Contingent Liabilities and Commitments (Notes)	12 Months Ended
Contingent Liabilities and Commitments (Notes)	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Contingent Liabilities and Commitments	Contingent Liabilities and Commitments Environmental Matters 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, 2023, two meter stations are still being remediated. During 2006 to 2018, 14 compressor stations were evaluated, of which 11 required remediation. As of December 31, 2023, four compressor stations are still being remediated. At December 31, 2023, we had accrued liabilities totaling approximately $1.0 million, $0.2 million of which is recorded in Accrued liabilities - Other and $0.8 million of which is recorded in Other Long-Term Liabilities - Other on the Balance Sheet. At December 31, 2022, we had accrued liabilities totaling approximately $1.0 million, $0.1 million of which is recorded in Accrued liabilities - Other and $0.9 million of which is recorded in Other Long-Term Liabilities - Other on the Balance Sheet. 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 propose and promulgate 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, review and updates to the National Ambient Air Quality Standards, and rules for new and existing source performance standards for volatile organic compounds and methane. We continuously monitor these regulatory changes and how they may impact our operations. Implementation of new or modified regulations may result in impacts to our operations and increase the cost of additions to Total property, plant, and equipment, net on the Balance Sheet for both new and existing facilities in affected areas; however, due to regulatory uncertainty on final rule content and applicability timeframes, we are unable to reasonably estimate the cost of these regulatory impacts at this time. 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. Washington State Climate Commitment Act In 2021, the state of Washington passed its Climate Commitment Act establishing a market-based cap-and-invest program to reduce carbon emissions. This program took effect on January 1, 2023, and sets a limit, or cap, on overall carbon emissions in the state and requires businesses like ours to obtain allowances equal to their annual covered carbon emissions. The state’s cap will be reduced over time to meet the state’s carbon emissions reduction targets, which means fewer carbon emissions allowances will be available to purchase each year. These allowances can be purchased through quarterly auctions hosted by the state or bought and sold on a secondary market. In 2023, we began purchasing allowances for the carbon emissions from nine of our thirteen compressor stations within the state whose annual carbon emissions exceed 25,000 metric tons of carbon dioxide equivalent. We also began purchasing allowances for our delivery of natural gas to certain of our customers and certain of our facilities in the state whose annual carbon emissions are insufficient to require their direct participation in the program. Our Settlement allows us to recover the costs of purchasing allowances under the program in our next rate case. As of December 31, 2023 a total of $22.3 million was included in Other Assets - Regulatory assets on our Balance Sheet and was comprised of the cost of the purchased allowances held, the estimated difference between the allowances held and the allowances required, and the interest income component of the regulatory asset. At December 31, 2023, $3.7 million was included in Accrued liabilities - Other on our Balance Sheet as the estimated difference. The interest income is reflected in Miscellaneous other (income) expense, net on our Statement of Net Income and was $0.6 million for the year ended December 31, 2023. Other Matters Various other proceedings are pending against us and are considered incidental to our operations. Summary We have disclosed all significant matters for which we are unable to reasonably estimate a range of possible loss. 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.

NORTHWEST PIPELINE LLC 10-K