Northwest Pipeline 10-K 24 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - shares	12 Months Ended
	Dec. 31, 2020	Feb. 24, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	1-7414
Entity Registrant Name	NORTHWEST PIPELINE LLC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1157701
Entity Address, Address Line One	295 Chipeta Way
Entity Address, City or Town	Salt Lake City
Entity Address, State or Province	UT
Entity Address, Postal Zip Code	84108
City Area Code	801
Local Phone Number	583-8800
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Central Index Key	0000110019
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		0

Statement of Comprehensive Inco

Statement of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Revenue
Natural gas transportation	$ 435,238	$ 428,656	$ 430,730
Natural gas storage	14,268	15,975	12,852
Other	(24)	(209)	36
Total operating revenues	449,482	444,422	443,618
OPERATING EXPENSES:
General and administrative	47,695	51,665	50,653
Operation and maintenance	69,970	81,736	75,237
Depreciation and amortization	112,004	107,951	106,073
Regulatory debits	1,148	1,107	2,334
Taxes, other than income taxes	16,290	14,388	17,507
Regulatory charges resulting from tax rate changes	23,580	23,580	35,680
Other expenses, net	(38)	47	444
Costs and Expenses	270,649	280,474	287,928
OPERATING INCOME	178,833	163,948	155,690
OTHER (INCOME) AND OTHER EXPENSES:
Interest expense	29,712	29,232	27,987
Allowance for equity and borrowed funds used during construction	(669)	(3,418)	(2,183)
Miscellaneous other (income) expenses, net	(1,296)	(4,533)	(1,575)
Total other (income) and other expenses	27,747	21,281	24,229
NET INCOME	$ 151,086	$ 142,667	$ 131,461

Balance Sheet

Balance Sheet - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Taxes, other than income taxes	$ 10,905	$ 9,913
CURRENT ASSETS:
Cash	0	0
Receivables:
Trade	37,365	40,066
Affiliated companies	51	109
Advances to affiliate	247,869	201,265
Other	840	1,962
Materials and supplies	9,807	9,683
Exchange gas due from others	4,466	5,534
Prepayments and other	4,078	3,346
Total current assets	304,476	261,965
PROPERTY, PLANT AND EQUIPMENT, at cost	3,653,792	3,593,186
Less-Accumulated depreciation	1,761,229	1,673,315
Total property, plant and equipment, net	1,892,563	1,919,871
OTHER ASSETS:
Deferred charges	3,775	1,075
Regulatory assets	20,022	21,935
Right-of-use assets	10,692	18,101
Total other assets	34,489	41,111
Total assets	2,231,528	2,222,947
Payables:
Trade	16,157	15,635
Affiliated companies	4,110	9,172
Accrued liabilities:
Taxes, other than income taxes	10,905	9,913
Interest	5,505	5,505
Exchange gas due to others	4,557	4,468
Customer advances	539	4,109
Other	6,786	6,990
Total current liabilities	48,559	55,792
LONG-TERM DEBT	578,018	577,045
OTHER NONCURRENT LIABILITIES:
Asset retirement obligations	95,777	91,251
Regulatory liabilities	355,725	323,032
Lease liability	8,341	16,823
Other	4,713	5,695
Total other noncurrent liabilities	464,556	436,801
MEMBER'S EQUITY:
Member's capital	1,073,892	1,073,892
Retained earnings	66,503	79,417
Total member's equity	1,140,395	1,153,309
Total liabilities and member's equity	$ 2,231,528	$ 2,222,947

Statement of Member's Equity St

Statement of Member's Equity Statement of Member's Equity - USD ($) $ in Thousands	Total	MEMBER'S CAPITAL	RETAINED EARINGS
Balance at beginning of period at Dec. 31, 2017		$ 1,073,892	$ 89,289
Net income	$ 131,461		131,461
Cash distributions to parent	(174,000)		(174,000)
Balance at end of period at Dec. 31, 2018	1,120,642	1,073,892	46,750
Net income	142,667		142,667
Cash distributions to parent	(110,000)		(110,000)
Balance at end of period at Dec. 31, 2019	1,153,309	$ 1,073,892	79,417
Net income	151,086		151,086
Cash distributions to parent	(164,000)		(164,000)
Balance at end of period at Dec. 31, 2020	$ 1,140,395		$ 66,503

Statement of Cash Flows

Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES:
Net income	$ 151,086	$ 142,667	$ 131,461
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	112,004	107,951	106,073
Regulatory debits	1,148	1,107	2,334
Regulatory charges resulting from tax rate changes	23,580	23,580	35,680
Amortization of deferred charges and credits	(3,526)	(2,806)	954
Allowance for equity funds used during construction (equity AFUDC)	(414)	(2,744)	(1,703)
Changes in current assets and liabilities:
Trade and other accounts receivable	3,606	(1,528)	(1,611)
Affiliated receivables	58	(91)	1,903
Materials and supplies	(124)	363	39
Other current assets	337	4,311	4,494
Trade accounts payable	5,140	(1,497)	1,753
Affiliated payables	(5,062)	(17,360)	15,234
Other accrued liabilities	(8,603)	(8,056)	881
Changes in noncurrent assets and liabilities:
Regulatory liabilities	4,185	3,493	2,530
Other, net	503	8,570	(4,894)
Net cash provided by operating activities	283,918	257,960	295,128
FINANCING ACTIVITIES:
Proceeds from long-term debt	0	0	246,395
Retirement of long-term debt	0	0	(250,000)
Payments for debt issuance costs	0	(59)	(2,498)
Cash distributions to parent	(164,000)	(110,000)	(174,000)
Net cash used in financing activities	(164,000)	(110,059)	(180,103)
Property, plant and equipment:
Capital expenditures	(78,497)	(128,440)	(73,487)
Contributions and advances for construction costs	6,103	4,805	2,075
Disposal of property, plant and equipment, net	(920)	(3,401)	(879)
Advances to affiliates, net	(46,604)	(20,865)	(42,734)
Net cash used in investing activities	(119,918)	(147,901)	(115,025)
NET (DECREASE) INCREASE IN CASH	0	0	0
CASH AT BEGINNING OF PERIOD	0	0	0
CASH AT END OF PERIOD	0	0	0
Supplemental Cash Flow Elements [Abstract]
Increases to property, plant and equipment, exclusive of equity AFUDC	(78,602)	(132,260)	(73,105)
Changes in related accounts receivable, accounts payable, and accrued liabilities	105	3,820	(382)
Capital expenditures	$ (78,497)	$ (128,440)	$ (73,487)

