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Business Combinations - Additional Information (Details) - USD ($) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 04, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 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 | $ 866,000 | 0 | $ 3,167,000 |  |
Professional services and legal |  | 1,047,000 | 801,000 | 1,994,000 | 1,324,000 |  |
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 | 900,000 | $ 0 | 3,200,000 |  |
Data processing, contract termination and conversion cost expenses |  |  | 400,000 |  | 1,300,000 |  |
Professional services and legal |  |  | 100,000 |  | 1,200,000 |  |
Severance payments |  |  | 100,000 |  | 400,000 |  |
Other expenses |  |  | $ 300,000 |  | $ 300,000 |  |
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 Percentage of voting equity interests acquired at the acquisition date in the business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463 + Details
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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=73530397&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=73531572&loc=d3e1043-128460 + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975 + Details
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- Definition Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=73531572&loc=d3e961-128460 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585 + Details
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