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Business Combinations - Additional Information (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 04, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Mar. 04, 2016 | Dec. 31, 2015
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  | $ 291,670,000 |  | $ 291,670,000 |  |  | $ 291,764,000
Merger related expenses |  | 0 | $ 0 | 0 | $ 3,167,000 |  |
Integrity Bancshares, Inc. |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition, percentage of voting interests acquired | 100.00% |  |  |  |  |  |
Business acquisition, cash per share offered in exchange of share (in dollars per 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 dollars per share) | $ 28.88 |  |  |  |  |  |
Purchase accounting adjustments, increase to goodwill |  |  |  |  |  | 1,100,000 |
Purchase accounting adjustments, decrease to deferred taxes |  |  |  |  |  | 300,000 |
Goodwill |  |  |  |  |  | 115,850,000 |
Carryover of allowance for loan losses | $ 0 |  |  |  |  |  |
Fair value of loans acquired | 788,700,000 |  |  |  |  | 788,687,000 |
Discount on loans acquired | 14,800,000 |  |  |  |  |  |
Merger related expenses |  | $ 0 | $ 0 | $ 0 | $ 3,200,000 |  |
Integrity Bancshares, Inc. | Commercial and Industrial |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 184,200,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Commercial Real Estate |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 331,600,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Commercial Construction |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 92,400,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Residential Mortgage |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 116,900,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Home Equity |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 25,600,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Installment and Other Consumer |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | 36,100,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Consumer Construction |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of loans acquired | $ 1,900,000 |  |  |  |  |  |
Integrity Bancshares, Inc. | Land |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Purchase accounting adjustments, decrease to land |  |  |  |  |  | $ 800,000 |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=73531572&loc=d3e1043-128460 + Details
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