Business Combinations Business Combinations (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Goodwill	$ 6,563	$ 0	$ 0
Falls Water Co [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Sep. 13, 2018
Business Acquisition, Name of Acquired Entity	Falls Water Co., Inc.
Business Acquisition, Description of Acquired Entity	a privately-owned water utility in the Pacific Northwest
Business Combination, Consideration Transferred	$ 8,500
Business Combination, Consideration Transferred, Shares	125,000
Business Combination, Reason for Business Combination	This acquisition aligns with our water sector strategy as the acquisition provides NWN Water entry into Idaho, expands service area, and opens further opportunity for growth.
Customers of Acquired Business	5,300 connections
Business Combination, Control Obtained Description	Through the purchase of all of the outstanding shares of Falls Water, NWN Water acquired the net assets and 100% control of Falls Water.
Goodwill	$ 6,563
Business Combination, Goodwill Recognized, Description	attributable to Falls Water's regulated service territory and experienced workforce as well as the strategic benefits expected from this high-growth service territory.