XML 36 R56.htm IDEA: XBRL DOCUMENT v2.4.1.9
401(k) Plan - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Defined Contribution Plan Disclosure [Line Items]    
Maximum employees contribution percentage 15.00%nrthf_PercentageOfAggregateEmployeeParticipantsUnderPlan  
Percentage of Company contribution 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch  
Company matching contributions $ 53us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount $ 44us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Maximum [Member]    
Defined Contribution Plan Disclosure [Line Items]    
Percentage of maximum contribution by employer 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_RangeAxis
= us-gaap_MaximumMember