Environmental Matters (Tables)

12 Months Ended

Dec. 31, 2023

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 at December 31:

	Current Liabilities				Non-Current Liabilities
In thousands	2023		2022		2023		2022
Portland Harbor site:
Gasco/Siltronic Sediments	$	12,428	$	9,744	$	42,550	$	42,120
Other Portland Harbor	3,035		2,634		11,270		11,270
Gasco/Siltronic Upland site	16,304		16,067		34,235		35,457
Front Street site	687		457		939		879
Oregon Steel Mills	—		—		179		179
Total	$	32,454	$	28,902	$	89,173	$	89,905

Environmental Regulatory Table

The following table presents information regarding the total regulatory asset deferred as of December 31:

In thousands	2023		2022
Deferred costs and interest(1)	$	57,758	$	47,666
Accrued site liabilities(2)	121,575		118,763
Insurance proceeds and interest	(50,764)		(54,784)
Total regulatory asset deferral(1)	128,569		111,645
Current regulatory assets(3)	9,950		7,392
Long-term regulatory assets(3)	118,619		104,253

(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 $53 thousand in 2023 and $43 thousand in 2022.

(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 nor 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. See "Oregon SRRM" below.