XML 61 R51.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Allowance for Loan Losses - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Jun. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |
Loans considered nonperforming (in days) |  |  |  | 90 days |
Threshold for evaluation for impairment of substandard and nonaccrual commercial loans | $ 500,000.0 |  |  | $ 500,000.0 |
Change-off arising from participation loan agreement | 1,234,000 |  | $ 2,660,000 | 11,311,000 | $ 11,423,000
Commercial real estate |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |
Change-off arising from participation loan agreement | 141,000 |  | 37,000 | 373,000 | 2,100,000
Other consumer |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |
Change-off arising from participation loan agreement | 425,000 |  | $ 425,000 | 1,298,000 | $ 1,228,000
Other consumer | Vehicles and Equipment Liens |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |
Change-off arising from participation loan agreement |  | $ 5,200,000 |  |  |
Participating loans, total exposure |  | 4,900,000 |  |  |
Participating loans, direct exposure |  | $ 950,000 |  |  |
Participating loans, recoveries | $ 100,000 |  |  |  |
Substandard |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |
Increase in substandard loans resulted in loans downgraded |  |  |  | $ 87,800,000 |

X
- Definition Minimum amount over which evaluation for impairment of substandard and nonaccrual commercial loans are made. + References No definition available. + Details
Name: | stba_MinimumAmountOverWhichEvaluationForImpairmentOfSubstandardAndNonaccrualCommercialLoansAreMade
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Participating Loans, Allowance for Loan and Lease Loss, Recovery of Bad Debts + References No definition available. + Details
Name: | stba_ParticipatingLoansAllowanceforLoanandLeaseLossRecoveryofBadDebts
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Participating Loans, Direct Exposure + References No definition available. + Details
Name: | stba_ParticipatingLoansDirectExposure
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Participating Loans, Total Exposure + References No definition available. + Details
Name: | stba_ParticipatingLoansTotalExposure
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Period interest and principal of loan due treated as loans considered nonperforming. + References No definition available. + Details
Name: | stba_PeriodInterestAndPrincipalOfLoanDueTreatedAsLoansConsideredNonperforming
Namespace Prefix: | stba_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in outstanding loans including accrued interest. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInFinanceReceivables
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=stba_OtherConsumerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CollateralAxis=stba_AutomobilesandEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |