Fair Value Measurements - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	$ 636,875	$ 734,919
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	48,867
Total marketable securities	636,875
Total cash equivalents and marketable securities	685,742
Fair Value, Measurements, Recurring | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	48,867
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	214,027
Fair Value, Measurements, Recurring | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	97,820
Fair Value, Measurements, Recurring | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	325,028
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	12,568
Total marketable securities	0
Total cash equivalents and marketable securities	12,568
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	12,568
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 1 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	36,299
Total marketable securities	636,875
Total cash equivalents and marketable securities	673,174
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	36,299
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	214,027
Fair Value, Measurements, Recurring | Level 2 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	97,820
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	325,028
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Total marketable securities	0
Total cash equivalents and marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	$ 0