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Summary of Significant Accounting Policies - Additional Information (Details) |  |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 30, 2020 USD ($) | Nov. 30, 2019 USD ($) | Dec. 31, 2020 USD ($) reportingUnit entity subsidiary | Jan. 01, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Jan. 01, 2018 | Dec. 31, 2017 USD ($)
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Number of wholly owned subsidiaries | subsidiary |  |  | 5 |  |  |  |  |
Percentage of outstanding common stock of investees accounted for using equity method of accounting |  |  | 100.00% |  |  |  |  |
Number of entities non-banking activities | entity |  |  | 8 |  |  |  |  |
Goodwill |  |  | $ 373,424,000 |  | $ 371,621,000 |  |  |
Prior period reclassification adjustment |  |  | $ 0 |  |  |  |  |
Period for satisfactory payment of TDRs (in months) |  |  | 6 months |  |  |  |  |
Threshold for evaluation for expected credit loss of commercial loans |  |  | $ 500,000 |  |  |  |  |
Evaluation for impairment of substandard and nonaccrual commercial loans |  |  | $ 500,000 |  |  |  |  |
Look-back period used for assumptions of allowance for loans and lease losses (in years) |  |  | 10 years 6 months |  |  |  |  |
Number of reporting units | reportingUnit |  |  | 1 |  |  |  |  |
Number of wholly owned trust subsidiaries | subsidiary |  |  | 3 |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 117,612,000 | $ 62,224,000 | 62,224,000 | $ 60,996,000 |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  | $ 1,154,711,000 |  | 1,191,998,000 | 935,761,000 |  | $ 884,031,000
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  | 27,353,000 | 27,346,000 |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  | (22,590,000) | 167,000 |  |
Commercial construction | Commercial |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 3 years |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 7,239,000 |  | 7,900,000 | 7,983,000 |  |
Commercial construction | Commercial | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  |  | (3,376,000) |  |  |
Commercial real estate | Commercial |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 3 years |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 65,656,000 |  | 30,577,000 | 33,707,000 |  |
Commercial real estate | Commercial | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  |  | 4,810,000 |  |  |
Commercial and industrial | Commercial |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 1 year 3 months |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 16,100,000 |  | 15,681,000 | 11,596,000 |  |
Commercial and industrial | Commercial | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  | 9,900,000 | 7,853,000 |  |  |
Consumer real estate | Consumer |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 2 years 9 months |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 10,014,000 |  | 6,337,000 | 6,187,000 |  |
Consumer real estate | Consumer | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  |  | 4,525,000 |  |  |
Other consumer | Consumer |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 1 year 3 months |  |  |  |  |
Adjustment for allowance for credit loss |  |  | $ 2,686,000 |  | 1,729,000 | 1,523,000 |  |
Other consumer | Consumer | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  |  | 636,000 |  |  |
S&T Legacy Loans | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  | 8,200,000 |  |  |  |
Maximum |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Amortization period of investments in joint ventures (in years) |  |  | 10 years |  |  |  |  |
Maximum | Commercial construction | Commercial |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 4 years |  |  |  |  |
Maximum | Core Deposits And Customers Lists |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Weighted average estimated useful of acquired intangibles (in years) |  |  | 20 years |  |  |  |  |
Minimum | Commercial construction | Commercial |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses (in years) |  |  | 3 years |  |  |  |  |
Minimum | Core Deposits And Customers Lists |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Weighted average estimated useful of acquired intangibles (in years) |  |  | 10 years |  |  |  |  |
Corporate Joint Venture | Common Wealth Trust Life Insurance Company |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Percentage of outstanding common stock of investees accounted for using equity method of accounting |  |  | 50.00% |  |  |  |  |
Subsidiaries | S&T Evergreen Insurance LLC |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Percentage of ownership in subsidiary sold |  |  |  |  |  |  | 70.00% |
New Partnership |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Percentage of outstanding common stock of investees accounted for using equity method of accounting |  |  |  |  |  |  | 30.00% |
Retained Earnings |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  | $ 710,061,000 |  | 761,083,000 | 701,819,000 |  | $ 628,107,000
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  | $ (22,590,000) | $ 167,000 |  |
Retained Earnings | ASU No. 2016-13 | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  | (22,600,000) |  |  |  |
Interest rate lock commitments |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Period for interest rate lock commitment (in days) |  |  | 60 days |  |  |  |  |
DNB |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Fair value of consideration | $ 200,991,000 | $ 200,991,000 |  |  |  |  |  |
Total assets acquired |  | 1,100,000,000 |  |  |  |  |  |
Loans acquired |  | 909,000,000.0 |  |  |  |  |  |
Goodwill | 85,978,000 | 84,175,000 | $ 86,000,000.0 |  |  |  |  |
Deposits acquired | $ 967,265,000 | $ 967,265,000 |  |  |  |  |  |
DNB | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Adjustment for allowance for credit loss |  |  |  | $ 9,300,000 |  |  |  |

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