Northwest Natural Holding (NWN) 10-K 1 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Feb. 22, 2019	Jun. 30, 2018
Document Information [Line Items]
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Central Index Key	1,733,998
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Registrant Name	NORTHWEST NATURAL HOLDING COMPANY
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding		28,896,471
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Public Float			$ 1,814,277
Entity Small Business	false
Entity Emerging Growth Company	false
Northwest Natural Gas Company [Member]
Document Information [Line Items]
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Central Index Key	73,020
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Registrant Name	NORTHWEST NATURAL GAS COMPANY
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Common Stock, Shares Outstanding		28,844,190
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Public Float			$ 1,814,277
Entity Small Business	false
Entity Emerging Growth Company	false

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues [Abstract]
Operating revenues	$ 706,143	$ 755,038	$ 668,173
Operating expenses:
Cost of gas	255,519	324,795	260,588
Operations and maintenance	156,698	152,358	136,723
Environmental remediation	11,127	15,291	13,298
General taxes	32,172	30,639	29,243
Revenue taxes	30,082	0	0
Depreciation and amortization	85,156	81,053	77,604
Other operating expenses	3,227	0	0
Total operating expenses	573,981	604,136	517,456
Income from operations	132,162	150,902	150,717
Other income (expense), net	(3,601)	(295)	(7,151)
Interest expense, net	37,059	37,526	38,136
Income before income taxes	91,502	113,081	105,430
Income tax expense	24,191	41,008	43,011
Net income from continuing operations	67,311	72,073	62,419
Loss from discontinued operations, net of tax	(2,742)	(127,696)	(3,524)
Net income (loss)	64,569	(55,623)	58,895
Other comprehensive income (loss):
Change in employee benefit plan liability, net of taxes of ($166) for 2018, $735 for 2017, and $452 for 2016	476	(2,059)	(744)
Amortization of non-qualified employee benefit plan liability, net of taxes of ($278) for 2018, ($374) for 2017, and ($624) for 2016	774	572	955
Comprehensive income (loss)	$ 65,819	$ (57,110)	$ 59,106
Average common shares outstanding:
Basic (in shares)	28,803	28,669	27,647
Diluted (in shares)	28,873	28,753	27,779
Earnings from continuing operations per share of common stock:
Basic (in dollars per share)	$ 2.34	$ 2.51	$ 2.26
Diluted (in dollars per share)	2.33	2.51	2.25
Loss from discontinued operations per share of common stock:
Basic (in dollars per share)	(0.10)	(4.45)	(0.13)
Diluted (in dollars per share)	(0.09)	(4.44)	(0.13)
Earnings (loss) per share of common stock:
Basic (in dollars per share)	2.24	(1.94)	2.13
Diluted (in dollars per share)	$ 2.24	$ (1.93)	$ 2.12
Northwest Natural Gas Company [Member]
Operating revenues [Abstract]
Operating revenues	$ 705,571	$ 755,038	$ 667,949
Operating expenses:
Cost of gas	255,743	325,019	260,588
Utility Operations and maintenance	155,225	152,180	135,979
Environmental remediation	11,127	15,291	13,298
General taxes	32,086	30,602	29,222
Revenue taxes	30,082	0	0
Depreciation and amortization	84,986	81,024	77,575
Other operating expenses	3,223	0	0
Total operating expenses	572,472	604,116	516,662
Income from operations	133,099	150,922	151,287
Other income (expense), net	(3,599)	(198)	(7,041)
Interest expense, net	36,992	37,526	38,136
Income before income taxes	92,508	113,198	106,110
Income tax expense	24,459	41,478	43,275
Net income from continuing operations	68,049	71,720	62,835
Loss from discontinued operations, net of tax	(1,723)	(127,343)	(3,940)
Net income (loss)	66,326	(55,623)	58,895
Other comprehensive income (loss):
Change in employee benefit plan liability, net of taxes of ($166) for 2018, $735 for 2017, and $452 for 2016	476	(2,059)	(744)
