Business Combinations - Additional Information (Details)					3 Months Ended		4 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2019 USD ($) banking_location $ / shares shares		Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill	$ 374,270,000	$ 371,621,000			$ 374,270,000		$ 374,270,000	$ 371,621,000
Adjustments, Loans	10,900,000	$ 12,300,000
Merger related expenses					2,342,000	$ 0
Professional services and legal					1,048,000	$ 1,184,000
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	85,983,000		$ 84,175,000		85,983,000		85,983,000
Preliminary estimates adjustments							1,808,000
Adjustments, Loans			8,143,000	[1]	(2,500,000)		2,496,000
Adjustments, Borrowings			276,000	[1]			257,000
Adjustments, Deferred income taxes			(3,298,000)	[1]			311,000
Carryover of allowance for credit losses	0		0		0		0
Fair value of loans acquired	906,488,000		908,984,000		906,488,000		906,488,000
Discount on loans acquired			$ 10,500,000
Direct costs related to merger	$ 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

[1] Management is continuing to evaluate the purchase accounting fair value adjustments related to loans, including loan classification, deferred and current income taxes until the final valuations are complete and final tax returns are filed. Any changes in preliminary estimates will be adjusted in goodwill in subsequent periods, but not extending beyond one year from the date of acquisition.