Business Combinations - Additional Information (Details)				3 Months Ended		6 Months Ended		7 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2019 USD ($) banking_location $ / shares shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill	$ 373,289,000	$ 371,621,000		$ 373,289,000		$ 373,289,000		$ 373,289,000	$ 371,621,000
Adjustments, Loans	10,500,000	$ 12,300,000
Merger related expenses				0	$ 618,000	2,342,000	$ 618,000
Professional services and legal				1,932,000	$ 1,145,000	2,980,000	$ 2,329,000
DNB
Business Acquisition [Line Items]
Number of banking locations | banking_location			14
Shares of S&T offered for each share of DNB (shares) | shares			1.22
Fair value of total consideration			$ 200,991,000
Cash			$ 360,000
S&T common shares issued (shares) | shares			5,318,964
S&T common shares issued, fair value (in dollars per share) | $ / shares			$ 37.72
Goodwill	85,842,000		$ 84,175,000	85,842,000		85,842,000		85,842,000
Preliminary estimates adjustments								1,667,000
Adjustments, Loans			8,143,000			(2,500,000)		2,377,000
Adjustments, Borrowings			276,000					257,000
Adjustments, Deferred income taxes			(3,298,000)					311,000
Carryover of allowance for credit losses	0		0	0		0		0
Fair value of loans acquired	906,607,000		908,984,000	906,607,000		906,607,000		906,607,000
Discount on loans acquired			$ 10,500,000
Direct costs related to merger	$ 13,700,000			$ 13,700,000		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