Number of persons employed by the Entity

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- Definition

The total percentage of investment held by the parent.

+ References

No definition available.

+ Details

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- Details

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- Definition

The regulatory debits or credits resulting from the differences between book depreciation and levelized depreciation on incremental projects.

+ References

No definition available.

+ Details

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- Definition

Current regulatory liabilities generally represent obligations to make refunds to customers for various reasons including overpayment.

+ References

No definition available.

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Data Type:	xbrli:monetaryItemType
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- Definition

Noncurrent regulatory liabilities generally represent obligations to make refunds to customers for various reasons including overpayment.

+ 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 210.5-02.24)

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- Definition

The amount of cash paid for interest during the period net of cash paid for interest that is capitalized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

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- Definition

Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 272

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

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Data Type:	num:percentItemType
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- Definition

The number of units or percentage investment held by one or more members or limited partners of the LLC or LP.

+ References

+ Details

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- Definition

The amount of a reclassification adjustment made to prior period financial statement amounts.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

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- Definition

Amount capitalized of allowance for funds used during construction.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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Balance Type:	debit
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- Definition

Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

+ References

+ Details

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- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

+ Details

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Name:	us-gaap_PublicUtilityPropertyPlantAndEquipmentLineItems
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Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Balance Type:	debit
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- Details

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Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014 employee	Dec. 31, 2013	Dec. 31, 2012	Feb. 25, 2015
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Entity number of employees	0dei_EntityNumberOfEmployees
Revenue Recognition [Abstract]
Reserve for rate refund, Current	$ 0us-gaap_CustomerRefundLiabilityCurrent
Reserve for rate refund, Noncurrent	0
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory debits (credits)	1,487,000nwp_RegulatoryDebitsCredits	921,000nwp_RegulatoryDebitsCredits	(494,000)nwp_RegulatoryDebitsCredits
Regulatory assets	49,923,000us-gaap_RegulatoryAssetsNoncurrent	58,141,000us-gaap_RegulatoryAssetsNoncurrent
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, debt	200,000	500,000	700,000
Allowance for funds used during construction, equity	391,000	945,000	1,564,000
Allowance for funds used during construction, composite rate	9.00%	9.00%	9.00%
Interest Payments [Abstract]
Cash payments for interest, net of interest capitalized	44,500,000us-gaap_InterestPaidNet	44,600,000us-gaap_InterestPaidNet	44,600,000us-gaap_InterestPaidNet
Levelized depreciation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory debits (credits)	1,500,000	900,000	(500,000)
Regulatory assets	29,775,000	33,261,000
Asset retirement obligation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets	63,700,000	57,900,000
Negative salvage [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets	(60,200,000)	(51,200,000)
Storage Facilities [Member] \| Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	1.60%	1.60%	1.60%
Storage Facilities [Member] \| Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	2.76%	2.76%	2.76%
Transmission Facilities [Member] \| Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	2.80%	2.80%	2.80%
Transmission Facilities [Member] \| Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	6.97%	6.97%	6.97%
Capital expenditures, net of equity AFUDC [Member]
Prior Period Reclassification Adjustment [Line Items]
Prior period reclassification adjustment		(6,300,000)	(1,800,000)
Contributions and advances for construction costs [Member]
Prior Period Reclassification Adjustment [Line Items]
Prior period reclassification adjustment		$ 6,300,000	$ 1,800,000
Subsequent Event [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Parent, total ownership percentage				60.00%
Parent, limited partner ownership percentage				58.00%
Parent, general partner ownership percentage				2.00%