Summary of Significant Accounti

Summary of Significant Accounting Policies (Notes)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Corporate Structure and Control In this report, Northwest is at times referred to in the first person as “we,” “us” or “our.” Northwest Pipeline LLC (Northwest) was indirectly owned by Williams Partners L.P. (WPZ), a publicly traded Delaware limited partnership, which was consolidated by The Williams Companies, Inc. (Williams). On August 10, 2018, Williams completed a merger with WPZ, pursuant to which Williams acquired all of the publicly held outstanding common units of WPZ in exchange for shares of Williams’ common stock (WPZ Merger). Williams continued as the surviving entity. Northwest is now indirectly owned by Williams. Northwest is a single member limited liability company, and as such, single member losses are limited to the amount of its investment. Nature of Operations We own and operate an interstate pipeline system for the mainline transmission of natural gas. This system extends from the San Juan Basin in northwestern New Mexico and southwestern Colorado through Colorado, Utah, Wyoming, Idaho, Oregon and Washington to a point on the Canadian border near Sumas, Washington. Regulatory Accounting We are regulated by the Federal Energy Regulatory Commission (FERC). Accounting Standards Codification (ASC) Regulated Operations, (Topic 980) provides that certain costs that would otherwise be charged to expense should be deferred as regulatory assets, based on the expected recovery from customers in future rates. Likewise, certain actual or anticipated credits that would otherwise reduce expense should be deferred as regulatory liabilities, based on the expected return to customers in future rates. Management's expected recovery of deferred costs and return of deferred credits generally results from specific decisions by regulators granting such ratemaking treatment. We record certain incurred costs and obligations as regulatory assets or liabilities if, based on regulatory orders or other available evidence, it is probable that the costs or obligations will be included in amounts allowable for recovery or refunded in future rates. Accounting for businesses that are regulated and apply the provisions of Topic 980 can differ from the accounting requirements for non-regulated businesses. Transactions that are recorded differently as a result of regulatory accounting requirements include the capitalization of an equity return component on regulated capital projects, capitalization of other project costs, retirements of general plant assets, employee related benefits, environmental costs, negative salvage, asset retirement obligations, and other costs and taxes included in, or expected to be included in, future rates. As a rate-regulated entity, our management has determined that it is appropriate to apply the accounting prescribed by Topic 980, and, accordingly, the accompanying financial statements include the effects of the types of transactions described above that result from regulatory accounting requirements. Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles 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. Estimates and assumptions which, in the opinion of management, are significant to the underlying amounts included in the financial statements and for which it would be reasonably possible that future events or information could change those estimates include: 1) litigation-related contingencies; 2) environmental remediation obligations; 3) impairment assessments of long-lived assets; 4) depreciation; 5) asset retirement obligations; and 6) regulatory deferred tax. Revenue Recognition Our customers are comprised of public utilities, municipalities, direct industrial users, electric power generators, and natural gas marketers and producers. Service revenue contracts from our gas pipeline business contain a series of distinct services, with the majority of our contracts having a single performance obligation that is satisfied over time as the customer simultaneously receives and consumes the benefits provided by our performance. Certain customers reimburse us for costs we incur associated with construction of property, plant, and equipment utilized in our operations. As a rate-regulated entity applying Topic 980, we follow FERC guidelines with respect to reimbursement of construction costs. FERC tariffs only allow for cost reimbursement and are non-negotiable in nature; thus, in our judgment, the construction activities do not represent an ongoing major and central operation of our gas pipelines business and are not within the scope of Accounting Standards Update (ASU) 2014-09, Revenues from Contracts with Customers (ASC 606). Accordingly, cost reimbursements are treated as a reduction to the cost of the constructed assets. Operating Revenues We are subject to regulation by certain state and federal authorities, including the FERC, with revenue derived from both firm and interruptible transportation and storage contracts. Most of our firm transportation and storage agreements provide for a fixed reservation charge based on the pipeline or storage capacity reserved, and a commodity charge based on the volume of natural gas scheduled, each at rates specified in our FERC tariffs or as negotiated with our customers, with contract terms that are generally long-term in nature. Most of our long-term contracts contain an evergreen provision, which allows the contracts to be extended beyond the specified contract term and until terminated generally by either us or the customer, but in certain cases unilaterally by the customer, with advance notice of termination ranging from one to five years. Interruptible transportation and storage agreements provide for a volumetric charge based on actual commodity transportation or storage utilized in the period in which those services are provided, and the contracts are generally limited to one month periods or less. Our performance obligations include the following: • Firm transportation or storage under firm transportation and storage contracts - an integrated package of services typically constituting a single performance obligation, which includes standing ready to provide such services and receiving, transporting or storing (as applicable), and redelivering commodities; • Interruptible transportation and storage under interruptible transportation and storage contracts - an integrated package of services typically constituting a single performance obligation once scheduled, which includes receiving, transporting or storing (as applicable), and redelivering commodities. In situations where, in our judgment, we consider the integrated package of services as a single performance obligation, which represents a majority of our contracts with customers, we do not consider there to be multiple performance obligations because the nature of the overall promise in the contract is to stand ready (with regard to firm transportation and storage contracts), receive, transport or store, and redeliver natural gas to the customer; therefore, revenue is recognized over time upon satisfaction of our daily stand ready performance obligation. We recognize revenues for reservation charges over the performance obligation period, which is the contract term, regardless of the volume of natural gas that is transported or stored. Revenues for commodity charges from both firm and interruptible transportation services and storage services are recognized based on volumes of natural gas scheduled for delivery at the agreed upon delivery point or based on volumes of natural gas scheduled for injection or withdrawn from the storage facility because they specifically relate to our efforts to provide these distinct services. Generally, reservation charges and commodity charges are recognized as revenue in the same period they are invoiced to our customers. As a result of the ratemaking process, certain amounts collected by us may be subject to refunds upon the issuance of final orders by the FERC in pending rate proceedings. We use judgment to record estimates of rate refund liabilities considering our and other third-party regulatory proceedings, advice of counsel, and other risks. At December 31, 2020, we had no such rate refund liabilities. In the course of providing transportation services to customers, we may receive different quantities of natural gas from customers than the quantities delivered on behalf of those customers or consumed in fuel to operate our system. The resulting customer imbalances are typically settled through the receipt or delivery of gas in the future based on the timelines outlined in Northwest’s Tariff, whereas the over/under recovery of fuel is cleared up through Northwest’s semi-annual fuel tracker. Customer imbalances to be repaid or recovered in-kind are recorded as Exchange gas due from others or Exchange gas due to others in our Balance Sheet. The under recovery of fuel is recorded as a Regulatory Asset and the over recovery is recorded as a Regulatory Liability. These imbalances are valued at the average of the spot market rates at the Canadian border and the Rocky Mountain market as published in the SNL Financial "Bidweek Index - Spot Rates." Contract Assets Our contract assets consist of discounts provided to customers in the beginning of the contract term that are recognized on a straight-line basis over the entire contract term resulting in revenue recognition occurring prior to actual billings. Contract assets are included within Deferred Charges in our Balance Sheet. Contract Liabilities Our contract liabilities consist of a fixed rate facility charge billed to customers that declines annually as specified per the tariff. These liabilities are classified as current or non-current according to when such amounts are expected to be recognized. Current and non-current liabilities are included within Accrued Liabilities and Other Non-current Liabilities , respectively, in our Balance Sheet. Leases We recognize a lease liability with an offsetting right-of-use asset in our Balance Sheet for operating leases based on the present value of the future lease payments. We have elected to combine lease and nonlease components for all classes of leased assets in our calculation of the lease liability and the offsetting right-of-use asset. Our lease agreements require both fixed and variable periodic payments, with initial terms typically ranging from one year to 15 years, with one having a term of up to 38 years. Payment provisions in certain of our lease agreements contain escalation factors which may be based on stated rates or a change in a published index at a future time. The amount by which a lease escalates based on the change in a published index, which is not known at lease commencement, is considered a variable payment and is not included in the present value of the future lease payments, which only includes those that are stated or can be calculated based on the lease agreement at lease commencement. In addition to the noncancellable periods, many of our lease agreements provide for one or more extensions of the lease agreement for periods ranging from one year in length to an indefinite number of times following the specified contract term. Other lease agreements provide for extension terms that allow us to utilize the identified leased asset for an indefinite period of time so long as the asset continues to be utilized in our operations. In consideration of these renewal features, we assess the term of the lease agreements, which includes using judgment in the determination of which renewal periods and termination provisions, when at our sole election, will be reasonably certain of being exercised. Periods after the initial term or extension terms that allow for either party to the lease to cancel the lease are not considered in the assessment of the lease term. Additionally, we have elected to exclude leases with an original term of one year or less, including renewal periods, from the calculation of the lease liability and the offsetting right-of-use asset. We used judgment in determining the discount rate upon which the present value of the future lease payments is determined. This rate is based on a collateralized interest rate corresponding to the term of the lease agreement using company, industry, and market information available. Environmental Matters We are subject to federal, state, and local environmental laws and regulations. Environmental expenditures are expensed or capitalized depending on their economic benefit and potential for rate recovery. We believe that expenditures required to meet applicable environmental laws and regulations are prudently incurred in the ordinary course of business and such expenditures would be permitted to be recovered through rates. Property, Plant, and Equipment Property, plant and equipment (plant), consisting principally of natural gas transmission facilities, is recorded at original cost. The FERC identifies installation, construction and replacement costs that are to be capitalized and included in our asset base for recovery in rates. Routine maintenance, repairs, and renewal costs are charged to income as incurred. Gains or losses from the ordinary sale or retirement of plant are charged or credited to accumulated depreciation; certain other gains or losses are recorded in operating income. We provide for depreciation under the composite (group) method at straight-line FERC prescribed rates that are applied to the cost of the group for transmission and storage facilities. Under this method, assets with similar lives and characteristics are grouped and depreciated as one asset. Included in our depreciation rates is a negative salvage component (net cost of removal) that we currently collect in rates. Our depreciation rates are subject to change each time we file a general rate case with the FERC. Depreciation rates used for major regulated gas plant facilities at December 31, 2020, 2019, and 2018 are as follows: Category of Property Storage Facilities 1.60 % — 2.76% Transmission Facilities 2.80 % — 6.97% The incrementally priced Evergreen Expansion Project, which was an expansion of our pipeline system, was placed in service on October 1, 2003. The levelized rate design of this project creates a consistent revenue stream over the related 25-year and 15-year customer contract terms. The related levelized depreciation is lower than book depreciation in the early years and higher than book depreciation in the later years of the contract terms. The depreciation component of the levelized incremental rates will equal the accumulated book depreciation by the end of the primary contract terms. The FERC has approved the accounting for the differences between book depreciation and the Evergreen Expansion Project’s levelized depreciation as a regulatory asset. The levelized period for the 15-year contracts ended September 30, 2018. We recorded regulatory debits totaling $1.1 million in 2020 and 2019, and $2.3 million in 2018 in the accompanying Statement of Net Income. These debits relate primarily to the levelized depreciation adjustment for the Evergreen Expansion Project discussed above. We record a liability and increase the basis in the underlying asset for the present value of each expected future asset retirement obligation (ARO) at the time the liability is initially incurred, typically when the asset is acquired or constructed. Measurement of AROs includes, as a component of future expected costs, an estimate of the price that a third party would demand, and could expect to receive, for bearing the uncertainties inherent in the obligations, sometimes referred to as market-risk premium. We measure changes in the liability due to passage of time by applying an interest method of allocation. This amount is recognized as an increase in the carrying amount of the liability and is offset by a regulatory asset. The gross regulatory asset balances associated with ARO as of December 31, 2020 and 2019 were $96.3 million and $90.9 million, respectively. The regulatory asset is expected to be fully recovered through the negative salvage component of depreciation included in our rates; as such, that amount of the negative salvage component of accumulated depreciation has been reclassified and netted against the amount of the ARO regulatory asset. Impairment of Long-lived Assets We evaluate our long-lived assets, including capitalized project development costs, for impairment when events or changes in circumstances indicate, in our management’s judgment, that the carrying value of such assets may not be recoverable. When an indicator of a potential impairment has occurred, we compare our management’s estimate of undiscounted future cash flows attributable to the assets to the carrying value of the assets to determine whether an impairment has occurred. We apply a probability-weighted approach to consider the likelihood of different cash flow assumptions. If an impairment of the carrying value has occurred, we determine the amount of the impairment recognized in the financial statements by estimating the fair value of the assets and recording a loss for the amount that the carrying value exceeds the estimated fair value. For assets identified to be disposed of in the future and considered held for sale in accordance with the ASC Property, Plant, and Equipment (Topic 360), we compare the carrying value to the estimated fair value less the cost to sell to determine if recognition of an impairment is required. Until the assets are disposed of, the estimated fair value, which includes estimated cash flows from operations until the assumed date of sale, is recalculated when related events or circumstances change. Judgments and assumptions are inherent in our estimate of undiscounted future cash flows used to determine recoverability of an asset and the estimate of an asset’s fair value used to calculate the amount of impairment to recognize. Allowance for Funds Used During Construction Allowance for funds used during construction (AFUDC) represents the estimated cost of borrowed and equity funds applicable to utility plant in process of construction and is included as a cost of property, plant and equipment because it constitutes an actual cost of construction under established regulatory practices. The FERC has prescribed a formula to be used in computing separate allowances for borrowed and equity AFUDC. The allowance for borrowed funds used during construction was $0.3 million for 2020, $0.7 million for 2019, and $0.5 million for 2018. The allowance for equity funds was $0.4 million, $2.7 million, and $1.7 million for 2020, 2019, and 2018, respectively. Income Taxes We are a natural gas company organized as a pass-through entity and our taxable income or loss is consolidated on the federal income tax return of our parent, Williams. We generally are treated as a pass-through entity for state and local income tax purposes, and those taxes are generally borne on a consolidated basis by Williams. Net income for financial statement purposes may differ significantly from taxable income of Williams as a result of differences between the tax basis and financial reporting basis of assets and liabilities. Accounts Receivable and Allowance for Doubtful Receivables Accounts receivable are stated at the historical carrying amount net of reserves or write-offs. We estimate the reserves, considering current expected credit losses (as discussed below in Accounting Standards Issued and Adopted). We do not offer extended payment terms and typically receive payment within one month. We consider receivables past due if full payment is not received by the contractual due date. Our credit risk exposure in the event of nonperformance by the other parties is limited to the face value of the receivables. We perform ongoing credit evaluations of our customers’ financial condition and require collateral from our customers, if necessary. Due to our customer base, we have not historically experienced recurring credit losses in connection with our receivables. Materials and Supplies Inventory All inventories are stated at average cost. We perform an annual review of materials and supplies inventories, including a quarterly analysis of parts that may no longer be useful due to planned replacements of compressor engines and other components on our system. Based on this assessment, we record a reserve for the value of the inventory which can no longer be used for maintenance and repairs on our pipeline. There was a minimal reserve at December 31, 2020 and 2019. Deferred Charges We amortize deferred charges over varying periods consistent with the FERC approved accounting treatment and recovery for such deferred items. Unamortized debt expense, debt discount and losses on reacquired long-term debt are amortized by the bonds outstanding method over the related debt repayment periods. Contingent Liabilities We record liabilities for estimated loss contingencies, including environmental matters, when we assess that a loss is probable and the amount of the loss can be reasonably estimated. These liabilities are calculated based upon our assumptions and estimates with respect to the likelihood or amount of loss and upon advice of legal counsel, engineers, or other third parties regarding the probable outcomes of the matters. These calculations are made without consideration of any potential recovery from third-parties. We recognize insurance recoveries or reimbursements from others when realizable. Revisions to these liabilities are generally reflected in income when new or different facts or information become known or circumstances change that affect the previous assumptions or estimates. Pension and Other Postretirement Benefits We do not have employees. Certain of the costs charged to us by Williams associated with employees who directly support us include costs related to Williams’ pension and other postretirement benefit plans (See Note 7. Benefit Plans ). Although the underlying benefit plans of Williams are single-employer plans, we follow multiemployer plan accounting whereby the amount charged to us, and thus paid by us, is based on our share of net periodic benefit cost. Cash Flows from Operating Activities and Cash Equivalents We use the indirect method to report cash flows from operating activities, which requires adjustments to net income to reconcile to net cash flows provided by operating activities. We include short-term, highly-liquid investments that have an original maturity of three months or less as cash equivalents. Interest Payments Cash payments for interest, net of interest capitalized, were $26.1 million in 2020, $26.2 million in 2019, and $25.4 million in 2018. Accounting Standards Issued and Adopted In June 2016, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) 2016-13 “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments” (ASU 2016-13). ASU 2016-13 changed the impairment model for most financial assets and certain other instruments. For trade and other receivables, held-to-maturity debt securities, loans, and other instruments, entities are required to use a forward-looking “expected loss” model that generally will result in the earlier recognition of allowances for losses. We adopted ASU 2016-13 effective January 1, 2020, which primarily applied to our short-term trade receivables. There was no cumulative effect adjustment to retained earnings upon adoption. The majority of our trade receivable balances are due within 30 days. We monitor the credit quality of our counterparties through review of collection trends, credit ratings, and other analysis, such as bankruptcy monitoring. Financial assets are evaluated as one pool. Changes in counterparty risk factors could lead to reassessment of the composition of our financial assets as one pool. We calculate our allowance for credit losses incorporating an aging method. In estimating our expected credit losses, we utilized historical loss rates over many years. Our expected credit loss estimate considered both internal and external forward-looking commodity price expectations, as well as counterparty credit ratings, and factors impacting their near-term liquidity. Past due accounts are generally written off against the allowance for doubtful accounts only after all collection attempts have been exhausted. We do not have a material amount of significantly aged receivables at December 31, 2020.

Revenue Recognition Revenue Rec

Revenue Recognition Revenue Recognition (Notes)	12 Months Ended
	Dec. 31, 2020
Revenue Recognition [Abstract]
Revenue from Contract with Customer [Text Block]	REVENUE RECOGNITION Revenue by Category Our revenue disaggregation by major service line includes Natural gas transportation , Natural gas storage , and Other, which are separately presented on the Statement of Net Income . Contract Assets The following table presents a reconciliation of our contract assets: Year ended December 31, 2020 2019 (Thousands) Balance at beginning of period $ 554 $ — Revenue recognized in excess of amounts invoiced 2,841 554 Balance at end of period $ 3,395 $ 554 Contract Liabilities The following table presents a reconciliation of our contract liabilities: Year ended December 31, 2020 2019 (Thousands) Balance at beginning of period $ 5,464 $ — Recorded contract liabilities — 5,464 Recognized in Revenue (854) — Balance at end of period $ 4,610 $ 5,464 Remaining Performance Obligations Our remaining performance obligations primarily include reservation charges on contracted capacity on our firm transportation and storage contracts with customers. Amounts from certain contracts included in the table below, which are subject to the periodic review and approval by the FERC, reflect the rates for such services in our current FERC tariffs for the life of the related contracts; however, these rates may change based on future rate cases or settlements approved by the FERC and the amount and timing of these changes is not currently known. This table excludes the variable consideration component for commodity charges. Certain of our contracts contain evergreen provisions for periods beyond the initial term of the contract. The remaining performance obligations as of December 31, 2020, do not consider potential future performance obligations for which the renewal has not been exercised. The table below also does not include contracts with customers for which the underlying facilities have not received FERC authorization to be placed into service. The following table presents the amount of the contract liabilities balance expected to be recognized as revenue when performance obligations are satisfied and the transaction price allocated to the remaining performance obligations under certain contracts as of December 31, 2020. Contract Liabilities Remaining Performance Obligations (Thousands) 2021 $ 938 $ 422,298 2022 1,029 408,593 2023 1,120 365,882 2024 1,218 335,757 2025 305 332,389 Thereafter — 2,659,570 Total $ 4,610 $ 4,524,489 Accounts Receivable Receivables from contracts with customers are included within Receivables - Trade and Receivables - Affiliated companies and receivables that are not related to contracts with customers are included within the balance of Receivables - Advances to affiliate and Receivables - Other in our Balance Sheet.

Leases (Notes)

Leases (Notes)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases of Lessee Disclosure [Text Block]	LEASES We are a lessee through noncancellable lease agreements for property and equipment consisting primarily of buildings, land, vehicles, and equipment used in both our operations and administrative functions. Lessee Activities Twelve Months Ended December 31, 2020 2019 (Thousands) Lease Cost: Operating lease cost $ 2,075 $ 2,234 Variable lease cost 1,122 1,107 Total lease cost $ 3,197 $ 3,341 Cash paid for amounts included in the measurement of operating lease liabilities $ 2,500 $ 2,064 December 31, 2020 2019 (Thousands) Other Information: Right-of-use assets $ 10,692 $ 18,101 Operating lease liabilities: Current (included in Accrued liabilities - Other in our Balance Sheet) $ 2,022 $ 1,356 Lease liability $ 8,341 $ 16,823 Weighted-average remaining lease term - operating leases (years) 15 13 Weighted-average discount rate - operating leases 3.96 % 4.80 % Prior to adopting ASU 2016-02 “Leases (Topic 842)”, which was effective January 1, 2019, total rent expense was $2.8 million in 2018 and primarily included in Operation and maintenance and Administrative and general expenses in the Statement of Net Income. As of December 31, 2020, the following table represents our operating lease maturities, including renewal provisions that we have assessed as being reasonably certain of exercise, for each of the years ended December 31: (Thousands) 2021 $ 2,315 2022 993 2023 911 2024 911 2025 927 Thereafter 8,474 Total future lease payments 14,531 Less amount representing interest 4,168 Total obligations under operating leases $ 10,363

Rate and Regulatory Matters (No

Rate and Regulatory Matters (Notes)	12 Months Ended
	Dec. 31, 2020
Public Utilities, Rate Matters [Abstract]
Public Utilities, Disclosure of Rate Matters [Text Block]	RATE AND REGULATORY MATTERS FERC Developments Notice of Inquiry (Docket No. PL19-4-000). On May 21, 2020, the FERC issued a “Policy Statement on Determining Return on Equity for Natural Gas and Oil Pipelines” (Policy Statement) in Docket No. PL19-4-000 adopting changes to its policies concerning the calculation of a public utility return on equity (ROE). In this Policy Statement, the FERC revised its ROE policy for natural gas pipelines and oil pipelines to provide that the FERC will determine a pipeline’s ROE by averaging the results of the Discounted Cash Flow model and the Capital Asset Pricing Model, with each weighted equally. The FERC declined to adopt any additional policy changes at this time and will address all other issues concerning the determination of a pipeline’s ROE as they arise in future proceedings.

