Basis of Presentation (Details) $ / shares in Units, $ in Thousands	Jun. 05, 2019 branch $ / shares shares	Sep. 30, 2019 USD ($) lease $ / shares	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($) lease	Dec. 31, 2018 $ / shares	Jan. 01, 2018
Schedule of Equity Method Investments [Line Items]
Common stock, par value (in dollars per share) | $ / shares		$ 2.50			$ 2.50
Lease Liabilities		$ 1,229
Operating lease, liabilities		42,244
Financing lease, right-of-use assets		1,168
Operating lease right-of-use assets		$ 37,534
Number of operating lease agreements | lease		43
ASU No. 2016-02
Schedule of Equity Method Investments [Line Items]
Lease Liabilities				$ 1,200
Operating lease, liabilities				33,700
Financing lease, right-of-use assets				1,200
Operating lease right-of-use assets				$ 33,400
Number of operating lease agreements | lease				1
New Partnership
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage						30.00%
S&T Evergreen Insurance LLC | Subsidiaries
Schedule of Equity Method Investments [Line Items]
Percentage of ownership in subsidiary sold						70.00%
DNB
Schedule of Equity Method Investments [Line Items]
Number of branches | branch	14
Total assets of DNB			$ 1,200,000
Shares of S&T offered for each share of DNB (shares) | shares	1.22
Common stock, par value (in dollars per share) | $ / shares	$ 2.50
DNB | DNB
Schedule of Equity Method Investments [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 1.00