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Description of Business and Summary of Significant Accounting Policies (Revenue Recognition, Advertising) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Advertising [Abstract]
Advertising expense, net	$ 164.9us-gaap_AdvertisingExpense	$ 166.4us-gaap_AdvertisingExpense	$ 135.7us-gaap_AdvertisingExpense
Manufacturer advertising reimbursements	47.1us-gaap_CooperativeAdvertisingAmount	42.4us-gaap_CooperativeAdvertisingAmount	38.3us-gaap_CooperativeAdvertisingAmount
Chargeback Reserves [Member]
Revenue Recognition [Abstract]
Chargeback liabilities	$ 84.9us-gaap_ValuationAllowancesAndReservesBalance / us-gaap_ValuationAllowancesAndReservesTypeAxis = an_ChargebacksReservesMember	$ 67.6us-gaap_ValuationAllowancesAndReservesBalance / us-gaap_ValuationAllowancesAndReservesTypeAxis = an_ChargebacksReservesMember	$ 56.0us-gaap_ValuationAllowancesAndReservesBalance / us-gaap_ValuationAllowancesAndReservesTypeAxis = an_ChargebacksReservesMember	$ 46.2us-gaap_ValuationAllowancesAndReservesBalance / us-gaap_ValuationAllowancesAndReservesTypeAxis = an_ChargebacksReservesMember

X

- Definition

Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 720

-SubTopic 35

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Total amount received by an entity from a vendor under a cooperative advertising arrangement that has been offset against advertising expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cooperative Advertising

-URI http://asc.fasb.org/extlink&oid=6509061



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 50

-Section 45

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6408645&loc=d3e63872-111659

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	duration

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Article 12

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Balance Type:	credit
Period Type:	instant

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- Details

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Namespace Prefix:
Data Type:	na
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