Other Items (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Other Items
Total assets	$ 7,890,359us-gaap_Assets	$ 7,973,313us-gaap_Assets	$ 7,775,033us-gaap_Assets
Net Income	16,170	14,611
LNB
Other Items
Percent of shares to be converted into Company common stock	50.00%
Percent of shares to be exchanged for cash	50.00%
Shares of Company's common stock, per share	1.461
Cash, in dollars per share	$ 18.70
Total assets	1,256,000
Net Income	$ 1,800

Represents the percentage of aquiree shares converted to common stock.

No definition available.

Name:	nwbi_BusinessCombinationPercentageOfAcquireeSharesConvertedToCommonStock
Namespace Prefix:	nwbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percent of acquiree shares exchanged for cash.

+ References

No definition available.

+ Details

Name:	nwbi_BusinessCombinationPercentageOfAcquireeSharesExchangedForCash
Namespace Prefix:	nwbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the ratio of company shares to acquiree shares that may be exchanged

+ References

No definition available.

+ Details

Name:	nwbi_BusinessCombinationsRatioOfCompanySharesToAcquireeShares
Namespace Prefix:	nwbi_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ 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 210.5-02.18)

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 12

-Article 7

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 18

-Article 5

+ Details

Name:	us-gaap_Assets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionSharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=nwbi_LorainNationalBankBancorpInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: