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Allowance for Credit Losses - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Jan. 01, 2020 | Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss | $ 108,348 | $ 120,998 | $ 108,348 | $ 120,998 | $ 109,636 | $ 117,612 | $ 114,609 |  | $ 62,224
Provision for credit losses on loans | 2,302 | 19,302 | 7,612 | 123,564 |  |  |  |  |
Provision for credit losses | 1,085 | (1,816) | 1,476 | 727 |  |  |  |  |
COVID-19 |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Financing receivable, credit loss, expense (reversal), period increase (decrease) | 14,100 | 17,500 | 115,600 | 124,300 |  |  |  |  |
Provision for credit losses on loans | 3,400 |  | 8,700 |  |  |  |  |  |
Provision for credit losses | 1,100 |  | $ 1,500 |  |  |  |  |  |
Forecast period |  |  | 2 years |  |  |  |  |  |
Commercial and industrial | Commercial |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss | 26,239 | 14,211 | $ 26,239 | 14,211 | $ 14,426 | $ 16,100 | $ 19,164 |  | 15,681
Provision for credit losses on loans | $ 11,599 | $ (4,155) | $ 14,552 | $ 62,949 |  |  |  |  |
Impact of new accounting standard |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  | $ 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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