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Related-Party Transactions - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
0 Months Ended 3 Months Ended 9 Months Ended
Apr. 16, 2014
Mar. 31, 2015
Mar. 31, 2014
Sep. 30, 2013
Related Party Transaction [Line Items]        
Issuance of notes to related parties $ 8,399us-gaap_RelatedPartyTransactionAmountsOfTransaction      
Cash portion of issuance of notes to related parties 4,765us-gaap_ProceedsFromRelatedPartyDebt      
Non-cash portion of issuance of notes to related parties 3,634us-gaap_DebtConversionConvertedInstrumentAmount1     5,250us-gaap_DebtConversionConvertedInstrumentAmount1
Number of voting common shares issued in conversion of notes 629,061us-gaap_DebtConversionConvertedInstrumentSharesIssued1     6,400,000us-gaap_DebtConversionConvertedInstrumentSharesIssued1
Interest expense   $ 0us-gaap_InterestExpenseDebt $ 122us-gaap_InterestExpenseDebt  
Class B Restricted Voting Common Shares [Member]        
Related Party Transaction [Line Items]        
Number of voting common shares issued in conversion of notes 1,678,786us-gaap_DebtConversionConvertedInstrumentSharesIssued1
/ us-gaap_StatementClassOfStockAxis
= nrthf_ClassbRestrictedVotingCommonShareMember