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Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jun. 30, 2020 | Jan. 01, 2020 | Sep. 30, 2019 | Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Accounting Standards Update [Extensible List] | us-gaap:AccountingStandardsUpdate201613Member | us-gaap:AccountingStandardsUpdate201613Member | us-gaap:AccountingStandardsUpdate201602Member |  |  |  |
Adjustment to allowance for credit loss | $ 120,998 | $ 62,224 | $ 60,996 | $ 114,609 | $ 62,224 | $ 62,115 | $ 61,479
Retained earnings | (1,142,115) | (1,191,998) | (935,761) | (1,135,777) |  | (982,447) | (964,953)
Threshold for evaluation for expected credit loss of commercial loans | 500 |  |  |  |  |  |
Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  | 27,352 |  |  | 27,353 |  |
Retained earnings |  | 22,590 | (167) |  |  |  |
Impact of ASU 2016-13 Adoption | DNB |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  | 9,300 |  |
Impact of ASU 2016-13 Adoption | S&T Legacy Loans |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Adjustment to allowance for credit loss |  |  |  |  | 8,200 |  |
Retained Earnings |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Retained earnings | $ (698,351) | (761,083) | (701,819) | $ (692,240) |  | $ (748,280) | $ (730,577)
Retained Earnings | Impact of ASU 2016-13 Adoption |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Retained earnings |  | $ 22,590 | $ (167) |  | $ 22,600 |  |

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