Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates (federal and state)	$ 13,105	$ 11,022
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	3,677	3,291
Differences required to be flowed-through by regulatory commissions	(2,525)	(5,260)
Other, net	(130)	(378)
Total provision for income taxes on continuing operations	$ 14,127	$ 8,675
Effective tax rate for continuing operations	22.80%	16.70%
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)	$ 13,355	$ 11,159
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	3,737	3,326
Differences required to be flowed-through by regulatory commissions	(2,525)	(5,260)
Other, net	(149)	(377)
Total provision for income taxes on continuing operations	$ 14,418	$ 8,848
Effective tax rate for continuing operations	22.80%	16.80%