Contingent Liabilities and Comm

Contingent Liabilities and Commitments (Notes)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Contingent Liabilities and Commitments	CONTINGENT LIABILITIES AND COMMITMENTS Environmental Matters We are subject to the National Environmental Policy Act and other federal and state legislation regulating the environmental aspects of our business. Except as discussed below, our management believes that we are in substantial compliance with existing environmental requirements. Environmental expenditures are expensed or capitalized depending on their future economic benefit and potential for rate recovery. We believe that, with respect to any expenditures required to meet applicable standards and regulations, the FERC would grant the requisite rate relief so that substantially all of such expenditures would be permitted to be recovered through rates. Beginning in the mid-1980s, we evaluated many of our facilities for the presence of toxic and hazardous substances to determine to what extent, if any, remediation might be necessary. We identified polychlorinated biphenyl (PCB) contamination in air compressor systems, soils, and related properties at certain compressor station sites. Similarly, we identified hydrocarbon impacts at these facilities due to the former use of earthen pits, lubricating oil leaks or spills, and excess pipe coating released to the environment. In addition, heavy metals have been identified at these sites due to the former use of mercury containing meters and paint and welding rods containing lead, cadmium, and arsenic. The PCBs were remediated pursuant to a Consent Decree with the U.S. Environmental Protection Agency (EPA) in the late 1980s, and we conducted a voluntary clean-up of the hydrocarbon and mercury impacts in the early 1990s. In 2005, the Washington Department of Ecology required us to re-evaluate our previous clean-ups in Washington. During 2006 to 2015, 129 meter stations were evaluated, of which 82 required remediation. As of December 31, 2020, all of the meter stations have been remediated. During 2006 to 2018, 14 compressor stations were evaluated, of which 11 required remediation. As of December 31, 2020, 10 compressor stations have been remediated. At December 31, 2020 we had accrued liabilities totaling approximately $1.1 million, $0.1 million in current liabilities and $1.0 million in other non-current liabilities and at December 31, 2019 approximately $1.2 million, $0.1 million in current liabilities and $1.1 million in other non-current liabilities for these costs. We are conducting environmental assessments and implementing a variety of remedial measures that may result in increases or decreases in the total estimated costs. The EPA and various state regulatory agencies routinely promulgate and propose new rules, and issue updated guidance to existing rules. These rulemakings include, but are not limited to, rules for reciprocating internal combustion engine and combustion turbine maximum achievable control technology, air quality standards for one-hour nitrogen dioxide emissions, and volatile organic compound and methane new source performance standards impacting design and operation of storage vessels, pressure valves, and compressors. The EPA previously issued its rule regarding National Ambient Air Quality Standards for ground-level ozone. We are monitoring the rule’s implementation as it will trigger additional federal and state regulatory actions that may impact our operations. Implementation of the regulations is expected to result in impacts to our operations and increase the cost of additions to Property, plant, and equipment - net in the Balance Sheet for both new and existing facilities in affected areas. We are unable to reasonably estimate the cost of additions that may be required to meet the regulations at this time due to uncertainty created by various legal challenges to these regulations and the need for further specific regulatory guidance. Other Matters Various other proceedings are pending against us and are considered incidental to our operations. Summary We estimate that for all matters for which we are able to reasonably estimate a range of loss, including those noted above and others that are not individually significant, our aggregate reasonably possible losses beyond amounts accrued for all of our contingent liabilities are immaterial to our expected future annual results of operations, liquidity and financial position. These calculations have been made without consideration of any potential recovery from third-parties. We have disclosed all significant matters for which we are unable to reasonably estimate a range of possible loss.

Debt, Financing Arrangements, a

Debt, Financing Arrangements, and Leases (Notes)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt Disclosure	DEBT AND FINANCING ARRANGEMENTS Long-Term Debt Long-term debt, presented net of unamortized discount and unamortized debt issuance costs, consists of the following: December 31, 2020 2019 (Thousands) 7.125% unsecured debentures due 2025 $ 85,000 $ 85,000 4.0% unsecured debentures due 2027 500,000 500,000 Unamortized debt issuance costs (3,581) (4,083) Unamortized debt discount (3,401) (3,872) Total long-term debt $ 578,018 $ 577,045 As of December 31, 2020, maturities of outstanding long-term debt (at face value) for the next five years are as follows: (Thousands) 7.125% unsecured debentures due 2025 $ 85,000 Total $ 85,000 There are no maturities applicable to long-term debt outstanding for the years 2021, 2022, 2023 and 2024. No property is pledged as collateral under any of our long-term debt. Restrictive Debt Covenants At December 31, 2020, none of our debt instruments restrict the amount of distributions to our parent, provided, however, that under the credit facility described below, we are restricted from making distributions to our parent during an event of default if we have directly incurred indebtedness under the credit facility. Our debt agreements contain restrictions on our ability to incur secured debt beyond certain levels. Credit Facility In 2018, we, along with Williams and Transco (the borrowers), the lenders named therein, and an administrative agent entered into a credit agreement (Credit Agreement) with aggregate commitments available of $4.5 billion, with up to an additional $500 million increase in aggregate commitments available under certain circumstances. We and Transco are each subject to a $500 million borrowing sublimit. The facility made available under the Credit Agreement is initially available for five years from the Credit Agreement Effective Date (the Maturity Date). The borrowers may request an extension of the Maturity Date for an additional one-year period up to two times, to allow a Maturity Date as late as the seventh anniversary of the Credit Agreement Effective Date, subject to certain conditions. The Credit Agreement allows for same day swingline borrowings up to an aggregate amount of $200 million, subject to other utilization of the aggregate commitments under the Credit Agreement. Letter of credit commitments of $1.0 billion are, subject to the $500 million borrowing sublimit applicable to us and Transco, available to the borrowers. At December 31, 2020, no letters of credit have been issued and no loans to Williams were outstanding under the credit facility. Measured as of December 31, 2020, we are in compliance with our financial covenant under the credit facility. Various covenants may limit, among other things, a borrower’s and its material subsidiaries’ ability to grant certain liens supporting indebtedness, merge or consolidate, sell all or substantially all of its assets, enter into certain affiliate transactions, make certain distributions during an event of default, enter into certain restrictive agreements, and allow any material change in the nature of its business. If an event of default with respect to a borrower occurs under the credit facility, the lenders will be able to terminate the commitments for the respective borrowers and accelerate the maturity of any loans of the defaulting borrower under the credit facility agreement and exercise other rights and remedies. Other than swing line loans, each time funds are borrowed, the applicable borrower may choose from two methods of calculating interest: a fluctuating base rate equal to Citibank N.A.'s alternate base rate plus an applicable margin or a periodic fixed rate equal to the London Interbank Offered Rate plus an applicable margin. We are required to pay a commitment fee based on the unused portion of the credit facility. The applicable margin and the commitment fee are determined by reference to a pricing schedule based on the applicable borrower's senior unsecured long-term debt ratings. Williams participates in a commercial paper program, and Williams management considers amounts outstanding under this program to be a reduction of available capacity under the credit facility. The program allows a maximum outstanding amount at anytime of $4.0 billion of unsecured commercial paper notes. At December 31, 2020, Williams had no outstanding commercial paper.

Employee Benefit Plans (Notes)

Employee Benefit Plans (Notes)	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefit Plans [Text Block]	BENEFIT PLANS Certain of the benefit costs charged to us by Williams associated with employees who directly support us are described below. Additionally, allocated corporate expenses from Williams to us also include amounts related to these same employee benefits, which are not included in the amounts presented below (See Note 9. Transactions With Major Customers and Affiliates ). Pension and Other Postretirement Benefit Plans Williams has noncontributory defined benefit pension plans (Williams Pension Plan, Williams Inactive Employees Pension Plan, and The Williams Companies Retirement Restoration Plan) that provide pension benefits for its eligible employees. Pension costs charged to us by Williams were $1.9 million in 2020, $2.2 million in 2019 and $3.9 million in 2018. Included in our pension costs are settlement charges of $0.7 million and $1.3 million in 2020 and 2018, respectively. Williams provides subsidized retiree health care and life insurance benefits to certain eligible participants. Generally, participants that were employed by Williams on or before December 31, 1991 are eligible for subsidized retiree health care benefits. During 2020, 2019, and 2018, we received credits from Williams related to retiree health care and life insurance benefits of $1.7 million, $1.5 million and $2.1 million, respectively. These credits were recorded as regulatory liabilities. We have been allowed by rate case settlements to collect or refund in future rates any differences between the actuarially determined costs and amounts currently being recovered in rates related to other postretirement benefits. Any differences between the annual actuarially determined cost and amounts currently being recovered in rates are recorded as regulatory assets or liabilities and collected or refunded through future rate adjustments. The amounts of other postretirement benefits costs deferred as regulatory liabilities at December 31, 2020 and 2019 are $39.4 million and $37.7 million, respectively. Defined Contribution Plan Williams maintains a defined contribution plan for substantially all of its employees. Williams charged us compensation expense of $2.3 million in 2020, $1.8 million in 2019 and $1.9 million in 2018 for Williams’ company contributions to this plan. Employee Stock-Based Compensation Plan Information The Williams Companies, Inc. 2007 Incentive Plan, as subsequently amended and restated (Plan), provides for Williams’ common stock-based awards to both employees and nonmanagement directors. The Plan permits the granting of various types of awards including, but not limited to, restricted stock units and stock options. Awards may be granted for no consideration other than prior and future services or based on certain financial performance targets achieved. Williams currently bills us directly for compensation expense related to stock-based compensation awards based on the fair value of the awards. We are also billed for our proportionate share of Williams’ and other affiliates’ stock-based compensation expense through various allocation processes.

