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Business Combinations - Additional Information (Details) |  | 12 Months Ended
--- | --- | ---
Nov. 30, 2019 USD ($) banking_location $ / shares shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |
Goodwill |  | $ 371,621,000 | $ 287,446,000 |
Merger related expenses |  | 11,350,000 | 0 | $ 0
Professional services and legal |  | 4,244,000 | $ 4,132,000 | $ 4,096,000
DNB |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of banking locations | banking_location | 14 |  |  |
Shares of S&T offered for each share of DNB (in shares) | shares | 1.22 |  |  |
Fair value of consideration | $ 200,991,000 |  |  |
Cash | $ 360,000 |  |  |
S&T common shares issued (in shares) | shares | 5,318,964 |  |  |
S&T common shares issued, fair value (in USD per share) | $ / shares | $ 37.72 |  |  |
Goodwill | $ 84,175,000 | 84,200,000 |  |
Carryover of allowance for loan losses | 0 |  |  |
Acquired loans | 908,984,000 | 899,300,000 |  |
Discount on loans acquired | $ 10,500,000 |  |  |
Merger related expenses |  | 11,400,000 |  |
Data processing, contract termination and conversion cost expenses |  | 4,700,000 |  |
Professional services and legal |  | 2,800,000 |  |
Severance payments |  | 3,400,000 |  |
Other expenses |  | 500,000 |  |
Net interest income |  | 3,200,000 |  |
Net Income |  | 2,100,000 |  |
DNB | Commercial real estate |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 455,600,000 |  |
DNB | Commercial and industrial |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 85,400,000 |  |
DNB | Commercial construction |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 77,100,000 |  |
DNB | Residential mortgage |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 219,700,000 |  |
DNB | Home equity |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 56,400,000 |  |
DNB | Installment and other consumer |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | 4,100,000 |  |
DNB | Consumer construction |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Acquired loans |  | $ 1,000,000.0 |  |

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