Business Combinations Additional Information (Details) - USD ($)		10 Months Ended	12 Months Ended
	Mar. 04, 2015	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill		$ 291,764,000	$ 291,764,000	$ 175,820,000
Merger Related Expenses			3,167,000	689,000	$ 838,000
Data Processing, Contract Termination and Conversion Cost Expenses			1,300,000	0	800,000
Professional Services and Legal			3,365,000	$ 3,717,000	$ 4,184,000
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 (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 (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 adjustment, decrease to land		800,000
Purchase accounting adjustment, Decrease to deferred tax assets		300,000
Goodwill	$ 115,900,000	115,944,000	115,944,000
Carryover of Allowance for Loan Losses	0	0	0
Acquired loans	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
Integrity Bancshares, Inc. | Commercial Real Estate
Business Acquisition [Line Items]
Acquired loans	331,600,000
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
Integrity Bancshares, Inc. | Commercial and Industrial
Business Acquisition [Line Items]
Acquired loans	$ 184,200,000