- Schedule of Credit Rating Downgrade Scenarios (Details) - Northwest Natural Gas Company [Member] $ in Thousands	Dec. 31, 2018 USD ($)
Moody's, A3 Rating [Member]
Description Of Credit Risk Related Contingent Features [Line Items]
With Adequate Assurance Calls	$ 0
Without Adequate Assurance Calls	0
Moody's, Baa1 Rating [Member]
Description Of Credit Risk Related Contingent Features [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Moody's, Baa2 Rating [Member]
Description Of Credit Risk Related Contingent Features [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
Moody's, Baa3 Rating [Member]
Description Of Credit Risk Related Contingent Features [Line Items]
With Adequate Assurance Calls	(3,940)
Without Adequate Assurance Calls	(3,940)
Speculative [Member]
Description Of Credit Risk Related Contingent Features [Line Items]
With Adequate Assurance Calls	(6,059)
Without Adequate Assurance Calls	$ (4,452)