Debt (details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At June 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%. The debt is secured by all of the membership interests in Gill Ranch and is nonrecourse to NW Natural. Under Gill Ranch’s amended loan agreement with Prudential, $20 million of the variable-rate debt was retired in June 2014. As part of the amended agreement, the EBITDA covenant requirement is suspended through March 31, 2015 with lower EBITDA hurdles thereafter, and the debt service reserve requirement is fixed at $3 million.
Long-term Debt, Maturities, Repayment Terms	Our utility segment has long-term debt, including current maturities of $701.7 million, consisting of FMBs at June 30, 2014, 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%. The utility has long-term debt due within the next 12 months totaling $100 million, consisting of $50 million of first rate mortgage bonds (FMB) with a coupon rate of 3.95% and maturity in July 2014, $10 million of FMBs with a coupon rate of 8.26% and maturity in September 2014, and $40 million of FMBs with a coupon rate of 4.70% and maturity in June 2015.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	$ 721,700	$ 741,700	$ 691,700
Estimated Fair Value	807,617	806,359	769,679
Short-term Debt [Abstract]
Short-term Debt	$ 74,200	$ 188,200	$ 136,000
Commercial Paper, Maximum Maturity	39 days
Commercial Paper, Average Maturity	28 days