Property and Equipment, Net (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
	Mar. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Feb. 29, 2020	Dec. 31, 2019
Property and equipment, net
Computer equipment		$ 11,416			$ 11,416			$ 11,453
Capitalized software		19,408			19,408			18,516
Fulfillment equipment		49,450			49,450			54,059
Furniture and fixtures		3,359			3,359			3,725
Leasehold improvements		32,076			32,076			41,735
Buildings		114,877			114,877			148,507
Construction in process		2,447			2,447			1,803
Property and equipment, gross		233,033			233,033			279,798
Less: accumulated depreciation and amortization		(99,810)			(99,810)			(97,992)
Property and equipment, net		133,223			133,223			181,806
Build-to-suit lease arrangements included in Buildings	$ 31,100	31,300			31,300			62,100
Cost incurred to date		80,800			80,800			82,300
Depreciation and amortization of property and equipment		6,175		$ 8,372	12,928	$ 16,976
Capitalized interest to date		2,800			2,800			4,200
Termination fee	1,500
Total non-cancelable minimum lease payments, pre adoption	32,900
Facility financing obligation	35,700	35,976			35,976			$ 71,689
Deferred rent	$ 1,800
Gain on derecognition			$ 4,900		4,936
Loss on impairment					7,603
Share based compensation		2,009		$ 1,622	4,249	$ 4,457
Arlington Fulfillment Center [Member]
Property and equipment, net
Property and equipment, net							$ 11,500
Total non-cancelable minimum lease payments, pre adoption		2,000			2,000
Fair value of long-lived assets							$ 4,100
Loss on impairment		$ 200	$ 7,400		$ 7,600