Business Combinations - Additional Information (Details) - USD ($)			3 Months Ended		6 Months Ended
	Jun. 30, 2015	Mar. 04, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 291,204,000		$ 291,204,000		$ 291,204,000		$ 175,820,000
Discount on loans acquired	14,700,000		14,700,000		14,700,000
Merger related expenses			866,000	$ 0	3,167,000	$ 0
Professional services and legal			801,000	$ 875,000	1,324,000	$ 1,538,000
Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Business acquisition, percentage of voting interests acquired		100.00%
Business acquisition, cash per share offered in exchange of share		$ 52.50
Shares of S&T offered for each share of Integrity (shares)		2.0627
Fair value of total consideration	171,979,000	$ 172,000,000
Cash		$ 29,500,000			29,510,000
S&T common shares issued (shares)		4,933,115
S&T common shares issued, fair value (in USD per share)		$ 28.88
Goodwill, purchase accounting adjustments			600,000
Goodwill	115,383,000	$ 115,400,000	115,383,000		115,383,000
Carryover of allowance for loan losses		0
Fair value of loans acquired	$ 788,687,000	788,700,000	$ 788,687,000		788,687,000
Discount on loans acquired		14,800,000
Merger related expenses					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
Commercial Real Estate [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		331,600,000
Commercial and Industrial [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		184,200,000
Construction Loans [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		92,400,000
Residential Mortgage [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		116,900,000
Home Equity [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		25,600,000
Consumer Borrower [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		36,100,000
Consumer Construction [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired		$ 1,900,000