Cover Page - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	001-34582
Entity Registrant Name	NORTHWEST BANCSHARES, INC.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	27-0950358
Entity Address, Address Line One	100 Liberty Street
Entity Address, City or Town	Warren,
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	16365
City Area Code	814
Local Phone Number	726-2140
Entity Shell Company	false
Title of 12(b) Security	Common Stock, 0.01 Par Value
Trading Symbol	NWBI
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding (in shares)		127,573,335
Entity Central Index Key	0001471265
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1