Property and Equipment - Summary of Capitalized Website Development and Internal-Use Software Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Movement in Capitalized Computer Software, Net [Roll Forward]
Balance as of the beginning of the period	$ 205,619
Balance as of the end of the period		$ 205,619
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Accumulated amortization balance as of the beginning of the period	85,440
Accumulated amortization balance as of the end of the period		85,440
Property and equipment, net		120,179
Developed technology
Movement in Capitalized Computer Software, Net [Roll Forward]
Balance as of the beginning of the period	69,201	48,333
Additions to website development	8,687	22,068
Acquisition of developed technology	30,300	0
Less: Retirements	1,973	1,200
Balance as of the end of the period	106,215	69,201	$ 48,333
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Accumulated amortization balance as of the beginning of the period	38,418	29,991
Amortization Expense	18,737	9,519	8,200
Less: Retirements	340	1,092
Accumulated amortization balance as of the end of the period	56,815	38,418	$ 29,991
Property and equipment, net	$ 49,400	$ 30,783