Business Combinations - Narrative (Details)			12 Months Ended
	Nov. 30, 2020 USD ($)	Nov. 30, 2019 USD ($) banking_location $ / shares shares	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 373,424,000	$ 373,424,000
Adjustments, loans			6,700,000	8,600,000
Merger related expenses			0	2,342,000	$ 11,350,000
Professional services and legal			$ 6,368,000	6,394,000	4,244,000
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	85,978,000	$ 84,175,000		86,000,000
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
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