Fair Value Measurements - Schedule of Major Categories of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 373,959	$ 257,302
Long-term investments	89,343	0
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	237,612	252,631
Short-term investments	373,959	257,302
Funds receivable and seller accounts	18,168	9,229
Long-term investments	89,343
Asset	719,082	519,162
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,794	7,775
Short-term investments	29,320	147,860
Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,959
Short-term investments	26,132
Long-term investments	4,729
Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	228,859	244,856
Funds receivable and seller accounts	18,168	9,229
Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	114,202	46,801
Long-term investments	38,563
Recurring | U.S. Government and agency bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	204,305	62,641
Long-term investments	46,051
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	228,859	244,856
Short-term investments	204,305	62,641
Funds receivable and seller accounts	18,168	9,229
Long-term investments	46,051
Asset	497,383	316,726
Level 1 | Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Level 1 | Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Short-term investments	0
Long-term investments	0
Level 1 | Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	228,859	244,856
Funds receivable and seller accounts	18,168	9,229
Level 1 | Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Long-term investments	0
Level 1 | Recurring | U.S. Government and agency bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	204,305	62,641
Long-term investments	46,051
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,753	7,775
Short-term investments	169,654	194,661
Funds receivable and seller accounts	0	0
Long-term investments	43,292
Asset	221,699	202,436
Level 2 | Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,794	7,775
Short-term investments	29,320	147,860
Level 2 | Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,959
Short-term investments	26,132
Long-term investments	4,729
Level 2 | Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Funds receivable and seller accounts	0	0
Level 2 | Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	114,202	46,801
Long-term investments	38,563
Level 2 | Recurring | U.S. Government and agency bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Long-term investments	0
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Funds receivable and seller accounts	0	0
Long-term investments	0
Asset	0	0
Level 3 | Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Level 3 | Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Short-term investments	0
Long-term investments	0
Level 3 | Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Funds receivable and seller accounts	0	0
Level 3 | Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Long-term investments	0
Level 3 | Recurring | U.S. Government and agency bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	$ 0
Long-term investments	$ 0