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Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Jan. 01, 2020	Sep. 30, 2019	Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Adjustment to allowance for credit loss	$ 120,998	$ 62,224	$ 60,996	$ 114,609	$ 62,224	$ 62,115	$ 61,479
Retained earnings	(1,142,115)	(1,191,998)	(935,761)	(1,135,777)		(982,447)	(964,953)
Threshold for evaluation for expected credit loss of commercial loans	500
Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to allowance for credit loss		27,352			27,353
Retained earnings		22,590	(167)
Impact of ASU 2016-13 Adoption | DNB
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to allowance for credit loss					9,300
Impact of ASU 2016-13 Adoption | S&T Legacy Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to allowance for credit loss					8,200
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ (698,351)	(761,083)	(701,819)	$ (692,240)		$ (748,280)	$ (730,577)
Retained Earnings | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 22,590	$ (167)		$ 22,600

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