Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to allowance for credit loss	$ 96,850	$ 62,224	$ 62,224
Retained earnings	(1,176,251)		(1,191,998)	$ (943,156)	$ (935,761)
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		27,353	27,352
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
Impact of ASU 2016-13 Adoption | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to allowance for credit loss		9,900
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ (740,726)		$ (761,083)	$ (716,078)	$ (701,819)
Retained Earnings | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 22,600