Amortization of non-qualified employee benefit plan liability, net of taxes of ($278) for 2018, ($374) for 2017, and ($624) for 2016	774	572	955
Comprehensive income (loss)	$ 67,576	$ (57,110)	$ 59,106

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in employee benefit plan liability, tax	$ (166)	$ 735	$ 452
Amortization of non-qualified employee benefit plan liability, tax	(278)	(374)	(624)
Northwest Natural Gas Company [Member]
Change in employee benefit plan liability, tax	(166)	735	452
Amortization of non-qualified employee benefit plan liability, tax	$ (278)	$ (374)	$ (624)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 12,633	$ 3,472
Accounts receivable	66,970	66,236
Accrued unbilled revenue	57,827	62,381
Allowance for uncollectible accounts	(977)	(956)
Regulatory assets	41,930	45,781
Derivative instruments	9,001	1,735
Inventories	44,149	47,577
Gas reserves	16,647	15,704
Income taxes receivable	6,000	0
Other current assets	28,472	24,949
Discontinued operations - current assets	13,269	3,057
Total current assets	295,921	269,936
Non-current assets:
Property, plant, and equipment	3,414,490	3,204,635
Less: Accumulated depreciation	993,118	960,477
Total property, plant, and equipment, net	2,421,372	2,244,158
Gas reserves	66,197	84,053
Regulatory assets	371,786	356,608
Derivative instruments	725	1,306
Other investments	63,558	66,363
Goodwill	8,954	0
Other non-current assets	14,149	6,505
Discontinued operations - non-current assets	0	10,817
Total non-current assets	2,946,741	2,769,810
Total assets	3,242,662	3,039,746
Current liabilities:
Short-term debt	217,620	54,200
Current maturities of long-term debt	29,989	96,703
Accounts payable	115,878	111,021
Taxes accrued	11,023	18,883
Interest accrued	7,306	6,773
Regulatory liabilities	47,436	34,013
Derivative instruments	12,381	18,722
Other current liabilities	54,492	39,942
Discontinued operations - current liabilities	12,959	1,593
Total current liabilities	509,084	381,850
Long-term debt	706,247	683,184
Deferred credits and other non-current liabilities:
Deferred tax liabilities	280,463	270,526
Regulatory liabilities	611,560	586,093
Pension and other postretirement benefit liabilities	221,886	223,333
Derivative instruments	3,025	4,649
Discontinued operations - non-current liabilities		13,900
Other non-current liabilities	147,763	135,292
Discontinued operations - non-current liabilities	0	12,043
Total deferred credits and other non-current liabilities	1,264,697	1,231,936
Commitments and contingencies (see Note 16 and Note 17)
Equity:
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively	457,640	448,865
Retained earnings	312,182	302,349
Accumulated other comprehensive loss	(7,188)	(8,438)
Total equity	762,634	742,776
Total liabilities and equity	3,242,662	3,039,746
Northwest Natural Gas Company [Member]
Current assets:
Cash and cash equivalents	7,947	3,110
Accounts receivable	66,824	66,236
Accrued unbilled revenue	57,773	62,381
Receivables from affiliates	4,166	266
Allowance for uncollectible accounts	(975)	(956)
Regulatory assets	41,930	45,781
Derivative instruments	9,001	1,735
Inventories	44,126	47,577
Gas reserves	16,647	15,704
Other current assets	25,347	24,862
Discontinued operations - current assets	0	7,170
Total current assets	272,786	273,866
Non-current assets:
Property, plant, and equipment	3,410,439	3,204,260
Less: Accumulated depreciation	992,855	960,285
Total property, plant, and equipment, net	2,417,584	2,243,975
Gas reserves	66,197	84,053
Regulatory assets	371,786	356,608
Derivative instruments	725	1,306
Other investments	49,922	52,654
Other non-current assets	13,736	6,505
Discontinued operations - non-current assets	0	24,709
Total non-current assets	2,919,950	2,769,810
Total assets	3,192,736	3,043,676
Current liabilities:
Short-term debt	217,500	54,200
Current maturities of long-term debt	29,989	96,703
