Debt (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At June 30, 2015, our utility segment had long-term debt of $601.7 million. Utility long-term debt consists of first mortgage bonds (FMBs) with maturity dates ranging from 2016 through 2042, interest rates ranging from 3.176% to 9.05%, and a weighted-average coupon rate of 5.70%. The utility redeemed $40 million of FMBs with a coupon rate of 4.70% in June 2015. At June 30, 2015, our gas storage segment’s long-term debt consisted of $20 million of fixed-rate senior collateralized debt with a maturity date of November 30, 2016 and an interest rate of 7.75%. This debt is collateralized by all of the membership interests in Gill Ranch and is nonrecourse to NW Natural. On April 28, 2015, Gill Ranch entered into an amendment to the loan agreement under which the earnings before interest, tax, depreciation, and amortization (EBITDA) covenant requirement is suspended through maturity of the loan. Previously, the covenant had been suspended through March 31, 2015, and the debt service reserve was set at $3 million. Under the amendment, the debt service reserve was fixed at $4.5 million as of June 30, 2015 with scheduled increases by contributions of $1.5 million on each of January 30, 2016 and August 30, 2016. Additionally, Gill Ranch must receive common equity contributions from its parent NWN Gas Storage of at least $2 million by August 31, 2015 and of at least $4 million by August 31, 2016.
Long-term Debt, Maturities, Repayment Terms	The utility has no long-term debt due within the next 12 months.
Long-term Debt, Unclassified [Abstract]
Carrying Amount	$ 621,700	$ 661,700	$ 721,700
Estimated Fair Value	695,902	756,808	807,617
Short-term Debt [Abstract]
Short-term Debt	$ 190,300	$ 234,700	$ 74,200
Commercial Paper, Maximum Maturity	111 days
Commercial Paper, Average Maturity	51 days