Business Combinations - Additional Information (Details)		12 Months Ended
	Nov. 30, 2019 USD ($) banking_location $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 371,621,000	$ 287,446,000
Merger related expenses		11,350,000	0	$ 0
Professional services and legal		4,244,000	$ 4,132,000	$ 4,096,000
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 consideration	$ 200,991,000
Cash	$ 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	$ 84,175,000	84,200,000
Carryover of allowance for loan losses	0
Acquired loans	908,984,000	899,300,000
Discount on loans acquired	$ 10,500,000
Merger related expenses		11,400,000
Data processing, contract termination and conversion cost expenses		4,700,000
Professional services and legal		2,800,000
Severance payments		3,400,000
Other expenses		500,000
Net interest income		3,200,000
Net Income		2,100,000
DNB | Commercial real estate
Business Acquisition [Line Items]
Acquired loans		455,600,000
DNB | Commercial and industrial
Business Acquisition [Line Items]
Acquired loans		85,400,000
DNB | Commercial construction
Business Acquisition [Line Items]
Acquired loans		77,100,000
DNB | Residential mortgage
Business Acquisition [Line Items]
Acquired loans		219,700,000
DNB | Home equity
Business Acquisition [Line Items]
Acquired loans		56,400,000
DNB | Installment and other consumer
Business Acquisition [Line Items]
Acquired loans		4,100,000
DNB | Consumer construction
Business Acquisition [Line Items]
Acquired loans		$ 1,000,000.0