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Fair Value Measurements - Assets and Liabilities Recorded at Fair Value (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Cash Flow Swaps Designated as Hedges [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Amount included in other accrued liabilities | $ 6.4 sah_DerivativeLiabilityInOtherAccruedLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsDesignatedAsHedgesMember | $ 7.5 sah_DerivativeLiabilityInOtherAccruedLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsDesignatedAsHedgesMember
Amount included in other long-term liabilities | 4.7 sah_DerivativeLiabilityInOtherLongTermLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsDesignatedAsHedgesMember | 2.8 sah_DerivativeLiabilityInOtherLongTermLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsDesignatedAsHedgesMember
Cash Flow Swaps Not Designated as Hedges [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Amount included in other accrued liabilities | 0.6 sah_DerivativeLiabilityInOtherAccruedLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsNotDesignatedAsHedgesMember | 0.7 sah_DerivativeLiabilityInOtherAccruedLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsNotDesignatedAsHedgesMember
Amount included in other long-term liabilities | $ 0.8 sah_DerivativeLiabilityInOtherLongTermLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsNotDesignatedAsHedgesMember | $ 0.8 sah_DerivativeLiabilityInOtherLongTermLiabilities / us-gaap_HedgingDesignationAxis = sah_CashFlowSwapsNotDesignatedAsHedgesMember

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- Definition Amount of derivative liability included in other accrued liabilities. + References No definition available. + Details
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Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of derivative liability included in other long-term liabilities. + References No definition available. + Details
Name: | sah_DerivativeLiabilityInOtherLongTermLiabilities
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |