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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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