Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015 Turbines	Mar. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Number of turbines sold	2nrthf_NumberOfWindTurbineGenerators
Total liabilities related to exposure	$ 411	$ 441
Service contract liability	562
Discount rate	9.00%
Time period for discount	10 years
Current liabilities related to exposure	68	83
Operating lease, rent expense	104us-gaap_OperatingLeasesRentExpenseNet	25us-gaap_OperatingLeasesRentExpenseNet
Operating lease term	5 years
Operating lease termination period	2 years
Annual lease payment obligation, 2016	201
Annual lease payment obligation, 2017	345
Annual lease payment obligation, 2018	345
Annual lease payment obligation, 2019	$ 144

Number Of Wind Turbine Generators

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- Definition

Operating Lease Termination Period

+ References

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- Definition

Time Period Used To Determine Average Discount Rate

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- Definition

Term of the lessor's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Amount of loss contingency liability.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

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- Definition

Amount of loss contingency liability expected to be resolved within one year or the normal operating cycle, if longer.

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- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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No definition available.

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- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

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- Definition

Fair value of a liability representing servicing arrangements under which the estimated future revenue from contractually specified servicing fees, late charges, and other ancillary revenues are not expected to adequately compensate the servicer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 45

-Paragraph 1

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-SubTopic 50

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-Paragraph 3

-Subparagraph (a)(1)

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- Definition

The average discount rate applied to insurance contracts to determine future policy benefits to be paid to or on behalf of policyholders.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 5

-Subparagraph (b)

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