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Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale Securities	$ 568,765	$ 401,132
Fair Value, Inputs, Level 1
Available-for-sale Securities	107,066	42,653
Fair Value, Inputs, Level 2
Available-for-sale Securities	461,699	358,479
Short-term Investments
Available-for-sale Securities	390,864	335,928
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	390,864	335,928
Short-term Investments | Commercial paper
Available-for-sale Securities	208,194	161,286
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	208,194	161,286
Short-term Investments | Municipal debt securities
Available-for-sale Securities	36,892	12,237
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	36,892	12,237
Short-term Investments | Federal agency debt securities
Available-for-sale Securities	14,375	13,515
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	14,375	13,515
Short-term Investments | Corporate debt securities
Available-for-sale Securities	128,344	145,975
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	128,344	145,975
Short-term Investments | US Treasury Bond Securities
Available-for-sale Securities	3,059	2,915
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	3,059	2,915
Long Term Investments
Available-for-sale Securities	0	15,542
Long Term Investments | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Long Term Investments | Fair Value, Inputs, Level 2
Available-for-sale Securities	0	15,542
Long Term Investments | Corporate debt securities
Available-for-sale Securities	0	15,396
Long Term Investments | Corporate debt securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Long Term Investments | Corporate debt securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	0	15,396
Long Term Investments | US Treasury Bond Securities
Available-for-sale Securities	0	146
Long Term Investments | US Treasury Bond Securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Long Term Investments | US Treasury Bond Securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	0	146
Cash Equivalents
Cash Equivalents	177,901	49,662
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents	107,066	42,653
Cash Equivalents | Fair Value, Inputs, Level 2
Cash Equivalents	70,835	7,009
Cash Equivalents | Money Market Funds
Cash Equivalents	107,066	42,653
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 1
Cash Equivalents	107,066	42,653
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 2
Cash Equivalents	0	0
Cash Equivalents | Commercial paper
Cash Equivalents	30,351	5,807
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 2
Cash Equivalents	30,351	5,807
Cash Equivalents | Municipal debt securities
Cash Equivalents	32,575	1,202
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	32,575	1,202
Cash Equivalents | Federal agency debt securities
Cash Equivalents	7,909	0
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	$ 7,909	$ 0

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- Definition

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