The ownership percentage NNG Financial holds in an 18 mile interest natural gas pipeline.

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The percenage of pre-tax income from gas storage credited to a deferred regulatory account for crediting back to core utility customers when the costs of the capacity have not been included in utility rates.

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The percenage of pre-tax income from gas storage credited to a deferred regulatory account for crediting back to core utility customers when the costs of the capacity have been included in utility rates.

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The percenage of pre-tax income from gas storage retained when the costs of the capacity have been included in utility rates.

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The percenage of pre-tax income from gas storage retained when the costs of the capacity have not been included in utility rates.

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No definition available.

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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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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

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The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

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Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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Amount after tax of income (loss) from continuing operations attributable to the parent.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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The net result for the period of deducting operating expenses from operating revenues.

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No definition available.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Disclosure of the extent of enterprise reliance on its major customers. For example, includes revenues from transactions with a single external customer amounting to 10 percent or more of the entity's revenues, the total amount of revenues from each such customer, and the identity of the segment or segments reporting the revenues. A group of entities that the entity knows to be under common control generally will be considered a single customer for inclusion in this item. The federal government, a state government, a local government (for example, a county or municipality), or a foreign government each will generally be considered as a single customer for inclusion in this item.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Amount of operating expense for purchased gas and petroleum of regulated operation.

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Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
Segment Information (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 231,705	$ 85,457	$ 97,077	$ 253,934	$ 755,038	$ 668,173	$ 717,888
Depreciation and amortization									81,053	77,604	75,722
Income (loss) from operations									145,259	144,109	131,037
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	34,488	$ (7,887)	$ 4,075	$ 41,397	29,620	$ (7,505)	$ 2,793	$ 37,511	72,073	62,419	60,026
Capital expenditures									213,325	138,357	116,312
Assets	3,039,746				3,079,801				3,039,746	3,079,801
Cost of gas									324,795	260,588	327,305
Environmental remediation									$ 15,291	13,298	3,513
Operating Segments [Member] \| Segments [Member]
Segment Reporting Information [Line Items]
Number of Reportable Segments									1
Segment Reporting, Disclosure of Major Customers									0
Operating Segments [Member] \| Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues									$ 732,942	650,477	702,210
Depreciation and amortization									79,734	76,289	74,410
Income (loss) from operations									132,807	130,570	119,215
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									60,509	54,567	53,391
Capital expenditures									211,672	138,074	115,272
Assets	2,961,326				2,806,627				2,961,326	2,806,627	2,791,623
Cost of gas									325,019	260,588	327,305
Environmental remediation									15,291	13,298	3,513
Gross Profit									392,632	376,591	371,392
Operating Segments [Member] \| Total Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									755,038	668,173	717,888
Depreciation and amortization									81,053	77,604	75,722
Income (loss) from operations									145,259	144,109	131,037
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									72,073	62,419	60,026
Capital expenditures									213,325	138,357	116,312
Assets	3,025,872				2,869,386				3,025,872	2,869,386	2,855,958
Corporate, Non-Segment [Member] \| Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues									22,096	17,696	15,678
Depreciation and amortization									1,319	1,315	1,312
Income (loss) from operations									12,452	13,539	11,822
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									11,564	7,852	6,635
Capital expenditures									1,653	283	1,040
Assets	$ 64,546				62,759				$ 64,546	62,759	$ 64,335
Mist Gas Storage Facility Member [Member] \| Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are Not In Utility Rates	80.00%								80.00%
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are In Utility Rates	33.00%								33.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account	20.00%								20.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account When In Rates	67.00%								67.00%
NNG Financial [Member]
Segment Reporting Information [Line Items]
Percent Interest In Pipeline	10.00%								10.00%
Assets	$ 400				$ 500				$ 400	$ 500
Customer Total [Member] \| Geographic Concentration Risk [Member] \| OREGON \| Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage									89.00%
Customer Total [Member] \| Geographic Concentration Risk [Member] \| WASHINGTON \| Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage									11.00%
Gross Profit Margin [Member] \| Residential and Commerical Customers [Member] \| Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage									90.00%
Volumes Delivered [Member] \| Residential and Commerical Customers [Member] \| Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage									60.00%