Fair Value Measurement - Financial Instruments at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Short-term investments	$ 665,567	$ 381,074
Fair Value on Recurring Basis
Assets:
Total assets measured and recorded at fair value	1,653,994	837,722
Fair Value on Recurring Basis \| Agency Securities
Assets:
Short-term investments		10,006
Long-term investments	39,031	15,005
Fair Value on Recurring Basis \| US Treasury Securities
Assets:
Short-term investments		44,286
Fair Value on Recurring Basis \| Corporate Bond Securities
Assets:
Short-term investments	461,397	319,293
Long-term investments	484,597	295,478
Fair Value on Recurring Basis \| Commercial paper
Assets:
Short-term investments	204,170	7,489
Fair Value on Recurring Basis \| Money Market Funds
Assets:
Cash equivalents	464,799	146,165
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured and recorded at fair value	464,799	190,451
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Agency Securities
Assets:
Short-term investments		0
Long-term investments	0	0
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| US Treasury Securities
Assets:
Short-term investments		44,286
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporate Bond Securities
Assets:
Short-term investments	0	0
Long-term investments	0	0
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Commercial paper
Assets:
Short-term investments	0	0
Fair Value on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Money Market Funds
Assets:
Cash equivalents	464,799	146,165
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured and recorded at fair value	1,189,195	647,271
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2) \| Agency Securities
Assets:
Short-term investments		10,006
Long-term investments	39,031	15,005
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2) \| US Treasury Securities
Assets:
Short-term investments		0
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2) \| Corporate Bond Securities
Assets:
Short-term investments	461,397	319,293
Long-term investments	484,597	295,478
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2) \| Commercial paper
Assets:
Short-term investments	204,170	7,489
Fair Value on Recurring Basis \| Significant Other Observable Inputs (Level 2) \| Money Market Funds
Assets:
Cash equivalents	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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