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Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale Securities	$ 675,824	$ 672,463
Fair Value, Inputs, Level 1
Available-for-sale Securities	160,730	5,340
Fair Value, Inputs, Level 2
Available-for-sale Securities	515,094	667,123
Short-term Investments
Available-for-sale Securities	426,381	532,477
Short-term Investments | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | Fair Value, Inputs, Level 2
Available-for-sale Securities	426,381	532,477
Short-term Investments | Commercial paper
Available-for-sale Securities	188,937	229,821
Short-term Investments | Commercial paper | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | Commercial paper | Fair Value, Inputs, Level 2
Available-for-sale Securities	188,937	229,821
Short-term Investments | Municipal debt securities
Available-for-sale Securities	84,294	87,290
Short-term Investments | Municipal debt securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | Municipal debt securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	84,294	87,290
Short-term Investments | Corporate debt securities
Available-for-sale Securities	115,812	166,768
Short-term Investments | Corporate debt securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | Corporate debt securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	115,812	166,768
Short-term Investments | US Treasury Bond Securities
Available-for-sale Securities	37,338	48,598
Short-term Investments | US Treasury Bond Securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | US Treasury Bond Securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	37,338	48,598
Cash Equivalents
Cash Equivalents	249,443	139,986
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents	160,730	5,340
Cash Equivalents | Fair Value, Inputs, Level 2
Cash Equivalents	88,713	134,646
Cash Equivalents | Money Market Funds
Cash Equivalents	160,730	5,340
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 1
Cash Equivalents	160,730	5,340
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 2
Cash Equivalents	0	0
Cash Equivalents | Commercial paper
Cash Equivalents	21,889	48,908
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 2
Cash Equivalents	21,889	48,908
Cash Equivalents | Municipal debt securities
Cash Equivalents	59,417	34,338
Cash Equivalents | Municipal debt securities | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Municipal debt securities | Fair Value, Inputs, Level 2
Cash Equivalents	59,417	34,338
Cash Equivalents | Federal agency debt securities
Cash Equivalents	0	51,400
Cash Equivalents | Federal agency debt securities | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Federal agency debt securities | Fair Value, Inputs, Level 2
Cash Equivalents	0	51,400
Cash Equivalents | Corporate debt securities
Cash Equivalents	7,407	0
Cash Equivalents | Corporate debt securities | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Corporate debt securities | Fair Value, Inputs, Level 2
Cash Equivalents	$ 7,407	$ 0

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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