Accounts payable	114,937	110,354
Payables to affiliates	523	3,664
Taxes accrued	10,990	18,844
Interest accrued	7,273	6,773
Regulatory liabilities	47,436	34,013
Derivative instruments	12,381	18,722
Other current liabilities	53,027	39,942
Discontinued operations - current liabilities	0	2,565
Total current liabilities	494,056	385,780
Long-term debt	704,134	683,184
Deferred credits and other non-current liabilities:
Deferred tax liabilities	294,739	287,388
Regulatory liabilities	611,560	586,093
Pension and other postretirement benefit liabilities	221,886	223,333
Derivative instruments	3,025	4,649
Discontinued operations - non-current liabilities	0	4,732
Other non-current liabilities	147,668	135,205
Total deferred credits and other non-current liabilities	1,278,878	1,231,936
Commitments and contingencies (see Note 16 and Note 17)
Equity:
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively	226,452	448,865
Retained earnings	496,404	302,349
Accumulated other comprehensive loss	(7,188)	(8,438)
Total equity	715,668	742,776
Total liabilities and equity	$ 3,192,736	$ 3,043,676

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - $ / shares shares in Thousands, $ / shares in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Par Value (in dollars per share)	$ 0	$ 0
Shares Authorized (in shares)	100,000	100,000
Shares outstanding (in shares)	28,880	28,736
Shares issued (in shares)	28,880	28,736

Consolidated Statement of Share

Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock Including APIC [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Northwest Natural Gas Company [Member]		Northwest Natural Gas Company [Member]Common Stock Including APIC [Member]		Northwest Natural Gas Company [Member]Retained Earnings [Member]		Northwest Natural Gas Company [Member]Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2015	$ 780,972	$ 383,144	$ 404,990	$ (7,162)	$ 780,972		$ 383,144		$ 404,990		$ (7,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	59,106	0	58,895	211	59,106		0		58,895		211
Dividends on common stock	(51,624)	0	(51,624)	0	(51,624)		0		(51,624)		0
Stock-based compensation	2,924	2,924	0	0	2,924		2,924		0		0
Shares issued pursuant to equity based plans	6,358	6,358	0	0	6,358		6,358		0		0
Issuance of common stock, net of issuance costs	52,761	52,761	0	0	52,761		52,761		0		0
Ending Balance at Dec. 31, 2016	$ 850,497	445,187	412,261	(6,951)	850,497		445,187		412,261		(6,951)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common stock (in dollars per share)	$ 1.87
Comprehensive income	$ (57,110)	0	(55,623)	(1,487)	(57,110)		0		(55,623)		(1,487)
Dividends on common stock	(54,289)	0	(54,289)	0	(54,289)		0		(54,289)		0
Stock-based compensation	2,882	2,882	0	0	2,882		2,882		0		0
Shares issued pursuant to equity based plans	796	796	0	0	796		796		0		0
Ending Balance at Dec. 31, 2017	$ 742,776	448,865	302,349	(8,438)	742,776		448,865		302,349		(8,438)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common stock (in dollars per share)	$ 1.88
Comprehensive income	$ 65,819	0	64,569	1,250	67,576		0		66,326		1,250
Dividends on common stock	(54,736)	0	(54,736)	0	(41,035)		0		(41,035)		0
Stock-based compensation	3,020	3,020	0	0	2,161	[1]	2,161	[1]	0	[1]	0	[1]
Shares issued pursuant to equity based plans	5,175	5,175	0	0	3,075	[1]	3,075	[1]	0	[1]	0	[1]
Cash purchase of shares for business combination	(7,945)	(7,945)	0	0
Value of shares transferred for business combination	8,525	8,525	0	0
Transfer of investments to NW Holdings					(58,885)		(227,649)		168,764		0
Ending Balance at Dec. 31, 2018	$ 762,634	$ 457,640	$ 312,182	$ (7,188)	$ 715,668		$ 226,452		$ 496,404		$ (7,188)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common stock (in dollars per share)	$ 1.89
[1]	Stock-based compensation is based on stock awards of NW Natural to be issued in shares of NW Holdings.

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income (loss)	$ 64,569	$ (55,623)	$ 58,895
Adjustments to reconcile net income (loss) to cash provided by operations:
Depreciation and amortization	85,156	81,053	77,604
Regulatory amortization of gas reserves	16,684	16,353	15,525
Deferred income taxes	14,356	(52,414)	32,056
Qualified defined benefit pension plan expense	8,108	5,364	5,274
Contributions to qualified defined benefit pension plans	(15,540)	(19,430)	(14,470)
Deferred environmental expenditures, net	(14,528)	(13,716)	(10,469)
Regulatory disallowance of prior environmental cost deferrals	0	0	3,287
Amortization of environmental remediation	11,127	15,291	13,298
Regulatory revenue deferral from the TCJA	7,929	0	0
Other	1,596	2,102	2,846
Changes in assets and liabilities:
Receivables, net	181	3,282	(6,395)
Inventories	3,207	5,600	16,565
Income and other taxes	(16,904)	6,734	9,467
Accounts payable	16,792	1,092	12,028
Interest accrued	526	807	93
Deferred gas costs	(14,395)	17,122	(10,204)
Other, net	552	(4,093)	11,727
Discontinued operations	(645)	197,180	5,020
Cash provided by operating activities	168,771	206,704	222,147
Investing activities:
Capital expenditures	(214,636)	(213,325)	(138,357)
Other	(3,390)	(577)	2,882
Discontinued operations	573	(270)	(1,154)
Cash used in investing activities	(217,453)	(214,172)	(136,629)
Financing activities:
Repurchases related to stock-based compensation	0	(2,034)	(1,042)
Proceeds from stock options exercised	1,546	4,819	8,404
Proceeds from common stock issued	0	0	52,760
Long-term debt issued	50,000	100,000	150,000
Long-term debt retired	(97,000)	(40,000)	(25,000)
Change in short-term debt	163,274	900	(216,735)
Cash dividend payments on common stock	(51,311)	(53,957)	(51,508)
Stock purchases related to acquisitions	(7,951)	0	0
Other	(715)	(2,309)	(3,087)
Cash provided by (used in) financing activities	57,843	7,419	(86,208)
Increase (decrease) in cash and cash equivalents	9,161	(49)	(690)
Cash and cash equivalents, beginning of period	3,472	3,521	4,211
Cash and cash equivalents, end of period	12,633	3,472	3,521
Supplemental disclosure of cash flow information:
Interest paid, net of capitalization	35,324	34,787	36,023
Income taxes paid (refunded)	27,370	14,780	(7,157)
Northwest Natural Gas Company [Member]
Operating activities:
Net income (loss)	66,326	(55,623)	58,895
Adjustments to reconcile net income (loss) to cash provided by operations:
Depreciation and amortization	84,986	81,024	77,575
Regulatory amortization of gas reserves	16,684	16,353	15,525
Deferred income taxes	12,330	15,894	30,772
Qualified defined benefit pension plan expense	8,108	5,364	5,274
Contributions to qualified defined benefit pension plans	(15,540)	(19,430)	(14,470)
Deferred environmental expenditures, net	(14,528)	(13,716)	(10,469)
Regulatory disallowance of prior environmental cost deferrals	0	0	3,287
Amortization of environmental remediation	11,127	15,291	13,298
Regulatory revenue deferral from the TCJA	7,929	0	0
Other	883	2,003	2,745
Changes in assets and liabilities:
Receivables, net	(3,920)	3,215	(6,319)
Inventories	3,212	5,601	16,565
Income and other taxes	(7,854)	6,730	9,467
Accounts payable	13,937	3,332	10,822
Interest accrued	500	807	93
Deferred gas costs	(14,395)	17,122	(10,204)
Other, net	539	(3,855)	12,342
Discontinued operations	3,184	126,371	7,041
Cash provided by operating activities	173,508	206,483	222,239
Investing activities:
Capital expenditures	(214,328)	(213,325)	(138,357)
Other	(3,517)	(577)	2,882
Discontinued operations	(20,617)	(270)	(1,154)
Cash used in investing activities	(238,462)	(214,172)	(136,629)
Financing activities:
Repurchases related to stock-based compensation	0	(2,034)	(1,042)
Proceeds from stock options exercised	1,368	4,819	8,404
Proceeds from common stock issued	0	0	52,760
Long-term debt issued	50,000	100,000	150,000
Long-term debt retired	(97,000)	(40,000)	(25,000)
Change in short-term debt	163,300	900	(216,735)
Cash dividend payments on common stock	(38,387)	(53,957)	(51,508)
Other	(1,539)	(2,309)	(3,087)
Discontinued operations	(7,951)	0	0
Cash provided by (used in) financing activities	69,791	7,419	(86,208)
Increase (decrease) in cash and cash equivalents	4,837	(270)	(598)
Cash and cash equivalents, beginning of period	3,110	3,380	3,978
Cash and cash equivalents, end of period	7,947	3,110	3,380
Supplemental disclosure of cash flow information:
Interest paid, net of capitalization	35,305	34,787	36,023
Income taxes paid (refunded)	$ 27,350	$ 14,780	$ (7,157)