Financial Instruments (Notes)

Financial Instruments (Notes)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Financial Instruments	FINANCIAL INSTRUMENTS Fair Value of Financial Instruments The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that value: Cash and advances to affiliate —The carrying amounts approximate fair value, because of the short-term nature of these instruments. Long-term debt – The disclosed fair value of our long-term debt, which we consider as a level 2 measurement, is determined by a market approach using broker quoted indicative period-end bond prices. The quoted prices are based on observable transactions in less active markets for our debt or similar instruments. The carrying amount and estimated fair value of our long-term debt, including current maturities, were $578.0 million and $680.6 million, respectively, at December 31, 2020, and $577.0 million and $637.8 million, respectively, at December 31, 2019.

Transactions with Major Custome

Transactions with Major Customers and Affiliates (Notes)	12 Months Ended
	Dec. 31, 2020
Transactions with Major Customers and Affiliates [Abstract]
Transactions with Major Customers and Affiliates	TRANSACTIONS WITH MAJOR CUSTOMERS AND AFFILIATES Major Customers During the periods presented, more than 10 percent of our operating revenues were generated from each of the following customers: Years Ended December 31, 2020 2019 2018 (Thousands) Puget Sound Energy, Inc. $ 132,747 $ 121,792 $ 115,238 Northwest Natural Gas Company 48,893 48,891 49,126 Cascade Natural Gas Corporation 47,493 46,969 46,164 Our major customers are located in the Pacific Northwest. As a general policy, collateral is not required for receivables, but customers’ financial condition and credit worthiness are regularly evaluated and historical collection losses have been minimal. Related Party Transactions We are a participant in Williams’ cash management program. At December 31, 2020 and 2019, the advances due to us by Williams totaled approximately $247.9 million and $201.3 million, respectively. These advances are represented by demand notes and are classified as Receivables - Advances to affiliate in the accompanying Balance Sheet. The interest rate on these intercompany demand notes is based upon the daily overnight investment rate paid on Williams’ excess cash at the end of each month, which was approximately 0.01 percent at December 31, 2020. The interest income from these advances was $0.6 million, $4.1 million, and $2.0 million for the years ended December 31, 2020, 2019, and 2018, respectively. Such interest income is included in Other (Income) and Other Expenses: Miscellaneous other (income) expenses, net on the accompanying Statement of Net Income. We have no employees. Services necessary to operate our business are provided to us by Williams and certain affiliates of Williams. We reimburse Williams and its affiliates for all direct and indirect expenses incurred or payments made (including salary, bonus, incentive compensation, and benefits) in connection with these services. Employees of Williams also provide general administrative and management services to us, and we are charged for certain administrative expenses incurred by Williams. These charges are either directly identifiable or allocated to our assets. Direct charges are for goods and services provided by Williams at our request. Allocated charges are based on a three factor formula, which considers revenues; property, plant, and equipment; and payroll. In management’s estimation, the allocation methodologies used are reasonable and result in a reasonable allocation to us of our costs of doing business incurred by Williams. We were billed $82.7 million, $96.2 million, and $92.7 million in the years ended December 31, 2020, 2019, and 2018, respectively, for these services. Such expenses are primarily included in General and administrative and Operation and maintenance expenses on the accompanying Statement of Net Income. During 2020, 2019, and 2018, we declared and paid cash distributions to our parent of $164.0 million, $110.0 million, and $174.0 million, respectively. During February 2021, we declared and paid cash distributions of $55.0 million to our parent.

Asset Retirement Obligations (N

Asset Retirement Obligations (Notes)	12 Months Ended
	Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	ASSET RETIREMENT OBLIGATIONS Our accrued asset retirement obligations relate to our gas storage and transmission facilities. At the end of the useful life of our facilities, we are legally obligated to remove or plug and abandon certain transmission facilities including underground pipelines, major river spans, compressor stations and meter station facilities. These obligations also include restoration of the property sites after removal of the facilities from above and below the ground. During 2020 and 2019, our overall asset retirement obligation changed as follows (in thousands): 2020 2019 Beginning balance $ 91,251 $ 69,350 Accretion 5,410 4,605 Changes in estimates of existing obligations (1) (884) 17,307 Property dispositions/obligations settled — (11) Ending balance $ 95,777 $ 91,251 (1) Changes in estimates of existing obligations are primarily due to the annual review process, which considers various factors including inflation rate, current estimates for removal costs, discount rates, and the estimated remaining life of assets. The decrease in 2020 is primarily due to a decrease in the inflation rate and an increase in the estimated remaining life of the ARO asset s.

