Cost and Fair Value Basis of Long-Term Debt (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt	$ 681,700,000	$ 601,700,000	$ 636,700,000
Debt Instrument Fair Value	808,724,000	690,126,000
Long-term Debt Current And Noncurrent Abstract
Long-term Debt, Description	In March 2009, the utility issued $75 million of 5.37 percent secured MTNs due February 1, 2020, and in July 2009 issued another $50 million of 3.95 percent secured MTNs due July 15, 2014.  The utility also issued $50 million of MTNs in September 2011 with an interest rate of 3.176 percent and a maturity date of September 15, 2021.
Subsidiary Debt Instrument, Description	In November 2011, Gill Ranch issued $40 million of subsidiary senior secured notes with an interest rate of 7.75 percent on the fixed portion and a 7.00 percent interest rate currently on the variable portion. The notes are secured by all of the membership interests in Gill Ranch Storage, LLC, and are nonrecourse notes to NW Natural. The maturity date of these notes is November 30, 2016.
Subsidiary Debt Instrument, Covenant Description	Under the note agreements, Gill Ranch is subject to certain covenants and restrictions, including but not limited to, a financial covenant that requires Gill Ranch to maintain minimum adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) at various levels over the term of the notes. The minimum adjusted EBITDA increases incrementally over the first few years, reaching its highest level in the 12-month period beginning April 1, 2015. Under the note agreements, Gill Ranch is also subject to a debt service reserve requirement of 10 percent of the outstanding principal amount, initially $4 million, certain prepayment penalties, restrictions on dividends out of Gill Ranch unless certain earnings ratios are met, and restrictions on incurrence of additional debt.
Maturities of Long-term Debt [Abstract]
Current maturities of long-term debt	40,000,000	10,000,000	35,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	60,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	40,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	65,000,000
Debt Instrument [Line Items]
Long-term Debt	681,700,000	601,700,000	636,700,000
Current maturities of long-term debt	40,000,000	10,000,000	35,000,000
Long-term Debt, Noncurrent	641,700,000	591,700,000	601,700,000
Note 4110 Series Due 2010 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	0	0	10,000,000
Note 7450 Series Due 2010 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	0	0	25,000,000
Note 6665 Series Due 2011 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	0	10,000,000	10,000,000
Note 7130 Series Due 2012 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	40,000,000	40,000,000	40,000,000
Note 8260 Series Due 2014 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 3950 Series Due 2014 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	50,000,000	50,000,000	50,000,000
Note 4700 Series Due 2015 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	40,000,000	40,000,000	40,000,000
Note 5150 Series Due 2016 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	25,000,000	25,000,000	25,000,000
Note 7000 Series Due 2017 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	40,000,000	40,000,000	40,000,000
Note 6600 Series Due 2018 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	22,000,000	22,000,000	22,000,000
Note 8310 Series Due 2019 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 5370 Series Due 2020 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	75,000,000	75,000,000	75,000,000
Note 7630 Series Due 2019 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	20,000,000	20,000,000	20,000,000
Note 9050 Series Due 2021 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 3176 Series Due 2021 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	50,000,000	0	0
Note 5620 Series Due 2023 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	40,000,000	40,000,000	40,000,000
Note 7720 Series Due 2025 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	20,000,000	20,000,000	20,000,000
Note 6520 Series Due 2025 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 7050 Series Due 2026 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	20,000,000	20,000,000	20,000,000
Note 7000 Series Due 2027 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	20,000,000	20,000,000	20,000,000
Note 6650 Series Due 2027 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	19,700,000	19,700,000	19,700,000
Note 6650 Series Due 2028 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 7740 Series Due 2030 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	20,000,000	20,000,000	20,000,000
Note 7850 Series Due 2030 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Note 5820 Series Due 2032 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	30,000,000	30,000,000	30,000,000
Note 5660 Series Due 2032 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	40,000,000	40,000,000	40,000,000
Note 5250 Series Due 2035 [Member]
Debt Instrument [Line Items]
Total MTN Carrying Amount	10,000,000	10,000,000	10,000,000
Secured Notes Due 2016 [Member]
Debt Instrument [Line Items]
Subsidiary Senior Secured Notes	$ 40,000,000	$ 0	$ 0