Organization and Principles of

Organization and Principles of Consolidation	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block]	ORGANIZATION AND PRINCIPLES OF CONSOLIDATION On October 1, 2018, we completed a reorganization into a holding company structure. In this reorganization, shareholders of NW Natural (the predecessor publicly held parent company) became shareholders of NW Holdings on a one-for-one basis; maintaining the same number of shares and ownership percentage as held in NW Natural immediately prior to the reorganization. NW Natural became a wholly-owned subsidiary of NW Holdings. Additionally, certain subsidiaries of NW Natural were transferred to NW Holdings. This reorganization was accounted for as a transaction among entities under common control. As required under accounting guidance, these subsidiaries are presented in this report as discontinued operations in the consolidated results of NW Natural. See Note 18 for additional information. The accompanying consolidated financial statements represent the respective, consolidated results and financial results of NW Holdings and NW Natural and all respective companies that each registrant directly or indirectly controls, either through majority ownership or otherwise. This is a combined report of NW Holdings and NW Natural, which includes separate consolidated financial statements for each registrant. NW Natural's natural gas distribution activities are reported in the natural gas distribution (NGD) segment, formerly titled and reported as the utility segment. The NGD segment is NW Natural's core operating business and serves residential, commercial, and industrial customers in Oregon and southwest Washington. The NGD segment is the only reportable segment for NW Holdings and NW Natural. All other business activities, including certain gas storage activities, water businesses, and other investments and activities are aggregated and reported as other at their respective registrant. In addition, NW Holdings has reported discontinued operations results related to the pending sale of Gill Ranch Storage, LLC (Gill Ranch). NW Holdings' direct and indirect wholly-owned subsidiaries include: • Northwest Natural Gas Company (NW Natural); ◦ Northwest Energy Corporation (Energy Corp); ▪ NWN Gas Reserves LLC (NWN Gas Reserves); • NW Natural Energy, LLC (NWN Energy); ◦ NW Natural Gas Storage, LLC (NWN Gas Storage); ▪ Gill Ranch Storage, LLC (Gill Ranch), which is presented as a discontinued operation; • NNG Financial Corporation (NNG Financial); ◦ KB Pipeline Company (KB); • NW Natural Water Company, LLC (NWN Water); ◦ Falls Water Co., Inc. (Falls Water); ◦ Salmon Valley Water Company; ◦ Cascadia Water, LLC (Cascadia); ◦ NW Natural Water of Oregon, LLC (NWN Water of Oregon); ◦ NW Natural Water of Washington, LLC (NWN Water of Washington); and ◦ NW Natural Water of Idaho, LLC (NWN Water of Idaho); and ▪ Gem State Water Company, LLC (Gem State) Investments in corporate joint ventures and partnerships that NW Holdings does not directly or indirectly control, and for which it is not the primary beneficiary, include NWN Financial's investment in Kelso-Beaver Pipeline and NWN Energy's investment in Trail West Holdings, LLC (TWH), which is accounted for under the equity method. NW Holdings and its direct and indirect subsidiaries are collectively referred to herein as NW Holdings, and NW Natural and its direct and indirect subsidiaries are collectively referred to herein as NW Natural. The consolidated financial statements of NW Holdings and NW Natural are presented after elimination of all intercompany balances and transactions. During the second quarter of 2018, we moved forward with our long-term strategic plans, which include a shift away from the California gas storage business. In June 2018, NWN Gas Storage, a wholly-owned subsidiary of NW Natural at the time and now a wholly-owned subsidiary of NW Holdings, entered into a Purchase and Sale Agreement that provides for the sale of all of the membership interests in its wholly-owned subsidiary, Gill Ranch, subject to various regulatory approvals and closing conditions. We have concluded that the pending sale of Gill Ranch qualifies as assets and liabilities held for sale and discontinued operations. As such, the results of Gill Ranch have been presented as a discontinued operation for NW Holdings for all periods presented and for NW Natural up until the holding company reorganization was effective on October 1, 2018 on