Exit or Disposal Cost Obligations (Details) - AutoNation Collision Parts Closure - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	$ 36.5		$ 36.7
Contract Termination Charges, Other Associated Closing Costs, and Involuntary Termination Benefits
Liability for Exit or Disposal Costs [Rollforward]
Liability for exit or disposal costs, beginning balance		$ 3.4
Costs Incurred		0.0
Costs paid or otherwise settled		(3.3)
Liability for exit or disposal costs, ending balance		0.1	3.4
Contract Termination Charges
Liability for Exit or Disposal Costs [Rollforward]
Liability for exit or disposal costs, beginning balance		0.2
Costs Incurred		0.0
Costs paid or otherwise settled		(0.2)
Liability for exit or disposal costs, ending balance		0.0	0.2
Contract Termination Charges | Other Operating Income (Expense)
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	7.3		3.2
Other Associated Closing Costs
Liability for Exit or Disposal Costs [Rollforward]
Liability for exit or disposal costs, beginning balance		2.4
Costs Incurred		0.0
Costs paid or otherwise settled		(2.4)
Liability for exit or disposal costs, ending balance		0.0	2.4
Other Associated Closing Costs | Selling, General and Administrative Expenses
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	0.0		2.6
Involuntary Termination Benefits
Liability for Exit or Disposal Costs [Rollforward]
Liability for exit or disposal costs, beginning balance		0.8
Costs Incurred		0.0
Costs paid or otherwise settled		(0.7)
Liability for exit or disposal costs, ending balance		$ 0.1	0.8
Involuntary Termination Benefits | Selling, General and Administrative Expenses
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	0.8		1.1
Inventory Valuation Adjustment | Parts and Service [Member] | Cost of Sales
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	20.6		17.6
Accelerated Depreciation | Depreciation and amortization
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	2.9		3.9
Accelerated Amortization | Selling, General and Administrative Expenses
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	2.1		3.2
Asset Impairment Charges | Other Operating Income (Expense)
Liability for Exit or Disposal Costs [Rollforward]
Costs Incurred	$ 2.8		$ 5.1