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Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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S&T |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Leverage Ratio, Actual Amount | $ 628,876 | $ 582,155
Leverage Ratio, Actual Ratio | 9.17% | 8.98%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount | $ 274,254 | $ 259,170
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio | 4.00% | 4.00%
Leverage Ratio, To be Well Capitalized Amount | $ 342,818 | $ 323,963
Leverage Ratio, To be Well Capitalized Ratio | 5.00% | 5.00%
Tier One Common Equity Capital | $ 608,876 | $ 562,155
Tier One Common Equity Capital to Risk Weighted Assets | 10.71% | 10.04%
Tier One Common Equity Capital Required for Capital Adequacy | $ 255,778 | $ 252,079
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets | 4.50% | 4.50%
Tier One Common Equity Capital Required to be Well Capitalized | $ 369,457 | $ 364,114
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets | 6.50% | 6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount | $ 628,876 | $ 582,155
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio | 11.06% | 10.39%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount | $ 341,037 | $ 336,105
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio | 6.00% | 6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount | $ 454,717 | $ 448,140
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio | 8.00% | 8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount | $ 713,056 | $ 664,184
Total Capital (to Risk-Weighted Assets), Actual Ratio | 12.55% | 11.86%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount | $ 454,717 | $ 448,140
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio | 8.00% | 8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount | $ 568,396 | $ 560,175
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio | 10.00% | 10.00%
S&T Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Leverage Ratio, Actual Amount | $ 582,929 | $ 542,048
Leverage Ratio, Actual Ratio | 8.52% | 8.39%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount | $ 273,538 | $ 258,460
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio | 4.00% | 4.00%
Leverage Ratio, To be Well Capitalized Amount | $ 341,922 | $ 323,075
Leverage Ratio, To be Well Capitalized Ratio | 5.00% | 5.00%
Tier One Common Equity Capital | $ 582,929 | $ 542,048
Tier One Common Equity Capital to Risk Weighted Assets | 10.29% | 9.71%
Tier One Common Equity Capital Required for Capital Adequacy | $ 255,024 | $ 251,213
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets | 4.50% | 4.50%
Tier One Common Equity Capital Required to be Well Capitalized | $ 368,368 | $ 362,864
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets | 6.50% | 6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount | $ 582,929 | $ 542,048
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio | 10.29% | 9.71%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount | $ 340,032 | $ 334,951
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio | 6.00% | 6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount | $ 453,375 | $ 446,601
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio | 8.00% | 8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount | $ 666,560 | $ 622,469
Total Capital (to Risk-Weighted Assets), Actual Ratio | 11.76% | 11.15%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount | $ 453,375 | $ 446,602
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio | 8.00% | 8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount | $ 566,719 | $ 558,252
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio | 10.00% | 10.00%

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- Definition The amount of total capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998 + Details
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- Definition The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 + Details
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- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
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- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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