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Commitments and Contingencies - Summary of Lease Exit Accruals (Parenthetical) (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract] |
Component of lease exit expense in interest expense, other, net | $ 0.1
Component of lease exit expense in selling, general and administrative expenses | 0.9
Component of lease exit expense in income (loss) from operations and the sale of dealerships | 1.2
Component of lease exit payments in selling, general and administrative expenses | 1.1
Component of lease exit payments in income (loss) from operations and the sale of dealerships | $ 2.5

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- Definition Lease exit expense interest. + References No definition available. + Details
Name: | sah_LeaseExitExpenseInterest
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Lease exit expense selling general and administrative. + References No definition available. + Details
Name: | sah_LeaseExitExpenseSellingGeneralAndAdministrative
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Lease exit income (expense) discontinued operations. + References No definition available. + Details
Name: | sah_LeaseExitIncomeExpenseDiscontinuedOperations
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Lease exit payment discontinued operations. + References No definition available. + Details
Name: | sah_LeaseExitPaymentDiscontinuedOperations
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Lease exit payment selling general and administrative. + References No definition available. + Details
Name: | sah_LeaseExitPaymentSellingGeneralAndAdministrative
Namespace Prefix: | sah_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration