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Environmental Matters (Tables)
 12 Months Ended
Dec. 31, 2021
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 LiabilitiesNon-Current Liabilities
In thousands2021202020212020
Portland Harbor site:
Gasco/Siltronic Sediments$7,582 $7,596 $42,076 $43,725 
Other Portland Harbor2,592 1,942 9,570 7,020 
Gasco/Siltronic Upland site15,711 14,887 36,215 40,250 
Front Street site1,100 3,816 811 1,107 
Oregon Steel Mills— — 179 179 
Total$26,985 $28,241 $88,851 $92,281 
Environmental Regulatory Table
The following table presents information regarding the total regulatory asset deferred as of December 31:
In thousands20212020
Deferred costs and interest(1)
$45,122 $44,516 
Accrued site liabilities(2)
115,773 120,352 
Insurance proceeds and interest(59,564)(69,253)
Total regulatory asset deferral(1)
$101,331 $95,615 
Current regulatory assets(3)
$6,694 $4,992 
Long-term regulatory assets(3)
$94,636 $90,623 
(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 or $0.1 million in 2021 and $0.2 million in 2020.
(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.