 187,509	$ 419,693
Marketable securities	37,827	71,507
Liabilities:
Deferred payments	104,742	90,828
Recurring
Assets:
Total assets at fair value on a recurring basis	40,668	102,200
Money market funds \| Recurring
Assets:
Cash and cash equivalents	2,841	30,693
Corporate debt securities
Assets:
Marketable securities	17,538	42,709
Corporate debt securities \| Recurring
Assets:
Marketable securities	17,538	42,709
Tax-exempt municipal securities
Assets:
Marketable securities	20,289	23,198
Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities	20,289	23,198
Variable rate demand notes
Assets:
Marketable securities		5,600
Variable rate demand notes \| Recurring
Assets:
Marketable securities		5,600
Deferred payments \| Recurring
Liabilities:
Deferred payments	1,658	3,257
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Recurring
Assets:
Total assets at fair value on a recurring basis	2,841	30,693
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) \| Money market funds \| Recurring
Assets:
Cash and cash equivalents	2,841	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
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	37,827	71,507
Significant Other Observable Inputs (Level 2) \| Money market funds \| Recurring
Assets:
Cash and cash equivalents	0	0
Significant Other Observable Inputs (Level 2) \| Corporate debt securities \| Recurring
Assets:
Marketable securities	17,538	42,709
Significant Other Observable Inputs (Level 2) \| Tax-exempt municipal securities \| Recurring
Assets:
Marketable securities	20,289	23,198
Significant Other Observable Inputs (Level 2) \| Variable rate demand notes \| Recurring
Assets:
Marketable securities		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:
Cash and cash equivalents	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
Significant Unobservable Inputs (Level 3) \| Deferred payments \| Recurring
Liabilities:
Deferred payments	$ 1,658	$ 3,257

Sum of the carrying values as of the balance sheet date of obligations incurred through that date, including liabilities incurred and payable to vendors for goods and services received, taxes, interest, rent and utilities, compensation costs, payroll taxes and fringe benefits (other than pension and postretirement obligations), contractual rights and obligations, and statutory obligations.

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