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v2.4.1.9

Summary of Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Significant Accounting Policies [Line Items] |  |
Percentage of dealership that is accounted for under the equity method | 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage |
Effective tax rate from continuing operations | 39.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 38.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Minimum [Member] |  |
Significant Accounting Policies [Line Items] |  |
Expected effective tax rate for continuing operations Range | 38.00%sah_EffectiveIncomeTaxRateContinuingOperationsFuturePeriod / us-gaap_RangeAxis = us-gaap_MinimumMember |
Maximum [Member] |  |
Significant Accounting Policies [Line Items] |  |
Expected effective tax rate for continuing operations Range | 40.00%sah_EffectiveIncomeTaxRateContinuingOperationsFuturePeriod / us-gaap_RangeAxis = us-gaap_MaximumMember |

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 2 -Article 4 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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