Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting [Abstract]
Revenues	$ 261,665us-gaap_Revenues	$ 293,386us-gaap_Revenues
Depreciation and Amortization	20,111us-gaap_DepreciationAndAmortization	19,589us-gaap_DepreciationAndAmortization
Operating Income (Loss)	53,001us-gaap_OperatingIncomeLoss	75,028us-gaap_OperatingIncomeLoss
Net Income (Loss) Attributable to Parent	28,486us-gaap_NetIncomeLoss	37,884us-gaap_NetIncomeLoss
Payments to Acquire Property, Plant, and Equipment	27,135	25,588
Assets	2,982,401us-gaap_Assets	2,830,384us-gaap_Assets	3,064,945us-gaap_Assets
Utility Margin [Abstract]
Cost of gas	125,705us-gaap_CostOfRevenue	155,201us-gaap_CostOfRevenue
Utility margin	130,601nwn_UtilityMargin	130,294nwn_UtilityMargin
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	256,306	285,495
Depreciation and Amortization	18,475	17,967
Operating Income (Loss)	51,880	71,457
Net Income (Loss) Attributable to Parent	28,335	36,019
Payments to Acquire Property, Plant, and Equipment	25,809	25,350
Assets	2,696,506	2,506,930	2,775,011
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	5,303	7,835
Depreciation and Amortization	1,636	1,622
Operating Income (Loss)	1,055	3,553
Net Income (Loss) Attributable to Parent	114	1,627
Payments to Acquire Property, Plant, and Equipment	1,326	238
Assets	270,992	307,055	273,813
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56	56
Depreciation and Amortization	0	0
Operating Income (Loss)	66	18
Net Income (Loss) Attributable to Parent	37	238
Payments to Acquire Property, Plant, and Equipment	0	0
Assets	$ 14,903	$ 16,399	$ 16,121

Utility operating revenues less utility cost of gas.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The aggregate cost of goods produced and sold and services rendered during the reporting period.

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