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Description of Business and Summary of Significant Accounting Policies - Range of Estimated Useful Lives (Detail) | 12 Months Ended
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Dec. 31, 2017
Leasehold and Land Improvements [Member] | Minimum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 10 years
Leasehold and Land Improvements [Member] | Maximum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 30 years
Buildings [Member] | Minimum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 10 years
Buildings [Member] | Maximum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 30 years
Parts and Service Equipment [Member] | Minimum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 7 years
Parts and Service Equipment [Member] | Maximum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 10 years
Office Equipment and Fixtures [Member] | Minimum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 3 years
Office Equipment and Fixtures [Member] | Maximum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 10 years
Company Vehicles [Member] | Minimum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 3 years
Company Vehicles [Member] | Maximum [Member] |
Property Plant And Equipment [Line Items] |
Estimated useful lives | 5 years

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=sah_LeaseholdAndLandImprovementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=sah_PartsAndServiceEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OfficeEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_VehiclesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |