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Fair Value Measurements - Assets or Liabilities Recorded at Fair Value (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2014 | Dec. 31, 2013
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Cash Flow Swaps Designated as Hedges [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Amount included in other accrued liabilities | $ 8.6 | $ 10.6
Amount included in other long-term liabilities | 2.5 | 7.4
Cash Flow Swaps Not Designated as Hedges [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Amount included in other accrued liabilities | 0.7 | 1.0
Amount included in other long-term liabilities | $ 0.9 | $ 1.0

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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

X
- 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
Period Type: | duration