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Allowance for Credit Losses - Additional Information (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2020 | Jan. 01, 2020 | Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update [Extensible List] |  |  |  | us-gaap:AccountingStandardsUpdate201613Member |  |  | us-gaap:AccountingStandardsUpdate201613Member | us-gaap:AccountingStandardsUpdate201602Member |  |  |
Adjustment to allowance for credit loss |  | $ 114,609 | $ 61,479 | $ 114,609 |  | $ 61,479 | $ 62,224 | $ 60,996 | $ 96,850 | $ 62,224 | $ 61,409
Charge-offs |  | 68,303 | 2,667 | 79,749 |  | 8,691 |  |  |  |  |
Total loans |  |  |  |  |  |  |  |  |  | 7,074,928 |
Restructured loans | [1],[2] | 1,304 | 6,427 | $ 7,853 |  | 11,248 |  |  |  |  |
Forecast period |  |  |  | 2 years |  |  |  |  |  |  |
Nonperforming |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  |  | $ 31,755 | [3] |  | 8,912 |  |  |  |
Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  | 27,352 |  |  | 27,353 |
Total loans |  |  |  |  |  |  |  |  |  | (27,353) |
S&T Legacy Loans | Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  |  |  | 8,200 |
Commercial and industrial |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  | 13,227 |  |  | 13,227 |  | $ 11,596 |  |  | $ 11,996
Charge-offs |  |  | 1,435 |  |  | 6,912 |  |  |  |  |
Restructured loans | [1] | 102 | $ 0 | 2,170 |  | $ 4,529 |  |  |  |  |
Commercial and industrial | Nonperforming |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  |  | 2,068 |  |  | $ 695 |  |  |  |
Commercial and industrial | Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  |  |  | 9,900 |
Customer fraud, check kiting |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Charge-offs |  | 58,700 |  |  |  |  |  |  |  |  |
Customer fraud |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Charge-offs |  | 4,200 |  |  |  |  |  |  |  |  |
Total loans |  | 15,100 |  | 15,100 |  |  |  |  |  |  |
Customer fraud | Nonperforming |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  |  | 10,900 |  |  |  |  |  |  |
CARES Act, Paycheck Protection Program Loans |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total loans |  | 547,600 |  | 547,600 |  |  |  |  |  |  |
Restructured loans |  |  |  | 1,400,000 |  |  |  |  |  |  |
ACL provision, COVID-19 assessment |  | 14,400 |  | 29,300 |  |  |  |  |  |  |
ACL provision, COVID-19 assessment, revised forecast |  | 14,800 |  | 26,000 |  |  |  |  |  |  |
Increase (decrease) in ACL provision, COVID-19 assessment, changes in current conditions |  | (400) |  | 3,300 |  |  |  |  |  |  |
Customer fraud, commercial real estate |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total loans |  | 14,300 |  | 14,300 |  |  |  |  |  |  |
Customer fraud, line of credit |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total loans |  | $ 800 |  | $ 800 |  |  |  |  |  |  |
DNB | Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  |  |  | $ 9,300 |
[1] | Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.
[2] | Refer to Note 1, Basis of Presentation for details of reclassification of our portfolio segments related to the adoption of ASU 2016-13 Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instrument.
[3] | Refer to Note 1, Basis of Presentation for details of reclassification of our portfolio segments related to the adoption of ASU 2016-13 Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.

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