Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates (federal and state)	$ 14,313	$ 15,368
Differences required to be flowed-through by regulatory commissions	(5,260)	849
Other, net	(378)	(585)
Total provision for income taxes on continuing operations	$ 8,675	$ 15,632
Effective tax rate for continuing operations	16.70%	27.10%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%
Northwest Natural Gas Company [Member]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates (federal and state)	$ 14,485	$ 15,372
Differences required to be flowed-through by regulatory commissions	(5,260)	849
Other, net	(377)	(586)
Total provision for income taxes on continuing operations	$ 8,848	$ 15,635
Effective tax rate for continuing operations	16.80%	27.10%