XML 48 R38.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Securities - Fair Values and Amortized Costs Securities (Details) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | $ 693,008 |  |  |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 2,858 |  | $ 5,645 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (12,950) |  | (3,967) |
Debt Securities, Available-for-Sale, Fair Value |  | 682,916 |  | 693,147 |
Equity Securities, Fair Value |  | 5,425 |  | 5,144 |
Total Securities, Amortized Cost |  |  |  | 695,284 |
Total Securities, Gross Unrealized Gains |  |  |  | 6,975 |
Total Securities, Gross Unrealized Losses |  |  |  | (3,968) |
Total Securities, Fair Value |  | 688,341 |  | 698,291 |
U.S. Treasury securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 9,953 |  | 19,943 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 0 |  | 0 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (334) |  | (154) |
Debt Securities, Available-for-Sale, Fair Value |  | 9,619 |  | 19,789 |
Obligations of U.S. government corporations and agencies |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 156,812 |  | 162,045 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 16 |  | 341 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (1,759) |  | (193) |
Debt Securities, Available-for-Sale, Fair Value |  | 155,069 |  | 162,193 |
Collateralized mortgage obligations of U.S. government corporations and agencies |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 121,631 |  | 109,916 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 61 |  | 93 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (2,950) |  | (1,321) |
Debt Securities, Available-for-Sale, Fair Value |  | 118,742 |  | 108,688 |
Residential mortgage-backed securities of U.S. government corporations and agencies |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 28,662 |  | 32,388 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 309 |  | 679 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (529) |  | (213) |
Debt Securities, Available-for-Sale, Fair Value |  | 28,442 |  | 32,854 |
Commercial mortgage-backed securities of U.S. government corporations and agencies |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 254,458 |  | 244,018 | [1]
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 0 |  | 247 | [1]
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (7,330) |  | (2,044) | [1]
Debt Securities, Available-for-Sale, Fair Value |  | 247,128 |  | 242,221 | [1]
Securities purchased pending settlement |  |  |  | 5,900 |
Obligations of states and political subdivisions |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 121,492 |  | 123,159 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 2,472 |  | 4,285 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (48) |  | (42) |
Debt Securities, Available-for-Sale, Fair Value |  | 123,916 |  | 127,402 |
Total Debt Securities Available-for-Sale |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost |  | 693,008 |  | 691,469 |
Debt Securities, Available-for-Sale, Gross Unrealized Gains |  | 2,858 |  | 5,645 |
Debt Securities, Available-for-Sale, Gross Unrealized Losses |  | (12,950) |  | (3,967) |
Debt Securities, Available-for-Sale, Fair Value |  | 682,916 |  | 693,147 |
Marketable equity securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Equity Securities, FV-NI, Cost | [2] | 0 |  | 3,815 |
Equity Securities, FV-NI, Unrealized Gain | [2] | 0 | $ 1,330 |  |
Equity Securities, FV-NI, Unrealized Loss | [2] | 0 | $ (1) |  |
Equity Securities, Fair Value | [2] | $ 0 |  | $ 5,144 |
[1] | Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.
[2] | ASU No. 2016-01 was adopted January 1, 2018, resulting in separate classification of our marketable equity securities previously included in available-for-sale securities

X
- Definition Debt Securities, Available-for-sale and Equity Securities, Accumulated Gross Unrealized Gain, before Tax + References No definition available. + Details
Name: | stba_DebtSecuritiesAvailableforsaleandEquitySecuritiesAccumulatedGrossUnrealizedGainbeforeTax
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Debt Securities, Available-for-sale and Equity Securities, Accumulated Gross Unrealized Loss, before Tax + References No definition available. + Details
Name: | stba_DebtSecuritiesAvailableforsaleandEquitySecuritiesAccumulatedGrossUnrealizedLossbeforeTax
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Available-for-sale and Equity Securities, FV-NI + References No definition available. + Details
Name: | stba_DebtSecuritiesAvailableforsaleandEquitySecuritiesFVNI
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Available-for-sale and Equity Securities, FV-NI, Cost + References No definition available. + Details
Name: | stba_DebtSecuritiesAvailableforsaleandEquitySecuritiesFVNICost
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Securities Purchased Pending Settlement + References No definition available. + Details
Name: | stba_SecuritiesPurchasedPendingSettlement
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. + References No definition available. + Details
Name: | us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714 + Details
Name: | us-gaap_EquitySecuritiesFvNiUnrealizedGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714 + Details
Name: | us-gaap_EquitySecuritiesFvNiUnrealizedLoss
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |