Guarantees (Details) (USD $)	3 Months Ended
Mar. 31, 2015
Guarantees
Maximum potential amount of future payments	$ 23,800,000
Maximum potential amount of future payments fully collateralized	23,200,000
Liability recognized for the obligations	1,100,000us-gaap_OtherDeferredCreditsCurrent
Recourse provisions for recovery of amounts from third parties	$ 0

Represents the maximum potential amount of future payments which is fully collateralized.

No definition available.

Name:	nwbi_GuaranteeObligationsMaximumExposureCollateralized
Namespace Prefix:	nwbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of recourse provisions that would enable the guarantor to recover from third parties any of the amounts paid under the guarantee (or each group of similar guarantees).

+ References

No definition available.

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_GuaranteesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of unearned revenue or income not otherwise specified in the taxonomy which is expected to be taken into income within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_OtherDeferredCreditsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant