Allowance for Credit Losses - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended			12 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Jan. 01, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Adjustment to allowance for credit loss		$ 114,609	$ 61,479	$ 114,609		$ 61,479	$ 62,224	$ 60,996	$ 96,850	$ 62,224	$ 61,409
Charge-offs		68,303	2,667	79,749		8,691
Total loans										7,074,928
Restructured loans	[1],[2]	1,304	6,427	$ 7,853		11,248
Forecast period				2 years
Nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Restructured loans				$ 31,755	[3]		8,912
Impact of ASU 2016-13 Adoption
Financing Receivable, Allowance for Credit Loss [Line Items]
Adjustment to allowance for credit loss							27,352			27,353
Total loans										(27,353)
S&T Legacy Loans | Impact of ASU 2016-13 Adoption
Financing Receivable, Allowance for Credit Loss [Line Items]
Adjustment to allowance for credit loss										8,200
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Adjustment to allowance for credit loss			13,227			13,227		$ 11,596			$ 11,996
Charge-offs			1,435			6,912
Restructured loans	[1]	102	$ 0	2,170		$ 4,529
Commercial and industrial | Nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Restructured loans				2,068			$ 695
Commercial and industrial | Impact of ASU 2016-13 Adoption
Financing Receivable, Allowance for Credit Loss [Line Items]
Adjustment to allowance for credit loss										9,900
Customer fraud, check kiting
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs		58,700
Customer fraud
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs		4,200
Total loans		15,100		15,100
Customer fraud | Nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Restructured loans				10,900
CARES Act, Paycheck Protection Program Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans		547,600		547,600
Restructured loans				1,400,000
ACL provision, COVID-19 assessment		14,400		29,300
ACL provision, COVID-19 assessment, revised forecast		14,800		26,000
Increase (decrease) in ACL provision, COVID-19 assessment, changes in current conditions		(400)		3,300
Customer fraud, commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans		14,300		14,300
Customer fraud, line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans		$ 800		$ 800
DNB | Impact of ASU 2016-13 Adoption
Financing Receivable, Allowance for Credit Loss [Line Items]
Adjustment to allowance for credit loss										$ 9,300

[1] Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.

[2] Refer to Note 1, Basis of Presentation for details of reclassification of our portfolio segments related to the adoption of ASU 2016-13 Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instrument.

[3] Refer to Note 1, Basis of Presentation for details of reclassification of our portfolio segments related to the adoption of ASU 2016-13 Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.