Debt (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Long-term Debt, Current and Noncurrent [Abstract]
Debt Instrument, Description	At March 31, 2015, our utility segment had 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%. At March 31, 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. 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. On April 28, 2015, Gill Ranch entered into a second amendment to the loan agreement under which the EBITDA covenant requirement is suspended through maturity of the loan. As part of the second amendment Gill Ranch will increase the debt reserve account by $4.5 million with contributions of $1.5 million by each of May 30, 2015, 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 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, Unclassified [Abstract]
Carrying Amount	$ 661,700	$ 661,700	$ 741,700
Estimated Fair Value	762,554	756,808	820,458
Short-term Debt [Abstract]
Short-term Debt	$ 156,200	$ 234,700	$ 32,600
Commercial Paper, Maximum Maturity	89 days
Commercial Paper, Average Maturity	57 days