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Employee Benefit Plans (Details Textual) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Employee Benefit Plans [Line Items] |  |  |
Administrative expense | $ 1,700 | $ 1,200 | $ 2,300
Discretionary contribution | 255,000 | 275,000 | 250,000
Deferred Compensation Expense Non Qualified Plan | $ 211,000 | $ 184,000 | $ 138,000

X
- Definition The compensation expense recognized during the period pertaining to non qualified deferred compensation plan. + References No definition available. + Details
Name: | sga_DeferredCompensationExpenseNonQualifiedPlan
Namespace Prefix: | sga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | sga_EmployeeBenefitPlansLineItems
Namespace Prefix: | sga_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of administrative expense incurred by defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanAdministrativeExpenses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration