Revenue - Adoption of ASC 605 (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Item Effected [Line Items]
Net sales	$ 1,637,652	$ 1,530,571
Cost merchandise sold	1,326,233	$ 1,230,742
Merchandise inventory	616,819		$ 579,350
Other current assets	22,820		34,227
Other non-current liabilities	22,052		30,361
Retained Earnings	635,933		$ 619,916
Calculated under Revenue Guidance in Effect before Topic 606
Item Effected [Line Items]
Net sales	1,637,341
Cost merchandise sold	1,325,883
Merchandise inventory	626,629
Other current assets	23,096
Other non-current liabilities	32,897
Retained Earnings	635,135
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Item Effected [Line Items]
Net sales	311
Cost merchandise sold	350
Merchandise inventory	(9,810)
Other current assets	(276)
Other non-current liabilities	(10,845)
Retained Earnings	$ 798