Subsequent Event (Details) - Subsequent Event [Member] - USD ($) $ in Millions	1 Months Ended
	Nov. 06, 2018	Oct. 02, 2018
Subsequent Event [Line Items]
Subsequent Event, Date	Oct. 01, 2018
Holding Company Reorganization Description	NW Holdings and NW Natural completed a reorganization into a holding company structure. NW Holdings is now the parent holding company of NW Natural, NWN Water, NWN Gas Storage and other subsidiaries previously held by NW Natural.
Northwest Natural Holding Company [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Initiation Date	Oct. 02, 2018
Line of Credit Facility, Expiration Date	Oct. 02, 2023
Line of Credit Facility, Frequency of Payment and Payment Terms	The principal amount of borrowings under the credit agreements are due and payable on the maturity date.
Line of Credit Facility, Covenant Terms	The credit agreements require NW Holdings and NW Natural to maintain a consolidated indebtedness to total capitalization ratio of 70% or less. Failure to comply with this covenant would entitle the lenders to terminate their lending commitments and accelerate the maturity of all amounts outstanding.
Line of Credit Facility, Additional Terms	The agreements also require NW Holdings and NW Natural to maintain credit ratings with Standard & Poor's (S&P) and Moody's Investors Service, Inc. (Moody’s) and notify the lenders of any change in the respective companies' senior unsecured debt ratings or senior secured debt ratings, as applicable, by such rating agencies, provided, however that in the event NW Holdings does not have a credit rating, its debt rating will be determined by a formula using NW Natural's credit rating. A change in debt ratings by S&P or Moody’s is not an event of default, nor is the maintenance of a specific minimum level of debt rating a condition of drawing upon the credit agreement. Rather, interest rates on any loans outstanding under the credit agreements are tied to debt ratings and therefore, a change in the debt rating would increase or decrease the cost of any loans under the credit agreements when ratings are changed.
Northwest Natural Holding Company [Member] | Line of Credit [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 100.0
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity		150.0
Northwest Natural Holding Company [Member] | Letter of Credit [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		40.0
Northwest Natural Gas Company [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Initiation Date	Oct. 02, 2018
Line of Credit Facility, Expiration Date	Oct. 02, 2023
Line of Credit Facility, Frequency of Payment and Payment Terms	The principal amount of borrowings under the credit agreements are due and payable on the maturity date.
Line of Credit Facility, Covenant Terms	The credit agreements require NW Holdings and NW Natural to maintain a consolidated indebtedness to total capitalization ratio of 70% or less. Failure to comply with this covenant would entitle the lenders to terminate their lending commitments and accelerate the maturity of all amounts outstanding.
Line of Credit Facility, Additional Terms	The agreements also require NW Holdings and NW Natural to maintain credit ratings with Standard & Poor's (S&P) and Moody's Investors Service, Inc. (Moody’s) and notify the lenders of any change in the respective companies' senior unsecured debt ratings or senior secured debt ratings, as applicable, by such rating agencies, provided, however that in the event NW Holdings does not have a credit rating, its debt rating will be determined by a formula using NW Natural's credit rating. A change in debt ratings by S&P or Moody’s is not an event of default, nor is the maintenance of a specific minimum level of debt rating a condition of drawing upon the credit agreement. Rather, interest rates on any loans outstanding under the credit agreements are tied to debt ratings and therefore, a change in the debt rating would increase or decrease the cost of any loans under the credit agreements when ratings are changed.
Northwest Natural Gas Company [Member] | Line of Credit [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		300.0
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity		450.0
Northwest Natural Gas Company [Member] | Letter of Credit [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 60.0
NW Water of Oregon [Member]
Subsequent Event [Line Items]
Business Acquisition, Date of Acquisition Agreement	Oct. 12, 2018
Business Acquisition, Name of Acquired Entity	Sunriver Water, LLC and Sunriver Environmental, LLC
Business Acquisition, Description of Acquired Entity	a water utility and wastewater treatment company providing a current combined 9,400 connections at the Sunriver Resort community in Central Oregon.
Expected Acquisition Close Period	the first half of 2019