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Shareholders' Equity (Shares Repurchased Under Share Repurchase Program) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Class of Stock [Line Items]
Aggregate purchase price	$ 9.6us-gaap_TreasuryStockValueAcquiredCostMethod
Treasury Stock [Member]
Class of Stock [Line Items]
Aggregate purchase price	9.6us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = us-gaap_TreasuryStockMember
Stock Repurchase Program Board Authorized Repurchases [Member]
Class of Stock [Line Items]
Remaining amount available for share repurchase	271.6us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1 / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Shares repurchased (in shares)	0.2us-gaap_TreasuryStockSharesAcquired / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	2.4us-gaap_TreasuryStockSharesAcquired / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Aggregate purchase price	$ 9.1us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	$ 115.7us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Average purchase price per share (in dollars per share)	$ 60.46us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	$ 47.92us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember

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- Definition

Amount remaining of a stock repurchase plan authorized.

+ References

No definition available.

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- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

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Period Type:	duration

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- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

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- Definition

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

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