Business Combinations - Additional Information (Details)					12 Months Ended
	Dec. 31, 2020 USD ($)	Nov. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2019 USD ($) banking_location $ / shares shares	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill	$ 373,424,000		$ 371,621,000		$ 373,424,000	$ 371,621,000
Adjustments, loans	8,600,000		$ 12,300,000
Merger related expenses					2,342,000	11,350,000	$ 0
Professional services and legal					6,394,000	4,244,000	$ 4,132,000
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 total consideration		$ 200,991,000		$ 200,991,000
Cash		360,000		$ 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	86,000,000.0	85,978,000		$ 84,175,000	86,000,000.0
Preliminary estimates adjustments		1,803,000
Adjustments, loans		2,377,000		8,143,000
Adjustments, borrowings		257,000		276,000
Adjustment, other liabilities		122,000		3,184,000
Adjustments, other assets		116,000		4,278,000
Adjustments, deferred income taxes		311,000		(3,298,000)
Carryover of allowance for credit losses		0		0
Fair value of loans acquired		$ 906,607,000		908,984,000
Discount on loans acquired				$ 10,500,000
Direct costs related to merger	$ 13,700,000				13,700,000
Merger related expenses					2,300,000	11,400,000
Professional services and legal					200,000	2,800,000
Severance payments					1,400,000	3,400,000
Data processing, contract termination and conversion cost expenses					400,000	4,700,000
Other expenses					$ 300,000	$ 500,000