the consolidated statements of comprehensive income and cash flows, and the assets and liabilities associated with Gill Ranch have been classified as discontinued operations assets and liabilities on the NW Holdings consolidated balance sheet. See Note 18 for additional information. Additionally, we reevaluated reportable segments and concluded that the remaining gas storage activities no longer meet the requirements to be separately reported as a segment. Interstate Storage Services is now reported in Other under NW Natural and all prior periods reflect this change. See Note 4 , which provides segment information. Notes to the consolidated financial statements reflect the activity of continuing operations for both NW Holdings and NW Natural for all periods presented, unless otherwise noted. Note 4 and Note 18 provide information regarding reportable segments and discontinued operations, respectively. All prior period amounts have been retrospectively adjusted to reflect the change in reportable segments and the designation of Gill Ranch as a discontinued operation for NW Holdings, and the designation of subsidiaries previously owned by NW Natural that are now owned by NW Holdings as discontinued operations for NW Natural. These reclassifications and the reorganization activities described above had no effect on the prior year’s consolidated results of operations, financial condition, or cash flows.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	SIGNIFICANT ACCOUNTING POLICIES Use of Estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America (U.S. GAAP) requires management to make estimates and assumptions that affect reported amounts in the consolidated financial statements and accompanying notes. Actual amounts could differ from those estimates, and changes would most likely be reported in future periods. Management believes the estimates and assumptions used are reasonable . Industry Regulation NW Holdings' principal business is to operate as a holding company for NW Natural, NWN Water and its other subsidiaries. NW Natural's principal business is the distribution of natural gas, which is regulated by the OPUC and WUTC. NW Natural also has natural gas storage services, which are regulated by the FERC, and to a certain extent by the OPUC and WUTC. Additionally, certain NW Holdings' subsidiaries own water businesses, which are regulated by the OPUC, WUTC, or IPUC. Accounting records and practices of the regulated businesses conform to the requirements and uniform system of accounts prescribed by these regulatory authorities in accordance with U.S. GAAP. The businesses in which customer rates are regulated by the OPUC, WUTC, IPUC, and FERC have approved cost-based rates which are intended to allow such businesses to earn a reasonable return on invested capital. In applying regulatory accounting principles, we capitalize or defer certain costs and revenues as regulatory assets and liabilities pursuant to orders of the OPUC, WUTC, or IPUC, which provide for the recovery of revenues or expenses from, or refunds to, utility customers in future periods, including a return or a carrying charge in certain cases. At December 31, NW Natural deferred the following amounts as regulatory assets and liabilities: Regulatory Assets In thousands 2018 2017 Current: Unrealized loss on derivatives (1) $ 12,381 $ 18,712 Gas costs 2,873 154 Environmental costs (2) 5,601 6,198 Decoupling (3) 9,140 11,227 Income taxes 2,218 2,218 Other (4) 9,717 7,272 Total current $ 41,930 $ 45,781 Non-current: Unrealized loss on derivatives (1) $ 3,025 $ 4,649 Pension balancing (5) 74,173 60,383 Income taxes 19,185 19,991 Pension and other postretirement benefit liabilities 174,993 179,824 Environmental costs (2) 76,149 72,128 Gas costs 9,978 84 Decoupling (3) 2,545 3,970 Other (4) 11,738 15,579 Total non-current $ 371,786 $ 356,608 Regulatory Liabilities In thousands 2018 2017 Current: Gas costs $ 17,182 $ 14,886 Unrealized gain on derivatives (1) 8,740 1,674 Decoupling (3) 2,264 322 Other (4) 19,250 17,131 Total current $ 47,436 $ 34,013 Non-current: Gas costs $ 552 $ 4,630 Unrealized gain on derivatives (1) 725 1,306 Decoupling (3) — 957 Income taxes (6) 225,408 213,306 Accrued asset removal costs (7) 380,464 360,929 Other (4) 4,411 4,965 Total non-current $ 611,560 $ 586,093 (1) Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through natural gas distribution rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement. (2) Refer to footnote (3) of the Deferred Regulatory Asset table in Note 17 for a description of environmental costs. (3) This deferral represents the margin adjustment resulting from differences between actual and expected volumes. (4) Balances consist of deferrals and amortizations under approved regulatory mechanisms and typically earn a rate of return or carrying charge. (5) Refer to footnote (1) of the Net Periodic Benefit Cost table in Note 9 for information regarding the deferral of pension expenses. (6) This balance represents estimated amounts associated with the Tax Cuts and Jobs Act. See Note 10 . (7) Estimated costs of removal on certain regulated properties are collected through rates. See "Accounting Policies— Plant, Property, and Accrued Asset Removal Costs " below. The amortization period for NW Natural's regulatory assets and liabilities ranges from less than one year to an indeterminable period. Regulatory deferrals for gas costs payable are generally amortized over 12 months beginning each November 1 following the gas contract year during which the deferred gas costs are recorded. Similarly, most other regulatory deferred accounts are amortized over 12 months . However, certain regulatory account balances, such as income taxes, environmental costs, pension liabilities, and accrued asset removal costs, are large and tend to be amortized over longer periods once NW Natural has agreed upon an amortization period with the respective regulatory agency. We believe all costs incurred and deferred at December 31, 2018 are prudent. All regulatory assets and liability are reviewed annually for recoverability, or more often if circumstances warrant. If it is determined that all or a portion of these regulatory assets or liabilities no longer meet the criteria for continued application of regulatory accounting, then NW Natural would be required to write-off the net unrecoverable balances in the period such determination is made. Environmental Regulatory Accounting See Note 17 for information about the SRRM and OPUC orders regarding implementation. New Accounting Standards NW Holdings and NW Natural consider the applicability and impact of all accounting standards updates (ASUs) issued by the Financial Accounting Standards Board (FASB). ASUs not listed below were assessed and determined to be either not applicable or are expected to have minimal impact on the consolidated financial position or results of operations. Recently Adopted Accounting Pronouncements STOCK COMPENSATION. On May 10, 2017, the FASB issued ASU 2017-09, "Stock Compensation - Scope of Modification Accounting." The purpose of the amendment is to provide clarity, reduce diversity in practice, and reduce the cost and complexity when applying the guidance in Topic 718, related to a change to the terms or conditions of a share-based payment award. Specifically, an entity would not apply modification accounting if the fair value, vesting conditions, and classification of the awards are the same immediately before and after the modification. The amendments in this update were effective beginning January 1, 2018, and will be applied prospectively to any award modified on or after the adoption date. The adoption did not have a material impact to the financial statements or disclosures of NW Holdings or NW Natural. RETIREMENT BENEFITS. On March 10, 2017, the FASB issued ASU 2017-07, "Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost." The ASU requires entities to disaggregate current service cost from the other components of net periodic benefit cost and present it with other current compensation costs for related employees in the income statement. Additionally, the other components of net periodic benefit costs are to be presented elsewhere in the income statement and outside of income from operations if that subtotal is presented. Only the service cost component of the net periodic benefit cost is eligible for capitalization. The amendments in this update were effective beginning January 1, 2018. Upon adoption, the ASU required that changes to the income statement presentation of net periodic benefit cost be applied retrospectively, while changes to amounts capitalized must be applied prospectively. As such, the interest cost, expected return on assets, amortization of prior service costs, and other costs have been reclassified from operations and maintenance expense to other income (expense), net on the consolidated statements of comprehensive income for the years ended