Basis of Presentation (Notes)	9 Months Ended
Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	BASIS OF PRESENTATION Northwest Pipeline LLC (Northwest) is indirectly owned by Williams Partners L.P. (WPZ), a publicly traded Delaware limited partnership, which is consolidated by The Williams Companies, Inc. (Williams). On February 2, 2015, WPZ was merged into Access Midstream Partners, L.P. (ACMP), another publicly traded limited partnership consolidated by Williams. ACMP was the surviving partnership and was subsequently renamed Williams Partners L.P. At September 30, 2015, Williams holds an approximate 60 percent interest in WPZ, comprised of an approximate 58 percent limited partner interest and all of the 2 percent general partner interest. On September 28, 2015, Williams publicly announced in a press release that it had entered into an Agreement and Plan of Merger (Merger Agreement) with Energy Transfer Equity, L.P. (Energy Transfer) and certain of its affiliates. The Merger Agreement provides that, subject to the satisfaction of customary closing conditions, Williams will be merged with and into Energy Transfer Corp LP (ETC) (ETC Merger) with ETC surviving the ETC Merger. Energy Transfer formed ETC as a limited partnership that will be treated as a corporation for U.S. federal income tax purposes. Immediately following the completion of the ETC Merger, ETC will contribute to Energy Transfer all of the assets and liabilities of Williams in exchange for the issuance by Energy Transfer to ETC of a number of Energy Transfer Class E common units equal to the number of ETC common shares issued to Williams stockholders in the ETC Merger. WPZ expects to retain its current name and remain a publicly traded limited partnership following the ETC Merger. In this report, Northwest is at times referred to in the first person as “we,” “us,” or “our.” General The accompanying interim financial statements do not include all the notes in our annual financial statements, and therefore, should be read in conjunction with the financial statements and notes thereto in our 2014 Annual Report on Form 10-K. The accompanying unaudited financial statements include all adjustments both normal recurring and others which, in the opinion of our management, are necessary to present fairly our interim financial statements. The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Accounting Standards Issued But Not Yet Adopted In September 2015, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2015-16 "Business Combinations (Topic 805): Simplifying the Accounting for Measurement-Period Adjustments" (ASU 2015-16). ASU 2015-16 requires an entity to: recognize adjustments to provisional amounts that are identified during the measurement period in the reporting period in which the adjustment amounts are determined; record, in the same period's financial statements, the effect on earnings of changes in depreciation, amortization, or other income effects, if any, as a result of the change to the provisional amounts, calculated as if the accounting had been completed at the acquisition date; and present separately on the face of the income statement or disclose in the notes the portion of the amount recorded in current-period earnings by line item that would have been recorded in previous reporting periods if the adjustment to the provisional amounts had been recognized as of the acquisition date. The new standard is effective for interim and annual periods beginning after December 15, 2015, with early adoption permitted for financial statements that have not been issued. We do not expect the new standard to have a significant impact on our financial statements. In August 2015, the FASB issued ASU 2015-15 "Interest-Imputation of Interest (Subtopic 835-30): Presentation and subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements--Amendments to SEC Paragraphs Pursuant to Staff Announcement at June 18, 2015 EITF Meeting" (ASU 2015-15). In ASU 2015-15, the FASB stated that the guidance in ASU 2015-03 did not address the presentation or subsequent measurement of debt issuance costs related to line-of-credit arrangements, and entities are permitted to defer and present debt issuance costs related to line-of-credit arrangements as assets. The new standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, and requires retrospective presentation, concurrent with ASU 2015-03. We do not expect the new standard will have a significant impact on our financial statements. In July 2015, the FASB issued ASU 2015-11 "Simplifying the Measurement of Inventory" (ASU 2015-11). ASU 2015-11 simplifies the guidance on the subsequent measurement of inventory, excluding inventory measured using last in, first out or the retail inventory method. Under the new standard, in scope inventory should be measured at the lower of cost and net realizable value. The new standard is effective for interim and annual periods beginning after December 15, 2016, with early adoption permitted. We are evaluating the impact of the new standard on our financial statements and our timing for adoption. In April 2015, the FASB issued ASU 2015-03 "Interest - Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs" (ASU 2015-03). ASU 2015-03 simplifies the presentation of debt issuance costs by requiring such costs be presented as a deduction from the corresponding debt liability. The standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, and requires retrospective presentation. Adoption of this standard would result in the presentation of $1.4 million and $1.9 million of debt issuance costs as of September 30, 2015 and December 31, 2014, respectively, as a direct reduction from debt in our balance sheet. The standard will have no impact on our statements of comprehensive income and cash flows. In February 2015, the FASB issued ASU 2015-02 "Amendments to the Consolidation Analysis" (ASU 2015-02). ASU 2015-02 alters the models used to determine consolidation conclusions for certain entities, including limited partnerships, and may require additional disclosures. The standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, with either retrospective or modified retrospective presentation allowed. We are evaluating the impact of the new standard on our financial statements. In May 2014, the FASB issued ASU 2014-09 establishing Accounting Standards Codification Topic 606, "Revenue from Contracts with Customers" (ASC 606). ASC 606 establishes a comprehensive new revenue recognition model designed to depict the transfer of goods or services to a customer in an amount that reflects the consideration the entity expects to be entitled to receive in exchange for those goods or services and requires significantly enhanced revenue disclosures. In August 2015, the FASB issued ASU 2015-14 "Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date" (ASU 2015-14). Per ASU 2015-14, the standard is effective for interim and annual reporting periods beginning after December 15, 2017. ASC 606 allows either full retrospective or modified retrospective transition and early adoption is permitted for annual periods beginning after December 15, 2016. We continue to evaluate both the impact of this new standard on our financial statements and the transition method we will utilize for adoption.

