Debt (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At March 31, 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 March 31, 2013, our utility's long-term debt consisted of $651.7 million of first mortgage bonds (FMBs) outstanding with maturity dates ranging from 2014 through 2042, interest rates ranging from 3.176% to 9.05%, and a weighted-average coupon rate of 5.71%. During the three months ended March 31, 2012, we did not issue or redeem any FMBs.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	$ 691,700	$ 691,700	$ 641,700
Estimated Fair Value	825,038	834,664	742,852
Short-term Debt [Abstract]
Commercial Paper, Maximum Maturity	254 days
Commercial Paper, Average Maturity	61 days
Short-term Debt	$ 130,750	$ 190,250	$ 113,700