Debt (details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At September 30, 2014, our gas storage segment’s long-term debt consisted of $20 million of fixed-rate senior secured debt with a maturity date of November 30, 2016 and an interest rate of 7.75%. This debt is secured by all of the membership interests in Gill Ranch and is nonrecourse to NW Natural. Under the amended loan agreement, $20 million of variable-rate debt was retired in June 2014. As part of the amended agreement, the earnings before interest, tax, depreciation, and amortization (EBITDA) covenant requirement was suspended through March 31, 2015 and the EBITDA hurdles thereafter were lowered. The debt service reserve requirement was fixed at $3 million. Our utility segment has long-term debt, including current maturities referred to above, of $641.7 million. Utility long-term debt consists of FMBs with maturity dates ranging from 2015 through 2042, interest rates ranging from 3.176% to 9.05%, and a weighted-average coupon rate of 5.64%.
Long-term Debt, Maturities, Repayment Terms	The utility has long-term debt due within the next 12 months consisting of $40 million of first mortgage bonds (FMBs) with a coupon rate of 4.70% and maturity in June 2015.
Long Term Debt Retirement	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%. As noted above, in June 2014 Gill Ranch retired $20 million of variable interest rate debt with a coupon rate of 7.00%.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	$ 661,700	$ 741,700	$ 741,700
Estimated Fair Value	748,902	806,359	828,360
Short-term Debt [Abstract]
Short-term Debt	$ 190,000	$ 188,200	$ 141,300
Commercial Paper, Maximum Maturity	209 days
Commercial Paper, Average Maturity	96 days