Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Number of stores Company owns and operates	3,740
Accounts receivable due from employees to the Company	$ 2.2	$ 2.2
Allowance for doubtful vendor receivables	0	0
Replacement cost of inventory	2,040.0	2,050.0
Total interest costs capitalized	4.7	5.1	6.7
Notes receivable from vendors and other third parties	15.0	22.2
Reserve for uncollectible notes receivables	0	0
Goodwill impairment	0	0
Discount rate of self-insurance reserves	4.75%	5.30%
Current portion of discounted self-insurance reserves	53.2	51.2
Instruments qualifying as cash flow or fair value hedges	0
Advertising expense charged to operations	73.8	70.0	72.9
Deferred debt issuance costs, net of amortization	9.0	21.6
Deferred debt costs, net of amortization, included in "Other current assets"	1.3	8.6
Valuation allowance for deferred tax assets	0	0
Vehicles [Member]
Gross value of capital lease assets	8.6	9.6
Accumulated amortization on all capital lease assets	$ 7.9	$ 7.5
Minimum [Member]
Note receivable interest rate	0.00%
Maximum [Member]
Note receivable interest rate	10.00%