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Summary of Significant Accounting Policies - Additional Information (Details) |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Nov. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) entity lease unit subsidiary | Jan. 01, 2019 USD ($) lease | Dec. 31, 2018 USD ($) | Jan. 01, 2018 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 |  |  | $ 371,621,000 |  | $ 287,446,000 |
Prior period reclassification adjustment |  |  | $ 0 |  |  |
Period for satisfactory payment of TDRs |  |  | 6 months |  |  |
Evaluation for impairment of substandard and nonaccrual commercial loans |  |  | $ 500,000 |  |  |
Look-back period used for assumptions of allowance for loans and lease losses |  |  | 10 years 6 months |  |  |
Number of reporting units | unit |  |  | 1 |  |  |
Number of wholly owned trust subsidiaries | subsidiary |  |  | 3 |  |  |
Financing leases, lease liabilities |  |  | $ 1,578,000 |  |  |
Operating leases, lease liabilities |  |  | 52,507,000 |  |  |
Financing leases, ROU assets |  |  | 1,513,000 |  |  |
Operating leases, ROU assets |  |  | $ 47,686,000 |  |  |
Number of operating lease agreements | lease |  |  | 48 |  |  |
Commercial construction |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 3 years |  |  |
Commercial real estate |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 3 years |  |  |
Commercial and industrial |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 1 year 3 months |  |  |
Consumer real estate |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 2 years 9 months |  |  |
Other consumer |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 1 year 3 months |  |  |
Minimum | Commercial construction |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 3 years |  |  |
Minimum | Core Deposits And Customers Lists |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Weighted average estimated useful of acquired intangibles |  |  | 10 years |  |  |
Maximum |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Amortization period of investments in joint ventures |  |  | 10 years |  |  |
Maximum | Commercial construction |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  |  | 4 years |  |  |
Maximum | Core Deposits And Customers Lists |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Weighted average estimated useful of acquired intangibles |  |  | 20 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%
ASU No. 2016-01 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements | [1] |  |  |  |  | $ 0
ASU No. 2016-02 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  | $ 167,000 |  |
Financing leases, lease liabilities |  |  |  | 1,200,000 |  |
Operating leases, lease liabilities |  |  |  | 33,700,000 |  |
Financing leases, ROU assets |  |  |  | 1,200,000 |  |
Operating leases, ROU assets |  |  |  | $ 33,400,000 |  |
Number of operating lease agreements | lease |  |  |  | 1 |  |
AOCI Attributable to Parent | ASU No. 2016-01 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements | [1] |  |  |  |  | (862,000)
Retained Earnings | ASU No. 2016-01 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements | [1] |  |  |  |  | $ 862,000
Retained Earnings | ASU No. 2016-02 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  | $ 167,000 |  |
Interest rate lock commitments |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Period for interest rate lock commitment |  |  | 60 days |  |  |
DNB |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Fair value of consideration |  | $ 200,991,000 |  |  |  |
Total assets acquired |  | 1,100,000,000 |  |  |  |
Loans acquired |  | 909,000,000.0 |  |  |  |
Goodwill |  | 84,175,000 | $ 84,200,000 |  |  |
Deposits acquired |  | $ 967,265,000 |  |  |  |
[1] | Reclassification due to the adoption of ASU No. 2016-01, related to changes in fair value for equity securities reclassified out of accumulated other comprehensive income.

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