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Basis of Presentation (Details) |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jan. 01, 2018 USD ($) | Sep. 30, 2018 USD ($) lease | Mar. 31, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) lease | Sep. 30, 2017 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Gain on sale of a majority interest of insurance business |  |  | $ 0 |  | $ 0 | $ 1,873,000 | $ 0 |  |  |
Unrealized (losses) gains on marketable equity securities |  |  | $ (111,000) |  | $ 318,000 | $ 171,000 | $ 1,555,000 |  |  |
Number of operating lease agreements | lease |  |  | 50 |  |  | 50 |  |  |  |
ASU No. 2018-02 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional estimate for reclassification of AOCI to retain earnings for other income tax effects of Tax Act |  |  |  | $ 3,400,000 |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  |  |  |  |  | $ 0 |
ASU No. 2018-02 | AOCI |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional estimate for reclassification of AOCI to retain earnings for other income tax effects of Tax Act | [1] |  |  |  |  | $ (3,427,000) |  |  |  |
ASU No. 2018-02 | Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional estimate for reclassification of AOCI to retain earnings for other income tax effects of Tax Act | [1] |  |  |  |  | 3,427,000 |  |  |  |
ASU No. 2017-07 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  |  |  |  |  | 0 |
ASU No. 2017-01 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  |  |  |  |  | 0 |
ASU No. 2016-16 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  |  |  |  |  | 0 |
ASU No. 2016-15 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  |  |  |  |  |  |  |  | 0 |
ASU No. 2014-09 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  | $ 0 |  |  |  |  |  |  |  |
ASU No. 2016-01 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Unrealized (losses) gains on marketable equity securities |  |  | $ (100,000) |  |  | $ 200,000 |  |  |  |
ASU No. 2016-01 | AOCI |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  | (900,000) |  |  |  |  |  |  | (862,000) | [2]
ASU No. 2016-01 | Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effect of new accounting pronouncement on Consolidated Financial Statements |  | $ 900,000 |  |  |  |  |  |  | $ 862,000 | [2]
ASU No. 2016-02 | Scenario, Forecast | Minimum |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Right-to-use assets |  |  |  |  |  |  |  | $ 30,000,000 |  |
Lease liabilities |  |  |  |  |  |  |  | 30,000,000 |  |
ASU No. 2016-02 | Scenario, Forecast | Maximum |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Right-to-use assets |  |  |  |  |  |  |  | 35,000,000 |  |
Lease liabilities |  |  |  |  |  |  |  | $ 35,000,000 |  |
New Partnership |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  | 30.00% |  |  |  |  |  |  |  |
Subsidiaries | ASU No. 2017-05 |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Gain on sale of a majority interest of insurance business |  | $ 1,900,000 |  |  |  |  |  |  |  |
S&T Evergreen Insurance LLC | Subsidiaries |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of ownership in subsidiary sold |  | 70.00% |  |  |  |  |  |  |  |
Gain on sale of a majority interest of insurance business |  | $ 1,900,000 |  |  |  |  |  |  |  |
[1] | Reclassification due to the adoption of ASU No. 2018-02, $(3,660) relates to funded status of pension and $233 relates to net unrealized gains on available-for-sale securities.
[2] | Reclassification due to the adoption of ASU No. 2016-01.

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