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Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale Securities	$ 1,020,182	$ 672,463
Fair Value, Inputs, Level 1
Available-for-sale Securities	100,965	5,340
Fair Value, Inputs, Level 2
Available-for-sale Securities	919,217	667,123
Cash Equivalents
Cash Equivalents	142,838	139,986
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents	100,965	5,340
Cash Equivalents | Fair Value, Inputs, Level 2
Cash Equivalents	41,873	134,646
Cash Equivalents | Money Market Funds
Cash Equivalents	100,965	5,340
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 1
Cash Equivalents	100,965	5,340
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 2
Cash Equivalents	0	0
Cash Equivalents | Commercial paper
Cash Equivalents	25,199	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	25,199	48,908
Cash Equivalents | Municipal debt securities
Cash Equivalents	16,674	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	16,674	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
Short-term Investments
Available-for-sale Securities	877,344	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	877,344	532,477
Short-term Investments | Commercial paper
Available-for-sale Securities	387,044	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	387,044	229,821
Short-term Investments | Municipal debt securities
Available-for-sale Securities	219,479	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	219,479	87,290
Short-term Investments | Federal agency debt securities
Available-for-sale Securities	3,000	48,598
Short-term Investments | Federal agency debt securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Short-term Investments | Federal agency debt securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	3,000	48,598
Short-term Investments | Corporate debt securities
Available-for-sale Securities	267,821	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	$ 267,821	$ 166,768

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