Contingent Liabilities and Commitments (Notes)	3 Months Ended
Mar. 31, 2017
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 Federal Energy Regulatory Commission 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 March 31, 2017, all of the meter stations have been remediated. Initial assessments have been completed at all thirteen compressor stations in Washington. Additional assessments are ongoing at two of these compressor stations. Remediation has been completed at eleven of the thirteen compressor stations. On the basis of the findings to date, we estimate that environmental assessment and remediation costs will total approximately $4.4 million, measured on an undiscounted basis, and are expected to be incurred through 2020. At March 31, 2017 and December 31, 2016, we had accrued liabilities totaling approximately $4.4 million and $4.4 million, respectively, 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. In March 2008, the EPA promulgated a new, lower National Ambient Air Quality Standard (NAAQS) for ground-level ozone. In May 2012, the EPA completed designation of new eight-hour ozone non-attainment areas. Based on the published designations, no Northwest facilities are located within the non-attainment areas. At this time, it is unknown whether future state regulatory actions associated with implementation of the 2008 ozone standard will impact our operations and increase the cost of additions to property, plant and equipment. Until any additional state regulatory actions are proposed, we are unable to estimate the cost of additions that may be required to meet any such new regulation. In December 2014, the EPA proposed to further reduce the ground-level ozone NAAQS from the March 2008 levels and subsequently finalized a rule on October 1, 2015. We are monitoring the rule's implementation as the reduction will trigger additional federal and state regulatory actions that may impact our operations. As a result, the cost of additions to property, plant, and equipment is expected to increase. We are unable at this time to estimate with any certainty the cost of additions that may be required to meet new regulations. On January 22, 2010, the EPA set a new one-hour nitrogen dioxide (NO2) NAAQS. The effective date of the new NO2 standard was April 12, 2010. On January 20, 2012, the EPA determined pursuant to available information that no area in the country is violating the 2010 NO2 NAAQS, and thus, designated all areas of the country as “unclassifiable/attainment.” Also, at that time, the EPA noted its plan to deploy an expanded NO2 monitoring network beginning in 2013. However, on October 5, 2012, the EPA proposed a graduated implementation of the monitoring network between January 1, 2014 and January 1, 2017. Once three years of data are collected from the new monitoring network, the EPA will reassess attainment status with the one-hour NO2 NAAQS. Until that time, the EPA or states may require ambient air quality modeling on a case by case basis to demonstrate compliance with the NO2 standard. Because we are unable to predict the outcome of the EPA’s or states’ future assessment using the new monitoring network, we are unable to estimate the cost of additions that may be required to meet this regulation. 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.

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 Federal Energy Regulatory Commission 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 March 31, 2017, all of the meter stations have been remediated. Initial assessments have been completed at all thirteen compressor stations in Washington. Additional assessments are ongoing at two of these compressor stations. Remediation has been completed at eleven of the thirteen compressor stations. On the basis of the findings to date, we estimate that environmental assessment and remediation costs will total approximately $4.4 million, measured on an undiscounted basis, and are expected to be incurred through 2020. At March 31, 2017 and December 31, 2016, we had accrued liabilities totaling approximately $4.4 million and $4.4 million, respectively, 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.

In March 2008, the EPA promulgated a new, lower National Ambient Air Quality Standard (NAAQS) for ground-level ozone. In May 2012, the EPA completed designation of new eight-hour ozone non-attainment areas. Based on the published designations, no Northwest facilities are located within the non-attainment areas. At this time, it is unknown whether future state regulatory actions associated with implementation of the 2008 ozone standard will impact our operations and increase the cost of additions to property, plant and equipment. Until any additional state regulatory actions are proposed, we are unable to estimate the cost of additions that may be required to meet any such new regulation.

In December 2014, the EPA proposed to further reduce the ground-level ozone NAAQS from the March 2008 levels and subsequently finalized a rule on October 1, 2015. We are monitoring the rule's implementation as the reduction will trigger additional federal and state regulatory actions that may impact our operations. As a result, the cost of additions to property, plant, and equipment is expected to increase. We are unable at this time to estimate with any certainty the cost of additions that may be required to meet new regulations.

On January 22, 2010, the EPA set a new one-hour nitrogen dioxide (NO2) NAAQS. The effective date of the new NO2 standard was April 12, 2010. On January 20, 2012, the EPA determined pursuant to available information that no area in the country is violating the 2010 NO2 NAAQS, and thus, designated all areas of the country as “unclassifiable/attainment.” Also, at that time, the EPA noted its plan to deploy an expanded NO2 monitoring network beginning in 2013. However, on October 5, 2012, the EPA proposed a graduated implementation of the monitoring network between January 1, 2014 and January 1, 2017. Once three years of data are collected from the new monitoring network, the EPA will reassess attainment status with the one-hour NO2 NAAQS. Until that time, the EPA or states may require ambient air quality modeling on a case by case basis to demonstrate compliance with the NO2 standard. Because we are unable to predict the outcome of the EPA’s or states’ future assessment using the new monitoring network, we are unable to estimate the cost of additions that may be required to meet this regulation.

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.