Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Abstract]
Short-term debt	$ 234,700,000	$ 188,200,000
Short-term Debt, Weighted Average Interest Rate	0.40%	0.30%
Commercial Paper, Maximum Maturity	209
Commercial Paper, Average Maturity	98
Bank Loans	0	0
Line of Credit Facility, Current Borrowing Capacity	300,000,000
Line of Credit Facility, Expiration Date	Dec. 20, 2019
Line of Credit Facility, Maximum Borrowing Capacity	450,000,000
Letters of Credit, Additional Allowed	100,000,000	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, 2014 and 2013.
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt	661,700,000	741,700,000
Debt Instrument, Fair Value Disclosure	756,808,000	806,359,000
Debt Instrument [Line Items]
Utility First Mortgage Bonds	641,700,000	701,700,000
Long-term Debt, Current Maturities	40,000,000	60,000,000
Long-term debt	621,700,000	681,700,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Current Maturities	40,000,000	60,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	45,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	40,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	22,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	30,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	484,700,000
Long-term Debt, Current and Noncurrent [Abstract]
Long-term Debt, Description	NW Natural issued $50 million of FMBs on August 19, 2013 with a coupon rate of 3.542% and a 10-year maturity. The utility redeemed $50 million of FMBs with a coupon rate of 3.95% in July 2014 and $10 million in September 2014 with a coupon rate of 8.26%. In June 2014, under the amended agreement Gill Ranch retired $20 million of variable interest rate debt with a coupon rate of 7.00%.
Debt Instrument, Description	Gill Ranch has $20 million of fixed-rate senior secured debt outstanding, which was issued in 2011 with a maturity date of November 30, 2016 and an interest rate of 7.75%.
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. As part of an amended agreement, the EBITDA covenant requirement is suspended through March 31, 2015 with lower EBITDA hurdles thereafter. The debt service reserve requirement was fixed at $3 million and is recorded on the balance sheet as restricted cash.
Note 8260 Series Due 2014 [Member]
Debt Instrument [Line Items]
Secured Debt	0	10,000,000
Note 3950 Series Due 2014 [Member]
Debt Instrument [Line Items]
Secured Debt	0	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 3542 Series Due 2023 [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	50,000,000
Secured Notes Due 2016 [Member]
Debt Instrument [Line Items]
Subsidiary Senior Secured Notes	$ 20,000,000	$ 40,000,000