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Significant Inputs Considered in Determining the Measurement of Credit Losses Recognized in Earnings for Securities (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]      
Current default rate 2.00%us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate [1]    
Prepayment rate 16.00%sti_InputsConsideredinDeterminingMeasurementofAFSSecuritiesCreditLossesRecognizedinEarningsPrepaymentRate [1]    
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity 46.00%sti_InputsConsideredInDeterminingMeasurementOfAFSSecuritiesCreditLossesRecognizedinEarningsLossSeverity [1]    
Minimum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Current default rate   2.00%us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 2.00%us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Prepayment rate   7.00%sti_InputsConsideredinDeterminingMeasurementofAFSSecuritiesCreditLossesRecognizedinEarningsPrepaymentRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 7.00%sti_InputsConsideredinDeterminingMeasurementofAFSSecuritiesCreditLossesRecognizedinEarningsPrepaymentRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity   46.00%sti_InputsConsideredInDeterminingMeasurementOfAFSSecuritiesCreditLossesRecognizedinEarningsLossSeverity
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 40.00%sti_InputsConsideredInDeterminingMeasurementOfAFSSecuritiesCreditLossesRecognizedinEarningsLossSeverity
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Current default rate   9.00%us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 9.00%us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Prepayment rate   21.00%sti_InputsConsideredinDeterminingMeasurementofAFSSecuritiesCreditLossesRecognizedinEarningsPrepaymentRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 21.00%sti_InputsConsideredinDeterminingMeasurementofAFSSecuritiesCreditLossesRecognizedinEarningsPrepaymentRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity   74.00%sti_InputsConsideredInDeterminingMeasurementOfAFSSecuritiesCreditLossesRecognizedinEarningsLossSeverity
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 56.00%sti_InputsConsideredInDeterminingMeasurementOfAFSSecuritiesCreditLossesRecognizedinEarningsLossSeverity
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[1] During the year ended December 31, 2014, all OTTI recognized in earnings related to one private MBS security with a fair value of approximately $16 million at December 31, 2014.