Cash and Cash Equivalents (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015
Cash and Due from Banks [Abstract]
Maturity period of securities at acquisition	90 days
Domestic certificates of deposit	$ 3.5

Domestic Certificates Of Deposit Excluded For Cash And Cash Equivalents

No definition available.

Name:	nrim_DomesticCertificatesOfDepositExcludedForCashAndCashEquivalents
Namespace Prefix:	nrim_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant