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Shareholders' Equity (Share Repurchase) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Aggregate purchase price	$ 487.7us-gaap_TreasuryStockValueAcquiredCostMethod	$ 55.7us-gaap_TreasuryStockValueAcquiredCostMethod	$ 582.3us-gaap_TreasuryStockValueAcquiredCostMethod
Treasury Stock [Member]
Class of Stock [Line Items]
Aggregate purchase price	487.7us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = us-gaap_TreasuryStockMember	55.7us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = us-gaap_TreasuryStockMember	582.3us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = us-gaap_TreasuryStockMember
Stock Repurchase Program Board Authorized Repurchases [Member]
Class of Stock [Line Items]
Shares repurchased (in shares)	9.4us-gaap_TreasuryStockSharesAcquired / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	1.1us-gaap_TreasuryStockSharesAcquired / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	16.6us-gaap_TreasuryStockSharesAcquired / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Aggregate purchase price	485.1us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	53.5us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	580.6us-gaap_TreasuryStockValueAcquiredCostMethod / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Average purchase price per share (in dollars per share)	$ 51.59us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	$ 47.37us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember	$ 34.89us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember
Remaining amount available for share repurchase	$ 280.6us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1 / us-gaap_StatementEquityComponentsAxis = an_StockRepurchaseProgramBoardAuthorizedRepurchasesMember

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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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- 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

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-Name Regulation S-X (SX)

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-Section 02

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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

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-Topic 505

-SubTopic 10

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

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-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

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



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