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities (Notes)	12 Months Ended
	Dec. 31, 2020
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	REGULATORY ASSETS AND LIABILITIES Our regulatory assets and liabilities result from our application of the provisions of ASC Topic 980 and are reflected on our Balance Sheet. Current regulatory assets are included in Prepayments and other . Current regulatory liabilities are included in Exchange gas due to others . These balances are presented on our Balance Sheet on a gross basis and are recoverable or refundable over various periods. Below are the details of our regulatory assets and liabilities as of December 31, 2020 and 2019: 2020 2019 (Thousands) Current regulatory assets: Levelized depreciation $ 1,360 $ 978 Fuel Recovery 52 — Total current regulatory assets 1,412 978 Non-current regulatory assets: Grossed-up deferred taxes on equity funds used during construction 5,114 5,498 Levelized depreciation 14,908 16,437 Total non-current regulatory assets 20,022 21,935 Total regulatory assets $ 21,434 $ 22,913 Current regulatory liabilities: Fuel recovery $ 3,232 $ 4,121 Non-current regulatory liabilities: Postretirement benefits 39,391 37,722 Deferred federal taxes-liability 206,527 206,527 Deferred state taxes - liability 12,120 12,120 Customer tax refund 75,694 49,598 Asset retirement obligations, net 21,993 17,065 Total non-current regulatory liabilities 355,725 323,032 Total regulatory liabilities $ 358,957 $ 327,153 The significant regulatory assets and liabilities include: Levelized Depreciation Levelized depreciation allows contract revenue streams to remain constant over the primary contract terms by recognizing lower than book depreciation in the early years and higher than book depreciation in later years. The depreciation component of the levelized incremental rates will equal the accumulated book depreciation by the end of the primary contract terms. The difference between levelized depreciation and straight-line book depreciation is recorded as a FERC approved regulatory asset or liability and is eliminated over the levelization period. Fuel Recovery These amounts reflect the value of the cumulative volumetric difference between the gas retained from our customers and the gas consumed in operations. These amounts are not included in the rate base, but are expected to be recovered or refunded by changing the fuel reimbursement factor in subsequent fuel filings. Grossed-Up Deferred Taxes on Equity Funds Used During Construction The regulatory asset balance was established to offset the deferred tax for the equity component of the allowance for funds used during the construction of long-lived assets. All amounts were generated during the period that we were a taxable entity. Taxes on capitalized funds used during construction and the offsetting deferred income taxes are included in the rate base and are recovered over the depreciable lives of the long-lived asset to which they relate. Postretirement Benefits We seek to recover the actuarially determined cost of postretirement benefits through rates that are set through periodic general rate filings. Any differences between the annual actuarially determined cost and amounts currently being recovered in rates are recorded as regulatory assets or liabilities and collected or refunded through future rate adjustments. These amounts are not included in the rate base, and we are not currently recovering postretirement benefit costs in our rates (See Note 7. Benefit Plans ). Deferred Federal Taxes-Liability This r e gulatory liability balance was established as a result of a decrease to rate base deferred taxes due to a decrease to the effective federal income tax rate. The timing of the refund of the regulatory liability to rate payers will be subject to future discussions and negotiations with our customers in our next rate case. Deferred State Taxes-Liability This regulatory liability balance, following the WPZ Merger, reflects a decrease to rate base deferred taxes due to a decrease to the estimated effective state income tax rates. The timing of the refund of the regulatory liability to rate payers will be subject to future discussions and negotiations with our customers in our next rate case. Customer Tax Refund In our 2017 Settlement, which became effective January 1, 2018, we agreed with our customers that if federal income tax rates decreased due to subsequent legislation, such as Tax Reform enacted in 2017, we would record a regulatory liability. As a result of Tax Reform, the regulatory liability will be $23.6 million annually plus accrued interest ($5.0 million at December 31, 2020) for the period beginning January 1, 2018 and continuing through the time that our current rates remain effective. Asset Retirement Obligations, net This regulatory liability balance reflects the amount that we have recovered in our rates related to our future retirement costs offset by depreciation of the ARO asset and changes in the ARO liability due to the passage of time. AROs are expected to be fully recovered through the net negative salvage component of depreciation included in our rates (See Note 10. Asset Retirement Obligations ).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Regulatory Accounting	Regulatory AccountingWe are regulated by the Federal Energy Regulatory Commission (FERC). Accounting Standards Codification (ASC) Regulated Operations, (Topic 980) provides that certain costs that would otherwise be charged to expense should be deferred as regulatory assets, based on the expected recovery from customers in future rates. Likewise, certain actual or anticipated credits that would otherwise reduce expense should be deferred as regulatory liabilities, based on the expected return to customers in future rates. Management's expected recovery of deferred costs and return of deferred credits generally results from specific decisions by regulators granting such ratemaking treatment. We record certain incurred costs and obligations as regulatory assets or liabilities if, based on regulatory orders or other available evidence, it is probable that the costs or obligations will be included in amounts allowable for recovery or refunded in future rates. Accounting for businesses that are regulated and apply the provisions of Topic 980 can differ from the accounting requirements for non-regulated businesses. Transactions that are recorded differently as a result of regulatory accounting requirements include the capitalization of an equity return component on regulated capital projects, capitalization of other project costs, retirements of general plant assets, employee related benefits, environmental costs, negative salvage, asset retirement obligations, and other costs and taxes included in, or expected to be included in, future rates. As a rate-regulated entity, our management has determined that it is appropriate to apply the accounting prescribed by Topic 980, and, accordingly, the accompanying financial statements include the effects of the types of transactions described above that result from regulatory accounting requirements.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles 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. Estimates and assumptions which, in the opinion of management, are significant to the underlying amounts included in the financial statements and for which it would be reasonably possible that future events or information could change those estimates include: 1) litigation-related contingencies; 2) environmental remediation obligations; 3) impairment assessments of long-lived assets; 4) depreciation; 5) asset retirement obligations; and 6) regulatory deferred tax.
Revenue Recognition	Revenue Recognition Our customers are comprised of public utilities, municipalities, direct industrial users, electric power generators, and natural gas marketers and producers. Service revenue contracts from our gas pipeline business contain a series of distinct services, with the majority of our contracts having a single performance obligation that is satisfied over time as the customer simultaneously receives and consumes the benefits provided by our performance. Certain customers reimburse us for costs we incur associated with construction of property, plant, and equipment utilized in our operations. As a rate-regulated entity applying Topic 980, we follow FERC guidelines with respect to reimbursement of construction costs. FERC tariffs only allow for cost reimbursement and are non-negotiable in nature; thus, in our judgment, the construction activities do not represent an ongoing major and central operation of our gas pipelines business and are not within the scope of Accounting Standards Update (ASU) 2014-09, Revenues from Contracts with Customers (ASC 606). Accordingly, cost reimbursements are treated as a reduction to the cost of the constructed assets. Operating Revenues We are subject to regulation by certain state and federal authorities, including the FERC, with revenue derived from both firm and interruptible transportation and storage contracts. Most of our firm transportation and storage agreements provide for a fixed reservation charge based on the pipeline or storage capacity reserved, and a commodity charge based on the volume of natural gas scheduled, each at rates specified in our FERC tariffs or as negotiated with our customers, with contract terms that are generally long-term in nature. Most of our long-term contracts contain an evergreen provision, which allows the contracts to be extended beyond the specified contract term and until terminated generally by either us or the customer, but in certain cases unilaterally by the customer, with advance notice of termination ranging from one to five years. Interruptible transportation and storage agreements provide for a volumetric charge based on actual commodity transportation or storage utilized in the period in which those services are provided, and the contracts are generally limited to one month periods or less. Our performance obligations include the following: • Firm transportation or storage under firm transportation and storage contracts - an integrated package of services typically constituting a single performance obligation, which includes standing ready to provide such services and receiving, transporting or storing (as applicable), and redelivering commodities; • Interruptible transportation and storage under interruptible transportation and storage contracts - an integrated package of services typically constituting a single performance obligation once scheduled, which includes receiving, transporting or storing (as applicable), and redelivering commodities. In situations where, in our judgment, we consider the integrated package of services as a single performance obligation, which represents a majority of our contracts with customers, we do not consider there to be multiple performance obligations because the nature of the overall promise in the contract is to stand ready (with regard to firm transportation and storage contracts), receive, transport or store, and redeliver natural gas to the customer; therefore, revenue is recognized over time upon satisfaction of our daily stand ready performance obligation. We recognize revenues for reservation charges over the performance obligation period, which is the contract term, regardless of the volume of natural gas that is transported or stored. Revenues for commodity charges from both firm and interruptible transportation services and storage services are recognized based on volumes of natural gas scheduled for delivery at the agreed upon delivery point or based on volumes of natural gas scheduled for injection or withdrawn from the storage facility because they specifically relate to our efforts to provide these distinct services. Generally, reservation charges and commodity charges are recognized as revenue in the same period they are invoiced to our customers. As a result of the ratemaking process, certain amounts collected by us may be subject to refunds upon the issuance of final orders by the FERC in pending rate proceedings. We use judgment to record estimates of rate refund liabilities considering our and other third-party regulatory proceedings, advice of counsel, and other risks. At December 31, 2020, we had no such rate refund liabilities. In the course of providing transportation services to customers, we may receive different quantities of natural gas from customers than the quantities delivered on behalf of those customers or consumed in fuel to operate our system. The resulting customer imbalances are typically settled through the receipt or delivery of gas in the future based on the timelines outlined in Northwest’s Tariff, whereas the over/under recovery of fuel is cleared up through Northwest’s semi-annual fuel tracker. Customer imbalances to be repaid or recovered in-kind are recorded as Exchange gas due from others or Exchange gas due to others in our Balance Sheet. The under recovery of fuel is recorded as a Regulatory Asset and the over recovery is recorded as a Regulatory Liability. These imbalances are valued at the average of the spot market rates at the Canadian border and the Rocky Mountain market as published in the SNL Financial "Bidweek Index - Spot Rates." Contract Assets Our contract assets consist of discounts provided to customers in the beginning of the contract term that are recognized on a straight-line basis over the entire contract term resulting in revenue recognition occurring prior to actual billings. Contract assets are included within Deferred Charges in our Balance Sheet. Contract Liabilities Our contract liabilities consist of a fixed rate facility charge billed to customers that declines annually as specified per the tariff. These liabilities are classified as current or non-current according to when such amounts are expected to be recognized. Current and non-current liabilities are included within Accrued Liabilities and Other Non-current Liabilities
Leases	Leases We recognize a lease liability with an offsetting right-of-use asset in our Balance Sheet for operating leases based on the present value of the future lease payments. We have elected to combine lease and nonlease components for all classes of leased assets in our calculation of the lease liability and the offsetting right-of-use asset. Our lease agreements require both fixed and variable periodic payments, with initial terms typically ranging from one year to 15 years, with one having a term of up to 38 years. Payment provisions in certain of our lease agreements contain escalation factors which may be based on stated rates or a change in a published index at a future time. The amount by which a lease escalates based on the change in a published index, which is not known at lease commencement, is considered a variable payment and is not included in the present value of the future lease payments, which only includes those that are stated or can be calculated based on the lease agreement at lease commencement. In addition to the noncancellable periods, many of our lease agreements provide for one or more extensions of the lease agreement for periods ranging from one year in length to an indefinite number of times following the specified contract term. Other lease agreements provide for extension terms that allow us to utilize the identified leased asset for an indefinite period of time so long as the asset continues to be utilized in our operations. In consideration of these renewal features, we assess the term of the lease agreements, which includes using judgment in the determination of which renewal periods and termination provisions, when at our sole election, will be reasonably certain of being exercised. Periods after the initial term or extension terms that allow for either party to the lease to cancel the lease are not considered in the assessment of the lease term. Additionally, we have elected to exclude leases with an original term of one year or less, including renewal periods, from the calculation of the lease liability and the offsetting right-of-use asset. We used judgment in determining the discount rate upon which the present value of the future lease payments is determined. This rate is based on a collateralized interest rate corresponding to the term of the lease agreement using company, industry, and market information available.
Environmental Matters	Environmental Matters We are subject to federal, state, and local environmental laws and regulations. Environmental expenditures are expensed or capitalized depending on their economic benefit and potential for rate recovery. We believe that expenditures required to meet applicable environmental laws and regulations are prudently incurred in the ordinary course of business and such expenditures would be permitted to be recovered through rates.
Property, Plant, and Equipment	Property, Plant, and Equipment Property, plant and equipment (plant), consisting principally of natural gas transmission facilities, is recorded at original cost. The FERC identifies installation, construction and replacement costs that are to be capitalized and included in our asset base for recovery in rates. Routine maintenance, repairs, and renewal costs are charged to income as incurred. Gains or losses from the ordinary sale or retirement of plant are charged or credited to accumulated depreciation; certain other gains or losses are recorded in operating income. We provide for depreciation under the composite (group) method at straight-line FERC prescribed rates that are applied to the cost of the group for transmission and storage facilities. Under this method, assets with similar lives and characteristics are grouped and depreciated as one asset. Included in our depreciation rates is a negative salvage component (net cost of removal) that we currently collect in rates. Our depreciation rates are subject to change each time we file a general rate case with the FERC. Depreciation rates used for major regulated gas plant facilities at December 31, 2020, 2019, and 2018 are as follows: Category of Property Storage Facilities 1.60 % — 2.76% Transmission Facilities 2.80 % — 6.97% The incrementally priced Evergreen Expansion Project, which was an expansion of our pipeline system, was placed in service on October 1, 2003. The levelized rate design of this project creates a consistent revenue stream over the related 25-year and 15-year customer contract terms. The related levelized depreciation is lower than book depreciation in the early years and higher than book depreciation in the later years of the contract terms. The depreciation component of the levelized incremental rates will equal the accumulated book depreciation by the end of the primary contract terms. The FERC has approved the accounting for the differences between book depreciation and the Evergreen Expansion Project’s levelized depreciation as a regulatory asset. The levelized period for the 15-year contracts ended September 30, 2018. We recorded regulatory debits totaling $1.1 million in 2020 and 2019, and $2.3 million in 2018 in the accompanying Statement of Net Income. These debits relate primarily to the levelized depreciation adjustment for the Evergreen Expansion Project discussed above. We record a liability and increase the basis in the underlying asset for the present value of each expected future asset retirement obligation (ARO) at the time the liability is initially incurred, typically when the asset is acquired or constructed. Measurement of AROs includes, as a component of future expected costs, an estimate of the price that a third party would demand, and could expect to receive, for bearing the uncertainties inherent in the obligations, sometimes referred to as market-risk premium. We measure changes in the liability due to passage of time by applying an interest method of allocation. This amount is recognized as an increase in the carrying amount of the liability and is offset by a regulatory asset. The gross regulatory asset balances associated with ARO as of December 31, 2020 and 2019 were $96.3 million and $90.9 million, respectively. The regulatory asset is expected to be fully recovered through the negative salvage component of depreciation included in our rates; as such, that amount of the negative salvage component of accumulated depreciation has been reclassified and netted against the amount of the ARO regulatory asset.
Impairment of Long-Lived Assets	Impairment of Long-lived Assets We evaluate our long-lived assets, including capitalized project development costs, for impairment when events or changes in circumstances indicate, in our management’s judgment, that the carrying value of such assets may not be recoverable. When an indicator of a potential impairment has occurred, we compare our management’s estimate of undiscounted future cash flows attributable to the assets to the carrying value of the assets to determine whether an impairment has occurred. We apply a probability-weighted approach to consider the likelihood of different cash flow assumptions. If an impairment of the carrying value has occurred, we determine the amount of the impairment recognized in the financial statements by estimating the fair value of the assets and recording a loss for the amount that the carrying value exceeds the estimated fair value. For assets identified to be disposed of in the future and considered held for sale in accordance with the ASC Property, Plant, and Equipment (Topic 360), we compare the carrying value to the estimated fair value less the cost to sell to determine if recognition of an impairment is required. Until the assets are disposed of, the estimated fair value, which includes estimated cash flows from operations until the assumed date of sale, is recalculated when related events or circumstances change.
Allowance for Funds Used During Construction	Allowance for Funds Used During ConstructionAllowance for funds used during construction (AFUDC) represents the estimated cost of borrowed and equity funds applicable to utility plant in process of construction and is included as a cost of property, plant and equipment because it constitutes an actual cost of construction under established regulatory practices. The FERC has prescribed a formula to be used in computing separate allowances for borrowed and equity AFUDC. The allowance for borrowed funds used during construction was $0.3 million for 2020, $0.7 million for 2019, and $0.5 million for 2018. The allowance for equity funds was $0.4 million, $2.7 million, and $1.7 million for 2020, 2019, and 2018, respectively.
Income Taxes	Income Taxes We are a natural gas company organized as a pass-through entity and our taxable income or loss is consolidated on the federal income tax return of our parent, Williams. We generally are treated as a pass-through entity for state and local income tax purposes, and those taxes are generally borne on a consolidated basis by Williams. Net income for financial statement purposes may differ significantly from taxable income of Williams as a result of differences between the tax basis and financial reporting basis of assets and liabilities.
Accounts Receivable and Allowance for Doubtful Receivables	Accounts Receivable and Allowance for Doubtful Receivables Accounts receivable are stated at the historical carrying amount net of reserves or write-offs. We estimate the reserves, considering current expected credit losses (as discussed below in Accounting Standards Issued and Adopted). We do not offer extended payment terms and typically receive payment within one month. We consider receivables past due if full payment is not received by the contractual due date. Our credit risk exposure in the event of nonperformance by the other parties is limited to the face value of the receivables. We perform ongoing credit evaluations of our customers’ financial condition and require collateral from our customers, if necessary. Due to our customer base, we have not historically experienced recurring credit losses in connection with our receivables.
Materials and Supplies Inventory	Materials and Supplies Inventory All inventories are stated at average cost. We perform an annual review of materials and supplies inventories, including a quarterly analysis of parts that may no longer be useful due to planned replacements of compressor engines and other components on our system. Based on this assessment, we record a reserve for the value of the inventory which can no longer be used for maintenance and repairs on our pipeline. There was a minimal reserve at December 31, 2020 and 2019.
Deferred Charges	Deferred Charges We amortize deferred charges over varying periods consistent with the FERC approved accounting treatment and recovery for such deferred items. Unamortized debt expense, debt discount and losses on reacquired long-term debt are amortized by the bonds outstanding method over the related debt repayment periods.
Contingent Liabilities	Contingent Liabilities We record liabilities for estimated loss contingencies, including environmental matters, when we assess that a loss is probable and the amount of the loss can be reasonably estimated. These liabilities are calculated based upon our assumptions and estimates with respect to the likelihood or amount of loss and upon advice of legal counsel, engineers, or other third parties regarding the probable outcomes of the matters. These calculations are made without consideration of any potential recovery from third-parties. We recognize insurance recoveries or reimbursements from others when realizable. Revisions to these liabilities are generally reflected in income when new or different facts or information become known or circumstances change that affect the previous assumptions or estimates.
Pension and Other Postretirement Benefits	Pension and Other Postretirement Benefits We do not have employees. Certain of the costs charged to us by Williams associated with employees who directly support us include costs related to Williams’ pension and other postretirement benefit plans (See Note 7. Benefit Plans ). Although the underlying benefit plans of Williams are single-employer plans, we follow multiemployer plan accounting whereby the amount charged to us, and thus paid by us, is based on our share of net periodic benefit cost.
Cash Equivalents	Cash Flows from Operating Activities and Cash Equivalents We use the indirect method to report cash flows from operating activities, which requires adjustments to net income to reconcile to net cash flows provided by operating activities. We include short-term, highly-liquid investments that have an original maturity of three months or less as cash equivalents.
Interest payments	$ 26.1	$ 26.2	$ 25.4