BASIS OF PRESENTATION

Northwest Pipeline LLC (Northwest) is indirectly owned by Williams Partners L.P. (WPZ), a publicly traded Delaware limited partnership, which is consolidated by The Williams Companies, Inc. (Williams). On February 2, 2015, WPZ was merged into Access Midstream Partners, L.P. (ACMP), another publicly traded limited partnership consolidated by Williams. ACMP was the surviving partnership and was subsequently renamed Williams Partners L.P. At September 30, 2015, Williams holds an approximate 60 percent interest in WPZ, comprised of an approximate 58 percent limited partner interest and all of the 2 percent general partner interest.

On September 28, 2015, Williams publicly announced in a press release that it had entered into an Agreement and Plan of Merger (Merger Agreement) with Energy Transfer Equity, L.P. (Energy Transfer) and certain of its affiliates. The Merger Agreement provides that, subject to the satisfaction of customary closing conditions, Williams will be merged with and into Energy Transfer Corp LP (ETC) (ETC Merger) with ETC surviving the ETC Merger. Energy Transfer formed ETC as a limited partnership that will be treated as a corporation for U.S. federal income tax purposes. Immediately following the completion of the ETC Merger, ETC will contribute to Energy Transfer all of the assets and liabilities of Williams in exchange for the issuance by Energy Transfer to ETC of a number of Energy Transfer Class E common units equal to the number of ETC common shares issued to Williams stockholders in the ETC Merger. WPZ expects to retain its current name and remain a publicly traded limited partnership following the ETC Merger.

In this report, Northwest is at times referred to in the first person as “we,” “us,” or “our.”

General

The accompanying interim financial statements do not include all the notes in our annual financial statements, and therefore, should be read in conjunction with the financial statements and notes thereto in our 2014 Annual Report on Form 10-K. The accompanying unaudited financial statements include all adjustments both normal recurring and others which, in the opinion of our management, are necessary to present fairly our interim financial statements.

The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.

Accounting Standards Issued But Not Yet Adopted

In September 2015, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2015-16 "Business Combinations (Topic 805): Simplifying the Accounting for Measurement-Period Adjustments" (ASU 2015-16). ASU 2015-16 requires an entity to: recognize adjustments to provisional amounts that are identified during the measurement period in the reporting period in which the adjustment amounts are determined; record, in the same period's financial statements, the effect on earnings of changes in depreciation, amortization, or other income effects, if any, as a result of the change to the provisional amounts, calculated as if the accounting had been completed at the acquisition date; and present separately on the face of the income statement or disclose in the notes the portion of the amount recorded in current-period earnings by line item that would have been recorded in previous reporting periods if the adjustment to the provisional amounts had been recognized as of the acquisition date. The new standard is effective for interim and annual periods beginning after December 15, 2015, with early adoption permitted for financial statements that have not been issued. We do not expect the new standard to have a significant impact on our financial statements.

In August 2015, the FASB issued ASU 2015-15 "Interest-Imputation of Interest (Subtopic 835-30): Presentation and subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements--Amendments to SEC Paragraphs Pursuant to Staff Announcement at June 18, 2015 EITF Meeting" (ASU 2015-15). In ASU 2015-15, the FASB stated that the guidance in ASU 2015-03 did not address the presentation or subsequent measurement of debt issuance costs related to line-of-credit arrangements, and entities are permitted to defer and present debt issuance costs related to line-of-credit arrangements as assets. The new standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, and requires retrospective presentation, concurrent with ASU 2015-03. We do not expect the new standard will have a significant impact on our financial statements.

In July 2015, the FASB issued ASU 2015-11 "Simplifying the Measurement of Inventory" (ASU 2015-11). ASU 2015-11 simplifies the guidance on the subsequent measurement of inventory, excluding inventory measured using last in, first out or the retail inventory method. Under the new standard, in scope inventory should be measured at the lower of cost and net realizable value. The new standard is effective for interim and annual periods beginning after December 15, 2016, with early adoption permitted. We are evaluating the impact of the new standard on our financial statements and our timing for adoption.

In April 2015, the FASB issued ASU 2015-03 "Interest - Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs" (ASU 2015-03). ASU 2015-03 simplifies the presentation of debt issuance costs by requiring such costs be presented as a deduction from the corresponding debt liability. The standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, and requires retrospective presentation. Adoption of this standard would result in the presentation of $1.4 million and $1.9 million of debt issuance costs as of September 30, 2015 and December 31, 2014, respectively, as a direct reduction from debt in our balance sheet. The standard will have no impact on our statements of comprehensive income and cash flows.

In February 2015, the FASB issued ASU 2015-02 "Amendments to the Consolidation Analysis" (ASU 2015-02). ASU 2015-02 alters the models used to determine consolidation conclusions for certain entities, including limited partnerships, and may require additional disclosures. The standard is effective for financial statements issued for interim and annual reporting periods beginning after December 15, 2015, with either retrospective or modified retrospective presentation allowed. We are evaluating the impact of the new standard on our financial statements.

In May 2014, the FASB issued ASU 2014-09 establishing Accounting Standards Codification Topic 606, "Revenue from Contracts with Customers" (ASC 606). ASC 606 establishes a comprehensive new revenue recognition model designed to depict the transfer of goods or services to a customer in an amount that reflects the consideration the entity expects to be entitled to receive in exchange for those goods or services and requires significantly enhanced revenue disclosures. In August 2015, the FASB issued ASU 2015-14 "Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date" (ASU 2015-14). Per ASU 2015-14, the standard is effective for interim and annual reporting periods beginning after December 15, 2017. ASC 606 allows either full retrospective or modified retrospective transition and early adoption is permitted for annual periods beginning after December 15, 2016. We continue to evaluate both the impact of this new standard on our financial statements and the transition method we will utilize for adoption.