Business Combinations Additional Information (Details) - USD ($)		10 Months Ended	12 Months Ended
	Mar. 04, 2015	Dec. 31, 2015	Dec. 31, 2016	Mar. 04, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill, adjustments during measurement period			$ (94,000)		$ 0
Goodwill		$ 291,764,000	291,670,000		291,764,000
Merger related expenses			0		3,167,000	$ 689,000
Data processing, contract termination and conversion cost expenses			0		1,300,000	0
Professional services and legal			4,212,000		3,365,000	$ 3,717,000
Integrity Bancshares, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Cash per share offered in exchange	$ 52.50
Shares of S&T offered for each share of Integrity (in shares)	2.0627
Fair value of consideration	$ 172,000,000			$ 171,979,000
Cash	$ 29,500,000			29,510,000
S&T common shares issued (in shares)	4,933,115
S&T common shares issued, fair value (in dollars per share)	$ 28.88
Goodwill, adjustments during measurement period		1,100,000	(100,000)
Purchase accounting adjustment, decrease to land		800,000
Purchase accounting adjustment, Decrease to deferred tax assets		$ 300,000
Goodwill	$ 115,900,000			115,850,000
Carryover of allowance for loan losses	0
Acquired loans	788,700,000			$ 788,687,000
Discount on Loans Acquired	14,800,000
Merger related expenses			$ 0		3,200,000
Data processing, contract termination and conversion cost expenses					1,300,000
Professional services and legal					1,200,000
Severance payments					400,000
Other expenses					$ 300,000
Integrity Bancshares, Inc. | Commercial Real Estate
Business Acquisition [Line Items]
Acquired loans	331,600,000
Integrity Bancshares, Inc. | Commercial and Industrial
Business Acquisition [Line Items]
Acquired loans	184,200,000
Integrity Bancshares, Inc. | Commercial Construction
Business Acquisition [Line Items]
Acquired loans	92,400,000
Integrity Bancshares, Inc. | Residential Mortgage
Business Acquisition [Line Items]
Acquired loans	116,900,000
Integrity Bancshares, Inc. | Home Equity
Business Acquisition [Line Items]
Acquired loans	25,600,000
Integrity Bancshares, Inc. | Installment and Other Consumer
Business Acquisition [Line Items]
Acquired loans	36,100,000
Integrity Bancshares, Inc. | Consumer Construction
Business Acquisition [Line Items]
Acquired loans	$ 1,900,000