Disclosures About Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2026	Dec. 31, 2025
Fair Value Disclosures [Abstract]
Significant unrealized appreciation or depreciation in financial instruments	$ 0	$ 0

The increase (decrease) of financial instruments measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period. Such increase (decrease) may be comprised of changes in fair value (gains or losses) unrealized.

No definition available.

Name:	nwbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisFinancialInstrumentsTotalUnrealizedGainsLossesPeriodIncreaseDecrease
Namespace Prefix:	nwbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration