Subsequent Event (Details)	12 Months Ended
Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Event, Description	As previously disclosed, in NW Natural’s 2012 Oregon general rate case, the OPUC adopted a Site Remediation and Recovery Mechanism (SRRM), through which NW Natural would track and recover past deferred and future environmental remediation costs. The OPUC ordered a separate docket to determine the following items: whether and how an earnings test should affect the recovery of already deferred environmental expenses, how an earnings test should apply to the recovery of future environmental expenditures through the SRRM, how to apply insurance proceeds received to offset past and/or future environmental expenses, and the prudence of environmental expenses and insurance recoveries through March 31, 2014. On February 20, 2015, the OPUC issued an Order addressing these outstanding items. In the Order, the OPUC determined that NW Natural’s environmental remediation expenses and associated carrying costs through March 31, 2014 were prudently incurred, and the Company’s settlement with insurance carriers resulting in insurance proceeds received between January 1, 2013 and March 31, 2014 was prudent. Under the Order, NW Natural will be required to forego collection of $15 million out of the approximate $95 million of environmental expenses and associated carrying costs that it had deferred through 2012. The OPUC disallowed this amount from rate recovery based on its determination of how an earnings test should apply to amounts deferred from 2003 to 2012, with adjustments for factors the OPUC deemed relevant. The disallowance is currently estimated to result in a net after-tax charge of $9.1 million taken through operating income in the first quarter of 2015. The OPUC applied one-third of the Company’s approximately $150 million of environmental insurance recoveries to amounts deferred through 2012, and will allow full recovery of the remainder of the amounts deferred through 2012, other than the disallowed amount discussed above and approximately $33 thousand, which the OPUC found was not specifically substantiated by company records. The remaining insurance recoveries will be applied against post-2012 environmental costs with the funds to be held in an account accruing interest with the interest also applied to future expenses as outlined below. The Order establishes that with respect to all environmental remediation expenses deferred after 2012, an aggregate of two-thirds of the environmental insurance receipts, plus interest, will be applied ratably over 20 years and the remainder will be collected through the SRRM, and subject to an earnings test as follows: The Company will recover the first $5 million of annual expense through an amount that will be collected from customers through a tariff rider. The Company will apply $5 million of insurance (plus interest) to the next portion of environmental expenses each year. Any amounts in excess of the annual $10 million (plus interest from insurance) described above would be fully recoverable through the SRRM, to the extent the Company earns at or below its authorized Return On Equity (ROE). To the extent the Company earns more than its authorized ROE in a year, the Company would be required to cover environmental expenses greater than the $10 million (plus interest from insurance) with those earnings that exceed its authorized ROE. For purposes of this earnings test, all earnings derived from utility assets, including gains and losses associated with NW Natural’s weighted average cost of gas incentive mechanism, plus 50% of the Company’s earnings derived from the Company’s portion of its asset management agreement with our independent energy marketing company for asset management services associated with utility assets will be included. In any year that environmental expenses are less than $10 million (plus the interest on insurance), any unused tariff rider amount will offset deferred amounts otherwise collected through the SRRM and any unused insurance proceeds (plus interest on insurance) will roll forward to offset the next year’s expenses. Any remaining funds will be used to offset environmental remediation costs at the end of the project. The Company is evaluating the results of the Order, including those noted above as well as the state allocations. At this time, the Company does not anticipate refunds for 2013 or 2014 based on the earnings test outlined above. In accordance with accounting guidance and the Company's policy, the Company expects to recognize deferred interest income, net of approximately $4.4 million or $2.7 million after-tax on the associated regulatory account balances in the first quarter of 2015. Under the Order, the OPUC will revisit the deferral and amortization of future remediation expenses, as well as the treatment of remaining insurance proceeds in three years, or earlier if the Company gains greater certainty about its future remediation costs. The Company continues to evaluate the effects of the Order and is required to file a compliance report with the OPUC within the next 30 days demonstrating how it will implement the Order. The compliance filing is subject to review and approval by the OPUC and, as a consequence thereof, additional or different implementation procedures could be required, which may, among other things, result in additional impacts on earnings. The Company anticipates filing the compliance report as required by the Order in March 2015.

