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Business Combinations Additional Information (Details) - USD ($) |  | 10 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 04, 2015 | Dec. 31, 2015 | Dec. 31, 2016 | Mar. 04, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Business Acquisition [Line Items] |  |  |  |  |  |
Goodwill, adjustments during measurement period |  |  | $ (94,000) |  | $ 0 |
Goodwill |  | $ 291,764,000 | 291,670,000 |  | 291,764,000 |
Merger related expenses |  |  | 0 |  | 3,167,000 | $ 689,000
Data processing, contract termination and conversion cost expenses |  |  | 0 |  | 1,300,000 | 0
Professional services and legal |  |  | 4,212,000 |  | 3,365,000 | $ 3,717,000
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 (in 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 (in shares) | 4,933,115 |  |  |  |  |
S&T common shares issued, fair value (in dollars per share) | $ 28.88 |  |  |  |  |
Goodwill, adjustments during measurement period |  | 1,100,000 | (100,000) |  |  |
Purchase accounting adjustment, decrease to land |  | 800,000 |  |  |  |
Purchase accounting adjustment, Decrease to deferred tax assets |  | $ 300,000 |  |  |  |
Goodwill | $ 115,900,000 |  |  | 115,850,000 |  |
Carryover of allowance for loan losses | 0 |  |  |  |  |
Acquired loans | 788,700,000 |  |  | $ 788,687,000 |  |
Discount on Loans Acquired | 14,800,000 |  |  |  |  |
Merger related expenses |  |  | $ 0 |  | 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. | Commercial and Industrial |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Acquired loans | 184,200,000 |  |  |  |  |
Integrity Bancshares, Inc. | Commercial Construction |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Acquired loans | 92,400,000 |  |  |  |  |
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 |  |  |  |  |

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- Definition Number of shares of equity interests issued or issuable to acquire entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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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 This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to property, plant, and equipment acquired in connection with a business combination for which the initial accounting was incomplete. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 4A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73530397&loc=SL65897772-128472 + 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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