Cost and Fair Value of Long-Term Debt (details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Long-term Debt, by Maturity [Abstract]
Commercial Paper, Maximum Maturity	254 days
Commercial Paper, Average Maturity	88 days
Short-term Debt	$ 175,800	$ 141,600	$ 181,200
Debt Instrument, Description	Our gas storage segment’s long-term debt consists of $40 million of senior secured notes with a maturity date of November 30, 2016. These senior secured notes consist of $20 million fixed rate notes, which have an interest rate of 7.75 percent and of $20 million variable rate notes, which currently have an interest rate of 7.00 percent. The notes are secured by our membership interests in Gill Ranch Storage, LLC and are nonrecourse to NW Natural. See Note 7 in our 2011 Form 10-K for more detail on our long-term debt.
Long-term Debt, Maturities, Repayment Terms	Our utility’s long-term debt consists of $601.7 million of first mortgage bonds with maturity dates ranging from 2014 through 2035, interest rates ranging from 3.176 percent to 9.05 percent, and a weighted-average coupon rate of 5.85 percent. In March of 2012, we redeemed $40 million of first mortgage bonds. In July of 2012, we entered into a bond purchase agreement with a delayed closing feature under which we agreed to sell to a group of investors $50 million of first mortgage bonds with a coupon rate of 4.00 percent and a 30-year maturity. The bond issuance closed on October 30, 2012 (see Note 14). The proceeds of the issuance will be used to reduce short-term debt and for other general corporate purposes.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	641,700	681,700	641,700
Estimated Fair Value	$ 786,496	$ 808,724	$ 774,186

Identification of the lender and information about a contractual promise to repay a short-term or long-term obligation, which includes borrowings under lines of credit, notes payable, commercial paper, bonds payable, debentures, and other contractual obligations for payment. This may include rationale for entering into the arrangement, significant terms of the arrangement, which may include amount, repayment terms, priority, collateral required, debt covenants, borrowing capacity, call features, participation rights, conversion provisions, sinking-fund requirements, voting rights, basis for conversion if convertible and remarketing provisions. The description may be provided for individual debt instruments, rational groupings of debt instruments, or by debt in total.

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Disclosure of timing of required repayments, sinking fund requirements, and other redeemable securities at fixed or determinable prices and dates in the five years immediately following the date of the latest balance sheet presented in the financial statements, and the amount thereafter to fully repay the principal of long-term debt. These disclosures may be made either on an individual debt or security basis, by type of debt or security basis, or on a combined basis.

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- Definition

Including the current and noncurrent portions, carrying value at the balance sheet date of a financial instrument used to receive debt funding on a regular basis with original maturities ranging from 5-10 years.

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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