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Inventories and Related Notes Payable - Floor Plan - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Inventory Disclosure [Abstract] |  |  |
Average interest rate for new vehicle floor plan facilities, for continuing operations and discontinued operations | 1.57%sah_WeightedAverageInterestRateNewVehicleFloorPlan | 1.86%sah_WeightedAverageInterestRateNewVehicleFloorPlan | 2.02%sah_WeightedAverageInterestRateNewVehicleFloorPlan
Amount recognized as reduction in cost of sales for continuing and discontinued operations | $ 39.7 sah_FloorPlanAssistanceRecognized | $ 37.9 sah_FloorPlanAssistanceRecognized | $ 33.0 sah_FloorPlanAssistanceRecognized
Average interest rate for used vehicle floor plan facilities, for continuing operations and discontinued operations | 1.80%sah_WeightedAverageInterestRateUsedVehicleFloorPlan | 2.78%sah_WeightedAverageInterestRateUsedVehicleFloorPlan | 2.80%sah_WeightedAverageInterestRateUsedVehicleFloorPlan

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- Definition Floor plan assistance recognized. + References No definition available. + Details
Name: | sah_FloorPlanAssistanceRecognized
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Weighted average interest rate new vehicle floor plan. + References No definition available. + Details
Name: | sah_WeightedAverageInterestRateNewVehicleFloorPlan
Namespace Prefix: | sah_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Weighted average interest rate used vehicle floor plan. + References No definition available. + Details
Name: | sah_WeightedAverageInterestRateUsedVehicleFloorPlan
Namespace Prefix: | sah_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_InventoryDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration