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Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Mar. 31, 2019 | Dec. 31, 2018
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New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Adjustment to allowance for credit loss | $ 96,850 | $ 62,224 | $ 62,224 |  |
Retained earnings | (1,176,251) |  | (1,191,998) | $ (943,156) | $ (935,761)
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 |  | 27,353 | 27,352 |  |
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 |  |  |
Impact of ASU 2016-13 Adoption | Commercial and industrial |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Adjustment to allowance for credit loss |  | 9,900 |  |  |
Retained Earnings |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Retained earnings | $ (740,726) |  | $ (761,083) | $ (716,078) | $ (701,819)
Retained Earnings | Impact of ASU 2016-13 Adoption |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Retained earnings |  | $ 22,600 |  |  |

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