Borrowed Funds - Narrative (Details)			3 Months Ended
	Dec. 15, 2005 USD ($) $ / shares shares	Dec. 05, 2005 USD ($) $ / shares shares	Mar. 31, 2020 USD ($) advance $ / shares shares	Dec. 31, 2019 USD ($)	Mar. 07, 2019 trust
Debt Instrument [Line Items]
Borrowed funds			$ 191,599,000	$ 246,336,000
Number of statutory business trusts owned | trust					5
Junior subordinated debentures			$ 121,813,000	121,800,000
Trust preferred investments
Debt Instrument [Line Items]
Maximum period for which interest payment on the subordinated debentures can be deferred			5 years
Interest deferral			$ 0
Trust preferred investments | Northwest Bancorp Capital Trust III
Debt Instrument [Line Items]
Cumulative trust preferred securities issued (in shares) | shares		50,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security) | $ / shares		$ 1,000
Total value of cumulative trust preferred securities issued		$ 50,000,000
Junior subordinated debentures			$ 51,547,000
Trust preferred investments | Northwest Bancorp Capital Trust III | LIBOR
Debt Instrument [Line Items]
Spread on variable rate basis (as a percent)		1.38%
Effective rate (as a percent)			2.75%
Trust preferred investments | Northwest Bancorp Statutory Trust IV
Debt Instrument [Line Items]
Cumulative trust preferred securities issued (in shares) | shares	50,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security) | $ / shares	$ 1,000
Total value of cumulative trust preferred securities issued	$ 50,000,000
Junior subordinated debentures			$ 51,547,000
Trust preferred investments | Northwest Bancorp Statutory Trust IV | LIBOR
Debt Instrument [Line Items]
Spread on variable rate basis (as a percent)	1.38%
Effective rate (as a percent)			2.12%
Trust preferred investments | Union National Capital Trust I
Debt Instrument [Line Items]
Cumulative trust preferred securities issued (in shares) | shares			8,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security) | $ / shares			$ 1,000
Total value of cumulative trust preferred securities issued			$ 8,000,000
Trust preferred investments | Union National Capital Trust I | LIBOR
Debt Instrument [Line Items]
Spread on variable rate basis (as a percent)			2.85%
Effective rate (as a percent)			4.62%
Trust preferred investments | Union National Capital Trust II
Debt Instrument [Line Items]
Cumulative trust preferred securities issued (in shares) | shares			3,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security) | $ / shares			$ 1,000
Total value of cumulative trust preferred securities issued			$ 3,000,000
Trust preferred investments | Union National Capital Trust II | LIBOR
Debt Instrument [Line Items]
Spread on variable rate basis (as a percent)			2.00%
Effective rate (as a percent)			3.68%
Interest rate swaps | LNB Trust II
Debt Instrument [Line Items]
Cumulative trust preferred securities issued (in shares) | shares			7,875
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security) | $ / shares			$ 1,000
Total value of cumulative trust preferred securities issued			$ 7,875,000
Junior subordinated debentures			$ 8,119,000
Interest rate swaps | LNB Trust II | LIBOR
Debt Instrument [Line Items]
Spread on variable rate basis (as a percent)			1.48%
Effective rate (as a percent)			2.22%
Interest rate swaps | Union National Capital Trust I
Debt Instrument [Line Items]
Junior subordinated debentures			$ 8,248,000
Interest rate swaps | Union National Capital Trust II
Debt Instrument [Line Items]
Junior subordinated debentures			3,093,000
Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity			250,000,000.0
Revolving line of credit			0	153,600,000
Securities Sold under Agreements to Repurchase
Debt Instrument [Line Items]
Securities sold under agreements to repurchase			91,599,000	$ 92,700,000
Term notes payable to FHLB of Pittsburgh
Debt Instrument [Line Items]
Borrowed funds			$ 100,000,000.0
Number of fixed rate advances | advance			4
Term notes payable to FHLB of Pittsburgh | Fixed Rate Advance Due June 16, 2020
Debt Instrument [Line Items]
Borrowed funds			$ 30,000,000.0
Fixed rate			0.91%
Term notes payable to FHLB of Pittsburgh | Fixed Rate Advance Due June 22, 2020
Debt Instrument [Line Items]
Borrowed funds			$ 25,000,000.0
Fixed rate			0.70%
Term notes payable to FHLB of Pittsburgh | Fixed Rate Advance Due June 22, 2020
Debt Instrument [Line Items]
Borrowed funds			$ 25,000,000.0
Fixed rate			0.70%
Term notes payable to FHLB of Pittsburgh | Fixed Rate Advance Due June 26, 2020
Debt Instrument [Line Items]
Borrowed funds			$ 20,000,000.0
Fixed rate			0.59%