Debt Fair Value of Long Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Gross long-term debt	$ 906,757	$ 741,813	$ 815,534
Unamortized debt issuance costs	(6,072)	(5,577)	(5,940)
Carrying amount	900,685	736,236	809,594
Estimated fair value	997,096	762,335	834,796
Northwest Natural Gas Company [Member]
Debt Instrument [Line Items]
Gross long-term debt	869,700	739,700	814,700
Unamortized debt issuance costs	(6,072)	(5,577)	(5,940)
Carrying amount	863,628	734,123	808,760
Estimated fair value	$ 959,906	$ 760,222	$ 833,962

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

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-Publisher FASB
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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