Investment Securities - Narrative (Details) $ in Millions	Jun. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Marketable Securities [Abstract]
Securities with unrealized losses	20	29
Number of securities in unrealized loss position greater than 12 months	2	0
Available-for-sale Securities Pledged as Collateral \| $	$ 57.5	$ 54.1
Available-for-sale Securities Pledged as Collateral, Percent of Portfolio	25.00%	19.00%

Available-for-sale Securities Pledged as Collateral, Percent of Portfolio

No definition available.

Name:	nrim_AvailableforsaleSecuritiesPledgedasCollateralPercentofPortfolio
Namespace Prefix:	nrim_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment securities with unrealized losses for less than 12 months.

+ References

No definition available.

+ Details

Name:	nrim_NumberOfInvestmentSecuritiesWithUnrealizedLosses
Namespace Prefix:	nrim_
Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	instant

- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5086-111524

+ Details

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

- Definition

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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