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Allowance for Credit Losses - Additional Information (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss | $ 109,636 | $ 114,609 | $ 109,636 | $ 114,609 | $ 115,101 | $ 117,612 | $ 96,850 |  | $ 62,224
Provision for credit losses on loans | 2,008 | 85,831 | 5,308 | 104,262 |  |  |  |  |
Provision for credit losses | 555 | 927 | 391 | 2,543 |  |  |  |  |
Charge-offs | 8,737 | 68,303 | 15,270 | 79,749 |  |  |  |  |
Total loans | 6,897,715 |  | 6,897,715 |  |  | 7,108,248 |  |  |
Recoveries | 1,264 | 231 | 1,985 | 520 |  |  |  |  |
COVID-19 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Financing receivable, credit loss, expense (reversal), period increase (decrease) | 84,200 | 86,800 | 101,100 | 106,800 |  |  |  |  |
Provision for credit losses on loans | 2,600 |  | 5,700 |  |  |  |  |  |
Provision for credit losses | 600 |  | $ 400 |  |  |  |  |  |
Forecast period |  |  | 2 years |  |  |  |  |  |
Commercial and industrial | Commercial |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss | 14,426 | 19,164 | $ 14,426 | 19,164 | $ 14,663 | $ 16,100 | 19,870 |  | 15,681
Provision for credit losses on loans | 225 | 60,906 | 2,953 | 67,104 |  |  |  |  |
Charge-offs | 473 | 61,616 | 4,774 | 71,496 |  |  |  |  |
Recoveries | 11 | 4 | $ 148 | 22 |  |  |  |  |
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 |  |  |  |  |
Recoveries | $ 900 |  |  |  |  |  |  |  |
Impact of new accounting standard |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  | 0 |  | 27,352
Impact of new accounting standard | ASU No. 2016-13 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  | $ 27,400 |
Impact of new accounting standard | S&T Legacy Loans | ASU No. 2016-13 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  | 8,200 |
Impact of new accounting standard | Commercial and industrial | Commercial |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  | $ 0 |  | $ 7,853
Impact of new accounting standard | Commercial and industrial | Commercial | ASU No. 2016-13 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  | 9,900 |
Impact of new accounting standard | DNB | ASU No. 2016-13 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  | $ 9,300 |

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