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Environmental Matters
 3 Months Ended
Mar. 31, 2024
Environmental Remediation Obligations [Abstract]  
Schedule of Environmental Loss Contingencies by Site
The following table summarizes information regarding liabilities related to environmental sites, which are recorded in other current liabilities and other noncurrent liabilities in NW Natural's balance sheet:
Current LiabilitiesNon-Current Liabilities
March 31,December 31,March 31,December 31,
In thousands202420232023202420232023
Portland Harbor site:
Gasco/Siltronic Sediments$9,989 $8,588 $12,428 $41,365 $42,766 $42,550 
Other Portland Harbor2,978 2,868 3,035 10,707 10,707 11,270 
Gasco/Siltronic Upland site13,875 13,275 16,304 33,307 34,519 34,235 
Front Street site546 456 687 513 939 939 
Oregon Steel Mills— — — 179 179 179 
Total$27,388 $25,187 $32,454 $86,071 $89,110 $89,173 
Environmental Regulatory Table
The following table presents information regarding the total regulatory asset deferred:
March 31,December 31,
In thousands202420232023
Deferred costs and interest (1)
$60,291 $47,651 $57,758 
Accrued site liabilities (2)
113,426 114,253 121,575 
Insurance proceeds and interest(51,389)(55,416)(50,764)
Total regulatory asset deferral(1)
122,328 106,488 128,569 
Current regulatory assets(3)
9,864 6,947 9,950 
Long-term regulatory assets(3)
112,464 99,541 118,619 
(1)     Includes pre-review and post-review deferred costs, amounts currently in amortization, and interest, net of amounts collected from customers.
(2)    Excludes 3.3% of the Front Street site liability as the OPUC only allows recovery of 96.7% of costs for those sites allocable to Oregon, including those that historically served only Oregon customers. Amounts excluded from regulatory assets were $34 thousand at March 31, 2024, $45 thousand at March 31, 2023, and $53 thousand at December 31, 2023.
(3)    Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, NW Natural earns a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. It also accrues a carrying charge on insurance proceeds for amounts owed to customers. In Washington, neither the cash paid for insurance proceeds received accrue a carrying charge. Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5.0 million tariff rider. The amounts allocable to Oregon are recoverable through NGD rates, subject to an earnings test.