Subsequent Event (Details)

12 Months Ended

Dec. 31, 2014

As previously disclosed, in NW Natural’s 2012 Oregon general rate case, the OPUC adopted a Site Remediation and Recovery Mechanism (SRRM), through which NW Natural would track and recover past deferred and future environmental remediation costs. The OPUC ordered a separate docket to determine the following items: whether and how an earnings test should affect the recovery of already deferred environmental expenses, how an earnings test should apply to the recovery of future environmental expenditures through the SRRM, how to apply insurance proceeds received to offset past and/or future environmental expenses, and the prudence of environmental expenses and insurance recoveries through March 31, 2014. On February 20, 2015, the OPUC issued an Order addressing these outstanding items. In the Order, the OPUC determined that NW Natural’s environmental remediation expenses and associated carrying costs through March 31, 2014 were prudently incurred, and the Company’s settlement with insurance carriers resulting in insurance proceeds received between January 1, 2013 and March 31, 2014 was prudent. Under the Order, NW Natural will be required to forego collection of $15 million out of the approximate $95 million of environmental expenses and associated carrying costs that it had deferred through 2012. The OPUC disallowed this amount from rate recovery based on its determination of how an earnings test should apply to amounts deferred from 2003 to 2012, with adjustments for factors the OPUC deemed relevant. The disallowance is currently estimated to result in a net after-tax charge of $9.1 million taken through operating income in the first quarter of 2015. The OPUC applied one-third of the Company’s approximately $150 million of environmental insurance recoveries to amounts deferred through 2012, and will allow full recovery of the remainder of the amounts deferred through 2012, other than the disallowed amount discussed above and approximately $33 thousand, which the OPUC found was not specifically substantiated by company records. The remaining insurance recoveries will be applied against post-2012 environmental costs with the funds to be held in an account accruing interest with the interest also applied to future expenses as outlined below. The Order establishes that with respect to all environmental remediation expenses deferred after 2012, an aggregate of two-thirds of the environmental insurance receipts, plus interest, will be applied ratably over 20 years and the remainder will be collected through the SRRM, and subject to an earnings test as follows: The Company will recover the first $5 million of annual expense through an amount that will be collected from customers through a tariff rider. The Company will apply $5 million of insurance (plus interest) to the next portion of environmental expenses each year. Any amounts in excess of the annual $10 million (plus interest from insurance) described above would be fully recoverable through the SRRM, to the extent the Company earns at or below its authorized Return On Equity (ROE). To the extent the Company earns more than its authorized ROE in a year, the Company would be required to cover environmental expenses greater than the $10 million (plus interest from insurance) with those earnings that exceed its authorized ROE. For purposes of this earnings test, all earnings derived from utility assets, including gains and losses associated with NW Natural’s weighted average cost of gas incentive mechanism, plus 50% of the Company’s earnings derived from the Company’s portion of its asset management agreement with our independent energy marketing company for asset management services associated with utility assets will be included. In any year that environmental expenses are less than $10 million (plus the interest on insurance), any unused tariff rider amount will offset deferred amounts otherwise collected through the SRRM and any unused insurance proceeds (plus interest on insurance) will roll forward to offset the next year’s expenses. Any remaining funds will be used to offset environmental remediation costs at the end of the project. The Company is evaluating the results of the Order, including those noted above as well as the state allocations. At this time, the Company does not anticipate refunds for 2013 or 2014 based on the earnings test outlined above. In accordance with accounting guidance and the Company's policy, the Company expects to recognize deferred interest income, net of approximately $4.4 million or $2.7 million after-tax on the associated regulatory account balances in the first quarter of 2015. Under the Order, the OPUC will revisit the deferral and amortization of future remediation expenses, as well as the treatment of remaining insurance proceeds in three years, or earlier if the Company gains greater certainty about its future remediation costs. The Company continues to evaluate the effects of the Order and is required to file a compliance report with the OPUC within the next 30 days demonstrating how it will implement the Order. The compliance filing is subject to review and approval by the OPUC and, as a consequence thereof, additional or different implementation procedures could be required, which may, among other things, result in additional impacts on earnings. The Company anticipates filing the compliance report as required by the Order in March 2015.