Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) common_stock_class brand	Dec. 31, 2017 USD ($)	Dec. 31, 2016 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.	$ 129,500,000	$ 63,400,000
Goods In-Transit
Goods in-transit	83,700,000	54,500,000
Property, plant and equipment
Property and equipment, net	606,977,000	361,141,000
Long-Lived Assets
Impairment of long-lived assets or identifiable intangibles	$ 0	0	$ 0
Revenue Recognition
Number of distinct sites | brand	5
Allowance for sales returns	$ 35,700,000	21,200,000
Contract liabilities	148,057,000	94,116,000
Cumulative-effect adjustment to reduce Accumulated deficit	1,082,689,000	583,266,000
Advertising Costs
Advertising expense	774,189,000	549,959,000	409,125,000
Prepaid advertising costs	700,000	600,000
Merchant Processing Fees
Merchant processing fees	133,400,000	88,700,000	66,000,000
Retail Partner Fees
Retail partner fees included in merchant processing fees	1,200,000	1,500,000	1,900,000
Retail partner fees included in advertising costs	$ 5,500,000	7,000,000	11,000,000
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)			$ 8,700,000
Deferred Revenue
Revenue Recognition
Contract liabilities	148,100,000	94,100,000
Revenue recognized	77,300,000
Other Current Liabilities
Revenue Recognition
Contract liabilities	$ 3,100,000	2,600,000
Revenue recognized		2,500,000
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	$ 62,900,000	45,400,000
Amortization expense	$ 51,300,000	$ 34,500,000	$ 21,600,000
Building
Property, plant and equipment
Estimated useful lives	30 years