Debt (details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At September 30, 2013, our gas storage segment’s long-term debt consisted of $40 million of senior secured debt with a maturity date of November 30, 2016. This debt consists of $20 million of fixed rate debt with an interest rate of 7.75% and $20 million of variable interest rate debt, which currently has an interest rate of 7.00%. The debt is secured by all of the membership interests in Gill Ranch and is nonrecourse to NW Natural.
Long-term Debt, Maturities, Repayment Terms	At September 30, 2013, our utility's long-term debt, including current maturities, consisted of $701.7 million of first mortgage bonds (FMBs) with maturity dates ranging from 2014 through 2042, interest rates ranging from 3.176% to 9.05%, and a weighted-average coupon rate of 5.55%. On August 19, 2013, we issued $50 million of FMBs with a 3.542% coupon rate and a 10-year maturity. We did not redeem any FMBs during the nine months ended September 30, 2013.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	$ 741,700	$ 691,700	$ 641,700
Estimated Fair Value	828,360	834,664	786,496
Short-term Debt [Abstract]
Commercial Paper, Maximum Maturity	75 days
Commercial Paper, Average Maturity	50 days
Short-term Debt	$ 141,300	$ 190,250	$ 175,800