Summary of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) stores states $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary of significant accounting policies
Number of stores | stores	5,019
Number of states, in which the Company operates | states	47
Accounts receivable due from employees to the Company	$ 900	$ 1,200
Allowance for doubtful supplier receivables	0	0
Replacement cost of inventory	3,010,000	2,780,000
LIFO inventory value in excess of replacement cost of inventory	157,300	132,000
Notes receivable from suppliers and other third parties	0	0
Goodwill impairment	0	0
Impairment of long-lived assets	0	0
Self-insurance reserves, current	71,695	67,921
Advertising expense, net	83,700	83,000	$ 79,300
Total interest costs capitalized	8,500	7,900	7,400
Deferred debt issuance costs, net of amortization	15,900	10,600
Original issuance discounts, net of accretion	3,700	3,100
Valuation allowance for deferred tax assets	$ 0	0
Adoption of ASU 2016-09 [Member]
Summary of significant accounting policies
Change in accounting policy, description	Upon adoption of ASU 2016-09, the Company elected to change its accounting policy to account for forfeitures as they occur; this change was applied using the modified retrospective transition method with a cumulative effect adjustment
Effect of accounting pronouncement adoption on Provision for income taxes	$ 48,700
Effect on diluted earnings per share | $ / shares	$ 0.50
Adoption of ASU 2016-09 [Member] | Reclassification or adjustment due to the adoption of an accounting pronouncement [Member]
Summary of significant accounting policies
Cumulative effective adjustment to opening Retained earnings	$ 300
Tax withholdings for share-based compensation, reclassified from operating activities		600	900
Excess tax benefit from share-based compensation, reclassified from financing activities		56,000	$ 63,100
Other assets, net [Member]
Summary of significant accounting policies
Deferred debt issuance costs, net of amortization	2,000	700
Loyalty program [Member]
Summary of significant accounting policies
Deferred revenue, loyalty program	4,700	4,800
Deferred revenue recognized, loyalty program	$ 17,600	$ 12,700