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