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.