Summary of Significant Accounting Policies - Additional Information (Details)			3 Months Ended	12 Months Ended
		Jan. 01, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) entity unit lease subsidiary	Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($) segment	Mar. 31, 2019 USD ($)
Summary of Significant Accounting Policies [Line Items]
Number of wholly owned subsidiaries | subsidiary				3
Percentage of outstanding common stock of investees accounted for using equity method of accounting				100.00%
Number of entities non-banking activities | entity				5
Number of operating segments | segment							3
Prior period reclassification adjustment				$ 0
Period for satisfactory payment of TDRs				6 months
Evaluation for impairment of substandard and nonaccrual commercial loans				$ 500,000
Look-back period used for assumptions of allowance for loans and lease losses				9 years 3 months
Number of reporting units | unit				3
Gain on sale of a majority interest of insurance business				$ 1,873,000	$ 0		$ 0
Unrealized losses on marketable equity securities				$ (1,001,000)	(1,329,000)		$ (3,670,000)
Number of operating lease agreements | lease				50
Number of financing leases | lease				2
Commercial real estate
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses				3 years
Commercial construction
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses				4 years
Commercial and industrial
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses				1 year 3 months
Consumer real estate
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses				2 years 9 months
Other consumer
Summary of Significant Accounting Policies [Line Items]
Loss emergence period for used for assumptions of allowance for loans and lease losses				1 year 3 months
Minimum | Core Deposits And Customers Lists
Summary of Significant Accounting Policies [Line Items]
Weighted average estimated useful of acquired intangibles				10 years
Maximum
Summary of Significant Accounting Policies [Line Items]
Amortization period of investments in joint ventures				10 years
Maximum | Core Deposits And Customers Lists
Summary of Significant Accounting Policies [Line Items]
Weighted average estimated useful of acquired intangibles				20 years
Corporate Joint Venture | Common Wealth Trust Life Insurance Company
Summary of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting				50.00%
Subsidiaries | S&T Evergreen Insurance LLC
Summary of Significant Accounting Policies [Line Items]
Percentage of ownership in subsidiary sold		70.00%
Gain on sale of a majority interest of insurance business		$ 1,900,000
New Partnership
Summary of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting		30.00%
ASU No. 2016-01
Summary of Significant Accounting Policies [Line Items]
Unrealized losses on marketable equity securities				$ 300,000
ASU No. 2018-02
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements					0
Reclassification of tax effects from the Tax Act			$ 3,400,000
ASU No. 2017-07
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements					0
ASU No. 2017-05 | Subsidiaries
Summary of Significant Accounting Policies [Line Items]
Gain on sale of a majority interest of insurance business		$ 1,900,000
ASU No. 2016-02 | Scenario, Forecast
Summary of Significant Accounting Policies [Line Items]
Financing lease, right-of-use assets								$ 1,000,000
Finance lease, liability								1,000,000
Operating lease, right-of-use assets								33,000,000
Operating lease, liability								$ 33,000,000
ASU No. 2017-01
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements					0
ASU No. 2016-16
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements					0
ASU No. 2016-15
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements					0
ASU No. 2014-09
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements		0
AOCI Attributable to Parent | ASU No. 2016-01
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements		(900,000)			(862,000)	[1]
AOCI Attributable to Parent | ASU No. 2018-02
Summary of Significant Accounting Policies [Line Items]
Reclassification of tax effects from the Tax Act	[2]			(3,427,000)
Retained Earnings | ASU No. 2016-01
Summary of Significant Accounting Policies [Line Items]
Effect of new accounting pronouncement on Consolidated Financial Statements		$ 900,000			$ 862,000	[1]
Retained Earnings | ASU No. 2018-02
Summary of Significant Accounting Policies [Line Items]
Reclassification of tax effects from the Tax Act	[2]			$ 3,427,000
Interest rate lock commitments
Summary of Significant Accounting Policies [Line Items]
Period for interest rate lock commitment				60 days

[1] Reclassification due to the adoption of ASU No. 2016-01, related to changes in fair value for equity securities reclassified out of accumulated other comprehensive income

[2] Reclassification of tax effects due to the adoption of ASU No. 2018-02, relating to $(3,660) relates to funded status of pension and $233 relates to net unrealized gains on available-for-sale securities.