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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 |  |  |  |  |

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- Definition For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the fair value of the receivables acquired, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472 + Details
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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=34752592&loc=d3e1043-128460 + Details
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