Financial Instruments (Policies

Financial Instruments (Policies)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair value of financial instruments	Fair Value of Financial Instruments The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that value: Cash and advances to affiliate —The carrying amounts approximate fair value, because of the short-term nature of these instruments. Long-term debt – The disclosed fair value of our long-term debt, which we consider as a level 2 measurement, is determined by a market approach using broker quoted indicative period-end bond prices. The quoted prices are based on observable transactions in less active markets for our debt or similar instruments. The carrying amount and estimated fair value of our long-term debt, including current maturities, were $578.0 million and $680.6 million, respectively, at December 31, 2020, and $577.0 million and $637.8 million, respectively, at December 31, 2019.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Depreciation rates	We provide for depreciation under the composite (group) method at straight-line FERC prescribed rates that are applied to the cost of the group for transmission and storage facilities. Under this method, assets with similar lives and characteristics are grouped and depreciated as one asset. Included in our depreciation rates is a negative salvage component (net cost of removal) that we currently collect in rates. Our depreciation rates are subject to change each time we file a general rate case with the FERC. Depreciation rates used for major regulated gas plant facilities at December 31, 2020, 2019, and 2018 are as follows: Category of Property Storage Facilities 1.60 % — 2.76% Transmission Facilities 2.80 % — 6.97%

Revenue Recognition Revenue R_2

Revenue Recognition Revenue Recognition (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue Recognition [Abstract]
Contract with Customer, Asset and Liability [Table Text Block]	Contract Assets The following table presents a reconciliation of our contract assets: Year ended December 31, 2020 2019 (Thousands) Balance at beginning of period $ 554 $ — Revenue recognized in excess of amounts invoiced 2,841 554 Balance at end of period $ 3,395 $ 554 Contract Liabilities The following table presents a reconciliation of our contract liabilities: Year ended December 31, 2020 2019 (Thousands) Balance at beginning of period $ 5,464 $ — Recorded contract liabilities — 5,464 Recognized in Revenue (854) — Balance at end of period $ 4,610 $ 5,464
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	The following table presents the amount of the contract liabilities balance expected to be recognized as revenue when performance obligations are satisfied and the transaction price allocated to the remaining performance obligations under certain contracts as of December 31, 2020. Contract Liabilities Remaining Performance Obligations (Thousands) 2021 $ 938 $ 422,298 2022 1,029 408,593 2023 1,120 365,882 2024 1,218 335,757 2025 305 332,389 Thereafter — 2,659,570 Total $ 4,610 $ 4,524,489

Leases Leases (Tables)

Leases Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lease, Cost [Table Text Block]	Twelve Months Ended December 31, 2020 2019 (Thousands) Lease Cost: Operating lease cost $ 2,075 $ 2,234 Variable lease cost 1,122 1,107 Total lease cost $ 3,197 $ 3,341 Cash paid for amounts included in the measurement of operating lease liabilities $ 2,500 $ 2,064 December 31, 2020 2019 (Thousands) Other Information: Right-of-use assets $ 10,692 $ 18,101 Operating lease liabilities: Current (included in Accrued liabilities - Other in our Balance Sheet) $ 2,022 $ 1,356 Lease liability $ 8,341 $ 16,823 Weighted-average remaining lease term - operating leases (years) 15 13 Weighted-average discount rate - operating leases 3.96 % 4.80 %
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	As of December 31, 2020, the following table represents our operating lease maturities, including renewal provisions that we have assessed as being reasonably certain of exercise, for each of the years ended December 31: (Thousands) 2021 $ 2,315 2022 993 2023 911 2024 911 2025 927 Thereafter 8,474 Total future lease payments 14,531 Less amount representing interest 4,168 Total obligations under operating leases $ 10,363

Debt, Financing Arrangements,_2

Debt, Financing Arrangements, and Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Long-term debt	Long-term debt, presented net of unamortized discount and unamortized debt issuance costs, consists of the following: December 31, 2020 2019 (Thousands) 7.125% unsecured debentures due 2025 $ 85,000 $ 85,000 4.0% unsecured debentures due 2027 500,000 500,000 Unamortized debt issuance costs (3,581) (4,083) Unamortized debt discount (3,401) (3,872) Total long-term debt $ 578,018 $ 577,045

Transactions with Major Custo_2

Transactions with Major Customers and Affiliates (Tables)	12 Months Ended
	Dec. 31, 2020
Transactions with Major Customers and Affiliates [Abstract]
Schedule of revenue by major customers	During the periods presented, more than 10 percent of our operating revenues were generated from each of the following customers: Years Ended December 31, 2020 2019 2018 (Thousands) Puget Sound Energy, Inc. $ 132,747 $ 121,792 $ 115,238 Northwest Natural Gas Company 48,893 48,891 49,126 Cascade Natural Gas Corporation 47,493 46,969 46,164

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]
Schedule of change in asset retirement obligation	During 2020 and 2019, our overall asset retirement obligation changed as follows (in thousands): 2020 2019 Beginning balance $ 91,251 $ 69,350 Accretion 5,410 4,605 Changes in estimates of existing obligations (1) (884) 17,307 Property dispositions/obligations settled — (11) Ending balance $ 95,777 $ 91,251 (1) Changes in estimates of existing obligations are primarily due to the annual review process, which considers various factors including inflation rate, current estimates for removal costs, discount rates, and the estimated remaining life of assets. The decrease in 2020 is primarily due to a decrease in the inflation rate and an increase in the estimated remaining life of the ARO asset s.

