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Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities	$ 385,421	$ 401,132
Fair Value, Inputs, Level 1
Available-for-sale Securities	47,774	42,653
Fair Value, Inputs, Level 2
Available-for-sale Securities	337,647	358,479
Short-term Investments
Available-for-sale Securities	314,946	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	314,946	335,928
Short-term Investments | Commercial paper
Available-for-sale Securities	142,667	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	142,667	161,286
Short-term Investments | Municipal debt securities
Available-for-sale Securities	8,678	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	8,678	12,237
Short-term Investments | Federal agency debt securities
Available-for-sale Securities	2,465	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	2,465	13,515
Short-term Investments | Corporate debt securities
Available-for-sale Securities	158,060	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	158,060	145,975
Short-term Investments | US Treasury Bond Securities
Available-for-sale Securities	3,076	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	3,076	2,915
Long Term Investments
Available-for-sale Securities	11,019	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	11,019	15,542
Long Term Investments | Federal agency debt securities
Available-for-sale Securities	11,019	0
Long Term Investments | Federal agency debt securities | Fair Value, Inputs, Level 1
Available-for-sale Securities	0	0
Long Term Investments | Federal agency debt securities | Fair Value, Inputs, Level 2
Available-for-sale Securities	11,019	0
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	59,456	49,662
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents	47,774	42,653
Cash Equivalents | Fair Value, Inputs, Level 2
Cash Equivalents	11,682	7,009
Cash Equivalents | Money Market Funds
Cash Equivalents	47,774	42,653
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 1
Cash Equivalents	47,774	42,653
Cash Equivalents | Money Market Funds | Fair Value, Inputs, Level 2
Cash Equivalents	0	0
Cash Equivalents | Commercial paper
Cash Equivalents	7,135	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	7,135	5,807
Cash Equivalents | Municipal debt securities
Cash Equivalents	4,547	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	$ 4,547	$ 1,202

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