Fair Value Measurement - Financial Instruments at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Short-term investments	$ 381,074	$ 93,345
Fair Value on Recurring Basis
Assets:
Total assets measured and recorded at fair value	837,722	132,716
Fair Value on Recurring Basis | Agency Securities
Assets:
Short-term investments	10,006
Long-term investments	15,005
Fair Value on Recurring Basis | US Treasury Securities
Assets:
Short-term investments	44,286	14,328
Long-term investments		1,628
Fair Value on Recurring Basis | Corporate Bond Securities
Assets:
Short-term investments	319,293	38,529
Long-term investments	295,478	14,424
Fair Value on Recurring Basis | Commercial paper
Assets:
Short-term investments	7,489	40,488
Fair Value on Recurring Basis | Commercial paper
Assets:
Cash equivalents		18,267
Fair Value on Recurring Basis | Money Market Funds
Assets:
Cash equivalents	146,165	5,052
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured and recorded at fair value	190,451	21,008
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
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury Securities
Assets:
Short-term investments	44,286	14,328
Long-term investments		1,628
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) | Commercial paper
Assets:
Cash equivalents		0
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds
Assets:
Cash equivalents	146,165	5,052
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured and recorded at fair value	647,271	111,708
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Agency Securities
Assets:
Short-term investments	10,006
Long-term investments	15,005
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | US Treasury Securities
Assets:
Short-term investments	0	0
Long-term investments		0
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Corporate Bond Securities
Assets:
Short-term investments	319,293	38,529
Long-term investments	295,478	14,424
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Commercial paper
Assets:
Short-term investments	7,489	40,488
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Commercial paper
Assets:
Cash equivalents		18,267
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Money Market Funds
Assets:
Cash equivalents	$ 0	$ 0