Debt and Senior Secured Convertible Notes - Foreign Working Capital Revolving Line of Credit - Additional Information (Detail) (USD $)	3 Months Ended
Mar. 31, 2015 Customer
Debt Instrument [Line Items]
Company to maintain unencumbered liquid assets	$ 1,500,000nrthf_UnencumberedLiquidityAssetValue
Outstanding on revolving line of credit	4,000,000us-gaap_LinesOfCreditCurrent
Outstanding performance letter of credit	2,000,000
Maximum supported borrowing base	0
Number of customers	4nrthf_NumberOfCustomers
Foreign Working Capital Revolving Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, renewal period	15 years
Revolving line of credit extended maturity date	September 30, 2016
Line of credit , maturity date	Jun. 30, 2014
Availability of revolving line of credit to support exports	$ 6,000,000

Debt Instrument Extension Option

No definition available.

Name:	nrthf_DebtInstrumentExtensionOption
Namespace Prefix:	nrthf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line Of credit facility extended maturity date.

+ References

No definition available.

+ Details

Name:	nrthf_LineOfCreditFacilityExtendedMaturityDate
Namespace Prefix:	nrthf_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of customers.

+ References

No definition available.

+ Details

Name:	nrthf_NumberOfCustomers
Namespace Prefix:	nrthf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Unencumbered Liquidity Asset Value

+ References

No definition available.

+ Details

Name:	nrthf_UnencumberedLiquidityAssetValue
Namespace Prefix:	nrthf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCapacityAvailableForSpecificPurposeOtherThanForTradePurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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 210.5-02.20)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Line-of-Credit Arrangement

-URI http://asc.fasb.org/extlink&oid=6517033

+ Details

Name:	us-gaap_LinesOfCreditCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_ForeignLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: