Revenue - Adoption of ASC 605 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Item Effected [Line Items]
Document Period End Date			Sep. 30, 2018
Cost merchandise sold	$ 1,516,849	$ 1,378,859	$ 4,324,258	$ 3,999,588
Merchandise inventory	661,530		661,530			$ 579,350
Other current assets	26,692		26,692			34,227
Other non-current liabilities	21,288		21,288			30,361
Retained Earnings	722,274		722,274			$ 619,916
Revenue, Net	1,870,251	$ 1,700,843	5,340,235	$ 4,944,294
Calculated under Revenue Guidance in Effect before Topic 606
Item Effected [Line Items]
Cost merchandise sold	1,518,311		4,327,342
Merchandise inventory	667,907		667,907
Other current assets	26,969		26,969
Other non-current liabilities	28,528		28,528
Retained Earnings	721,688		721,688
Revenue, Net	1,871,849		5,343,529
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Item Effected [Line Items]
Cost merchandise sold	(1,462)		(3,084)
Merchandise inventory	(6,377)		(6,377)
Other current assets	(277)		(277)
Other non-current liabilities	(7,240)		(7,240)
Retained Earnings	586		586		$ 798
Revenue, Net	$ 1,598		$ 3,294