Regulatory Assets and Liabili_2

Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Regulatory Assets and Liabilities Disclosure [Abstract]
Schedule of Regulatory Assets and Liabilities [Table Text Block]	Our regulatory assets and liabilities result from our application of the provisions of ASC Topic 980 and are reflected on our Balance Sheet. Current regulatory assets are included in Prepayments and other . Current regulatory liabilities are included in Exchange gas due to others . These balances are presented on our Balance Sheet on a gross basis and are recoverable or refundable over various periods. Below are the details of our regulatory assets and liabilities as of December 31, 2020 and 2019: 2020 2019 (Thousands) Current regulatory assets: Levelized depreciation $ 1,360 $ 978 Fuel Recovery 52 — Total current regulatory assets 1,412 978 Non-current regulatory assets: Grossed-up deferred taxes on equity funds used during construction 5,114 5,498 Levelized depreciation 14,908 16,437 Total non-current regulatory assets 20,022 21,935 Total regulatory assets $ 21,434 $ 22,913 Current regulatory liabilities: Fuel recovery $ 3,232 $ 4,121 Non-current regulatory liabilities: Postretirement benefits 39,391 37,722 Deferred federal taxes-liability 206,527 206,527 Deferred state taxes - liability 12,120 12,120 Customer tax refund 75,694 49,598 Asset retirement obligations, net 21,993 17,065 Total non-current regulatory liabilities 355,725 323,032 Total regulatory liabilities $ 358,957 $ 327,153

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)employee	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Entity number of employees | employee	0
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory Assets	$ 21,434	$ 22,913
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, debt	300	700	$ 500
Allowance for funds used during construction, equity	400	2,700	1,700
Interest Payments [Abstract]
Cash payments for interest, net of interest capitalized	26,100	26,200	25,400
Levelized depreciation
Public Utility, Property, Plant and Equipment [Line Items]
regulatory debits	1,100	1,100	$ 2,300
Asset retirement obligation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory Assets	$ 96,300	$ 90,900
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%
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lessee, Operating Lease, Term of Contract	38 years
Accounting Standards Update 2016-02 [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Accounting Standards Update 2016-02 [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lessee, Operating Lease, Term of Contract	15 years

Revenue Recognition Revenue r_3

Revenue Recognition Revenue recognition contract liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue Recognition [Abstract]
Contract with Customer, Liability	$ 5,464	$ 0
Recognized in Revenue	0	5,464
Increase (Decrease) in Contract with Customer, Liability	(854)	0
Contract with Customer, Liability - End of Period	4,610	5,464
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	4,524,489
Revenue Recognition and Deferred Revenue [Abstract]
Contract with Customer, Asset, Net - Beginning of Period	554	0
Contract with Customer, Asset, Cumulative Catch-up Adjustment to Revenue, Change in Measure of Progress	2,841	554
Contract with Customer, Asset, Net - End of Period	3,395	$ 554
Revenue, Remaining Performance Obligation, Amount	4,524,489
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	938
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	1,029
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	1,120
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	1,218
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	305
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue Recognition [Abstract]
Contract with Customer, Liability - End of Period	0
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 422,298
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 422,298
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 408,593
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 408,593
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 365,882
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 365,882
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 335,757
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 335,757
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 332,389
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 332,389
Remaining Performance Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 2,659,570
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue Recognition and Deferred Revenue [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 2,659,570
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Performance Obligations Related To Contract Liabilities [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year

Leases (Details)

Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 2,075	$ 2,234
Variable lease cost	1,122	1,107
Total lease cost	3,197	3,341
Cash paid for amounts included in the measurement of operating lease liabilities	2,500	2,064
Right-of-use assets	10,692	18,101
Lease liability	$ 8,341	$ 16,823
Weighted-average remaining lease term - operating lease	15 years	13 years
Weighted-average discount rate - operating lease	3.96%	4.80%
Prior to adopting ASU 2016-02, total rent expense			$ 2,800
2021	$ 2,315
2022	993
2023	911
2024	911
2025	927
Thereafter	8,474
Total future lease payments	14,531
Less amount representing interest	4,168
Total obligations under operating leases	10,363
Other Current Liabilities
Lessee, Lease, Description [Line Items]
Current (included in Other in our Balance Sheet	$ 2,022	$ 1,356

Contingent Liabilities and Co_2

Contingent Liabilities and Commitments (Details) - Environmental assessment and remediation - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total	$ 1.1	$ 1.2
Accrued environmental assessment and remediation costs, current	0.1	0.1
Accrued environmental assessment and remediation costs, noncurrent	$ 1	$ 1.1

Debt, Financing Arrangements,_3

Debt, Financing Arrangements, and Leases Debt Disclosure (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term Debt	$ 578,018,000	$ 577,045,000
Debt issuance costs	(3,581,000)	(4,083,000)
Unamortized debt discount	(3,401,000)	(3,872,000)
Property pledged as collateral	0
Long-term Debt, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	85,000,000
7.125 percent unsecured debentures due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	85,000,000	85,000,000
4.0% unsecured debentures due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 500,000,000	$ 500,000,000

Debt, Financing Arrangements,_4

Debt, Financing Arrangements, and Leases Line of Credit Facility (Details) - USD ($)	Dec. 31, 2020	Jul. 13, 2018
Williams Companies Inc [Member]
Line of Credit Facility [Line Items]
Commercial paper, amount outstanding	$ 0
Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 500,000,000
Line of Credit [Member] | Williams Companies Inc [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		4,500,000,000
Additional Amount By Which Credit Facility Can Be Increased		500,000,000
Long-term Line of Credit	0
Swing Line Loan | Williams Companies Inc [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		200,000,000
Commercial Paper [Member] | Williams Companies Inc [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		4,000,000,000
Letter of Credit [Member] | Williams Companies Inc [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,000,000,000
Letters of Credit Outstanding, Amount	$ 0

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ (355,725)	$ (323,032)
Regulatory Liabilities	358,957	327,153
Salary and Wage, Excluding Cost of Good and Service Sold [Abstract]
Stock-based compensation expense	1,300	1,500	$ 1,800
Other Postretirement Benefit Costs
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	(39,400)	(37,700)
Williams Companies Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost	1,900	2,200	3,900
Defined contribution plan matching contributions expense	2,300	1,800	1,900
Williams Companies Inc [Member] | Retiree health care and life insurance benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	(1,700)	$ (1,500)	(2,100)
Williams Companies Inc [Member] | Pension Plan [Member] | Directly charged [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost charges to us by Williams	$ 700		$ 1,300

Financial Instruments (Details)

Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 578	$ 577
Estimate of fair value of long-term debt, including current maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 680.6	$ 637.8

Transactions with Major Custo_3

Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2021USD ($)	Dec. 31, 2020USD ($)employee	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Related Party Transaction [Line Items]
Advances to affiliate		$ 247,869	$ 201,265
Related party transaction, rate		0.01%
Interest Income, affiliate		$ 600	4,100	$ 2,000
Entity number of employees | employee		0
Related party transaction, expenses from transactions with related party		$ 82,700	96,200	92,700
Cash distributions to parent		(164,000)	(110,000)	(174,000)
Williams Companies Inc [Member]
Related Party Transaction [Line Items]
Advances to affiliate		247,900	201,300
Cash distributions to parent		164,000	110,000	174,000
Subsequent Event [Member]
Related Party Transaction [Line Items]
Cash distributions to parent	$ 55,000
Puget Sound Energy, Inc.
Revenue, Major Customer [Line Items]
Operating revenues		132,747	121,792	115,238
Northwest Natural Gas Company
Revenue, Major Customer [Line Items]
Operating revenues		48,893	48,891	49,126
Cascade Natural Gas Corporation
Revenue, Major Customer [Line Items]
Operating revenues		$ 47,493	$ 46,969	$ 46,164

Asset Retirement Obligations (D

Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 91,251	$ 69,350
Accretion	5,410	4,605
Changes in estimates of existing obligations (1)	(884)	17,307
Property dispositions/obligations settled	0	(11)
Ending balance	$ 95,777	$ 91,251

Regulatory Assets and Liabili_3

Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Current regulatory assets	$ 1,412	$ 978
Regulatory assets, noncurrent	20,022	21,935
Total regulatory assets	21,434	22,913
Regulatory Liabilities [Line Items]
Regulatory liabilities	355,725	323,032
Total regulatory liabilities	358,957	327,153
Fuel recovery
Regulatory Liabilities [Line Items]
Current regulatory liabilities	3,232	4,121
Postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory liabilities	39,391	37,722
Deferred federal taxes-liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	206,527	206,527
Deferred state taxes - liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,120	12,120
Customer tax refund [Domain]
Regulatory Liabilities [Line Items]
Regulatory liabilities	75,694	49,598
Asset retirement obligations, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21,993	17,065
Accrued Interest on Deferred Regulatory Tax Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5,000
Regulatory charges resulting from tax rate changes [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	23,600
Levelized depreciation
Regulatory Assets [Line Items]
Current regulatory assets	1,360	978
Regulatory assets, noncurrent	14,908	16,437
Fuel recovery
Regulatory Assets [Line Items]
Current regulatory assets	52	0
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets, noncurrent	$ 5,114	$ 5,498