Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) common_stock_class	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2017 USD ($)
Concentrations of Credit Risk
Cash and cash equivalents and short-term investments held in banks located outside the U.S.	$ 48,200,000	$ 129,500,000
Goods In-Transit
Goods in-transit	104,900,000	83,700,000
Property, plant and equipment
Property and equipment, net	624,544,000	606,977,000		$ 379,268,000
Amortization expense	81,600,000	51,300,000	$ 34,500,000
Long-Lived Assets
Impairment of long-lived assets or identifiable intangibles	0	0	0
Revenue Recognition
Sales return reserve	38,042,000	35,707,000
Allowance for sales returns	38,000,000.0	35,700,000
Contract liabilities	167,641,000	148,057,000
Cumulative-effect adjustment to reduce Accumulated deficit	2,065,423,000	1,082,689,000		$ 1,080,839,000
Advertising Costs
Advertising expense	1,095,840,000	774,189,000	549,959,000
Prepaid advertising costs	3,800,000	700,000
Merchant Processing Fees
Merchant processing fees	$ 179,700,000	133,400,000	$ 88,700,000
Earnings Per Share
Number of classes of common stock | common_stock_class	2
ASU 2014-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU					$ (4,657,000)
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue Recognition
Cumulative-effect adjustment to reduce Accumulated deficit					4,700,000
ASU 2014-09 | Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU					$ (4,657,000)
ASU 2016-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU						$ 0
ASU 2016-09 | Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU						$ 8,712,000
Deferred Revenue
Revenue Recognition
Contract liabilities	$ 167,600,000	148,100,000
Revenue recognized	126,000,000.0
Other Current Liabilities
Revenue Recognition
Contract liabilities	4,600,000	3,100,000
Revenue recognized	$ 3,100,000
Other Long-term Investments | Minimum
Marketable Securities [Abstract]
Marketable securities, term	1 year
Other Long-term Investments | Maximum
Marketable Securities [Abstract]
Marketable securities, term	2 years
Furniture and computer equipment | Minimum
Property, plant and equipment
Estimated useful lives	3 years
Furniture and computer equipment | Maximum
Property, plant and equipment
Estimated useful lives	7 years
Site and software development costs
Property, plant and equipment
Estimated useful lives	2 years
Property and equipment, net	$ 123,500,000	$ 62,900,000
Building
Property, plant and equipment
Estimated useful lives	30 years