Debt (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Short-term Debt [Abstract]
Commercial Paper	$ 190,300,000	$ 141,600,000
Short-term Debt, Weighted Average Interest Rate	0.30%	0.30%
Bank Loans	0	0
Commercial Paper, Maximum Maturity	254
Commercial Paper, Average Maturity	84
Line of Credit Facility, Current Borrowing Capacity	300,000,000	250,000,000
Line of Credit Facility, Expiration Date	Dec. 20, 2017
Line of Credit Facility, Maximum Borrowing Capacity	450,000,000
Letters of Credit, Additional Allowed	200,000,000
Letters of Credit Outstanding, Amount	0	0
Line of Credit Facility, Amount Outstanding	0	0
Line of Credit Facility, Covenant Terms	The credit agreement requires that we maintain credit ratings with Standard & Poor’s (S&P) and Moody’s Investors Service, Inc. (Moody’s) and notify the lenders of any change in our senior unsecured debt ratings or senior secured debt ratings, as applicable, by such rating agencies. A change in our debt ratings is not an event of default, nor is the maintenance of a specific minimum level of debt rating a condition of drawing upon the credit facility. However, interest rates on any loans outstanding under the credit facility are tied to debt ratings, which would increase or decrease the cost of any loans under the credit facility when ratings are changed.The credit agreement also requires us to maintain a consolidated indebtedness to total capitalization ratio of 70% or less. Failure to comply with this covenant would entitle the lenders to terminate their lending commitments and accelerate the maturity of all amounts outstanding. We were in compliance with this covenant at December 31, 2012 and 2011.
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt	691,700,000	681,700,000
Debt Instrument, Fair Value Disclosure	834,664,000	808,724,000
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	0	40,000,000
Long-term Debt, Excluding Current Maturities	691,700,000	641,700,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Current Maturities	0	40,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	60,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	40,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	65,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	40,000,000
Long-term Debt, Current and Noncurrent [Abstract]
Long-term Debt, Description	The utility issued $50 million of FMBs in October 2012 with a coupon rate of 4.00% and a maturity date of October 31, 2042. In September 2011, the utility issued $50 million of FMBs due September 15, 2021.
Debt Instrument, Description	In November 2011, Gill Ranch issued $40 million of senior secured debt, which consists of $20 million of fixed rate debt with an interest rate of 7.75% and $20 million of variable interest rate debt with an interest rate of LIBOR plus 5.50%, or 7.00%, whichever is higher. At December 31, 2012, the variable interest rate was 7.00%. This debt is secured by all of the membership interests in Gill Ranch and is nonrecourse to NW Natural. The maturity date of this debt is November 30, 2016.
Debt Instrument, Covenant Description	Under the debt agreements, Gill Ranch is subject to certain covenants and restrictions including, but not limited to, a financial covenant that requires Gill Ranch to maintain minimum adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) at various levels over the term of the debt. The minimum adjusted EBITDA increases incrementally over the first few years, reaching its highest level in the 12-month period beginning April 1, 2015. Under the debt agreements, Gill Ranch is also subject to a debt service reserve requirement of 10% of the outstanding principal amount, initially $4 million, certain prepayment penalties, restrictions on dividends out of Gill Ranch unless certain earnings ratios are met, and restrictions on incurrence of additional debt. Gill Ranch was in compliance with all existing debt provisions and covenants for the year ended December 31, 2012.
Note 6665 Series Due 2011 [Member]
Debt Instrument [Line Items]
Secured Debt	0	0
Note 7130 Series Due 2012 [Member]
Debt Instrument [Line Items]
Secured Debt	0	40,000,000
Note 8260 Series Due 2014 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 3950 Series Due 2014 [Member]
Debt Instrument [Line Items]
Secured Debt	50,000,000	50,000,000
Note 4700 Series Due 2015 [Member]
Debt Instrument [Line Items]
Secured Debt	40,000,000	40,000,000
Note 5150 Series Due 2016 [Member]
Debt Instrument [Line Items]
Secured Debt	25,000,000	25,000,000
Note 7000 Series Due 2017 [Member]
Debt Instrument [Line Items]
Secured Debt	40,000,000	40,000,000
Note 6600 Series Due 2018 [Member]
Debt Instrument [Line Items]
Secured Debt	22,000,000	22,000,000
Note 8310 Series Due 2019 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 7630 Series Due 2019 [Member]
Debt Instrument [Line Items]
Secured Debt	20,000,000	20,000,000
Note 5370 Series Due 2020 [Member]
Debt Instrument [Line Items]
Secured Debt	75,000,000	75,000,000
Note 9050 Series Due 2021 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 3176 Series Due 2021 [Member]
Debt Instrument [Line Items]
Secured Debt	50,000,000	50,000,000
Note 5620 Series Due 2023 [Member]
Debt Instrument [Line Items]
Secured Debt	40,000,000	40,000,000
Note 7720 Series Due 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	20,000,000	20,000,000
Note 6520 Series Due 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 7050 Series Due 2026 [Member]
Debt Instrument [Line Items]
Secured Debt	20,000,000	20,000,000
Note 7000 Series Due 2027 [Member]
Debt Instrument [Line Items]
Secured Debt	20,000,000	20,000,000
Note 6650 Series Due 2027 [Member]
Debt Instrument [Line Items]
Secured Debt	19,700,000	19,700,000
Note 6650 Series Due 2028 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 7740 Series Due 2030 [Member]
Debt Instrument [Line Items]
Secured Debt	20,000,000	20,000,000
Note 7850 Series Due 2030 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 5820 Series Due 2032 [Member]
Debt Instrument [Line Items]
Secured Debt	30,000,000	30,000,000
Note 5660 Series Due 2032 [Member]
Debt Instrument [Line Items]
Secured Debt	40,000,000	40,000,000
Note 5250 Series Due 2035 [Member]
Debt Instrument [Line Items]
Secured Debt	10,000,000	10,000,000
Note 4000 Series Due 2042 [Member]
Debt Instrument [Line Items]
Secured Debt	50,000,000	0
Secured Notes Due 2016 [Member]
Debt Instrument [Line Items]
Subsidiary Senior Secured Notes	$ 40,000,000	$ 40,000,000