Business Combinations - Additional Information (Details) - USD ($)		3 Months Ended		7 Months Ended	9 Months Ended
	Mar. 04, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill		$ 291,683,000		$ 291,683,000	$ 291,683,000		$ 175,820,000
Discount on loans acquired		12,000,000		12,000,000	12,000,000		$ 2,000,000
Merger related expenses		0	$ 0		3,167,000	$ 0
Professional services and legal		946,000	$ 950,000		2,270,000	$ 2,488,000
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	$ 172,000,000				171,979,000
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
Purchase accounting adjustments, increase to goodwill				1,100,000
Purchase accounting adjustments, decrease to land				800,000
Purchase accounting adjustments, decrease to deferred taxes				300,000
Goodwill	$ 115,900,000	115,862,000		115,862,000	115,862,000
Carryover of allowance for loan losses	0
Fair value of loans acquired	788,700,000	$ 788,687,000		$ 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 Construction [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
Installment and Other Consumer [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
Commercial and Industrial [Member] | Integrity Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Fair value of loans acquired	$ 184,200,000