Guaranteed Preferred Beneficial Interests in Company's Junior Subordinated Deferrable Interest Debentures (Trust-Preferred Securities) and Interest Rate Swap Agreements (Details) (USD $)	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Dec. 31, 2006	Dec. 31, 2014	Dec. 05, 2006	Dec. 15, 2006
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Number of statutory business trusts owned	2
Junior subordinated deferrable interest debentures held by trusts that issued guaranteed capital debt securities	$ 103,094,000		$ 103,094,000
Interest rate swap agreements
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Number of agreements	3
Northwest Bancorp Statutory Trust IV | Interest rate swap agreements
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Number of agreements	2
Trust preferred investments
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
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
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Cumulative trust preferred securities issued (in shares)				50,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security)				$ 1,000
Total value of cumulative trust preferred securities issued				50,000,000
Variable rate basis		three-month LIBOR
Spread on variable rate basis (as a percent)		1.38%
Junior subordinated deferrable interest debentures held by trusts that issued guaranteed capital debt securities	51,547,000
Trust preferred investments | Northwest Bancorp Capital Trust III | Swap 1 | Interest rate swap agreements
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Original term of swaps	10 years
Fixed rate (as a percent)	4.61%
Notional amount	25,000,000
Trust preferred investments | Northwest Bancorp Statutory Trust IV
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Cumulative trust preferred securities issued (in shares)					50,000
Cumulative trust preferred securities issued, liquidation value per preferred security (in dollars per security)					$ 1,000
Total value of cumulative trust preferred securities issued					50,000,000
Variable rate basis		three-month LIBOR
Spread on variable rate basis (as a percent)		1.38%
Junior subordinated deferrable interest debentures held by trusts that issued guaranteed capital debt securities	51,547,000
Trust preferred investments | Northwest Bancorp Statutory Trust IV | Swap 2 | Interest rate swap agreements
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Original term of swaps	7 years
Fixed rate (as a percent)	3.85%
Notional amount	25,000,000
Trust preferred investments | Northwest Bancorp Statutory Trust IV | Swap 3 | Interest rate swap agreements
Guaranteed Preferred Beneficial Interests in the Company's Junior Subordinated Deferrable Interest Debentures (Trust Preferred Securities) and Interest Rate Swaps
Original term of swaps	10 years
Fixed rate (as a percent)	4.09%
Notional amount	$ 25,000,000