December 31, 2017 and 2016. We did not elect the practical expedient which would have allowed for the reclassification of amounts disclosed previously in the pension and other postretirement benefits footnote disclosure as the basis for applying retrospective presentation. As mentioned above, on a prospective basis, the other components of net periodic benefit cost will not be eligible for capitalization. The retrospective presentation requirement related to the other components of net periodic benefit cost affected the operations and maintenance expense and other income (expense), net lines on the NW Natural consolidated statements of comprehensive income. For the years ended December 31, 2017 and 2016, $5.6 million and $6.6 million of expense was reclassified from operations and maintenance expense and included in other income (expense), net, respectively. GOODWILL. On January 26, 2017, the FASB issued ASU 2017-04, "Simplifying the Test for Goodwill Impairment." The ASU removes Step 2 from the goodwill impairment test and under the amended guidance an entity should perform its annual goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the amount in which the carrying amounts exceed the fair value of the reporting unit. The amendments in this standard are effective beginning January 1, 2020 and early adoption is permitted for interim or annual goodwill impairment tests performed after January 1, 2017. ASU 2017-04 was early adopted in the third quarter ended September 30, 2018. The adoption of this ASU did not materially affect the financial statements or disclosures of NW Holdings or NW Natural and is currently not applicable to NW Natural. STATEMENT OF CASH FLOWS. On August 26, 2016, the FASB issued ASU 2016-15, "Classification of Certain Cash Receipts and Cash Payments." The ASU adds guidance pertaining to the classification of certain cash receipts and payments on the statement of cash flows. The purpose of the amendment is to clarify issues that have been creating diversity in practice. The amendments in this standard were effective beginning January 1, 2018, and the adoption did not have a material impact to financial statements or disclosures as our historical practices and presentation were consistent with the directives of this ASU for NW Holdings and NW Natural. FINANCIAL INSTRUMENTS. On January 5, 2016, the FASB issued ASU 2016-01, "Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities." The ASU enhances the reporting model for financial instruments, which includes amendments to address aspects of recognition, measurement, presentation, and disclosure. The new standard was effective beginning January 1, 2018, and the adoption did not materially impact the financial statements or disclosures of NW Holdings or NW Natural. REVENUE RECOGNITION. On May 28, 2014, the FASB issued ASU 2014-09 "Revenue From Contracts with Customers." The underlying principle of the guidance requires entities to recognize revenue depicting the transfer of goods or services to customers at amounts the entity is expected to be entitled to in exchange for those goods or services. The ASU also prescribes a five-step approach to revenue recognition: (1) identify the contract(s) with the customer; (2) identify the separate performance obligations in the contract(s); (3) determine the transaction price; (4) allocate the transaction price to separate performance obligations; and (5) recognize revenue when, or as, each performance obligation is satisfied. The guidance also requires additional disclosures, both qualitative and quantitative, regarding the nature, amount, timing and uncertainty of revenue and cash flows. The new accounting standard and all related amendments were effective beginning January 1, 2018. The accounting standard was applied to all contracts using the modified retrospective method. The new standard is primarily reflected in the consolidated statements of comprehensive income and Note 6 . The implementation of the new revenue standard did not result in changes to how NW Holdings and NW Natural currently recognize revenue, and therefore, no cumulative effect or adjustment to the opening balances of retained earnings was required. The implementation did result in changes to the disclosures and presentation of revenues and expenses. The comparative information for prior years has not been restated. There is no material impact to the financial results of NW Holdings or NW Natural and no significant changes to our control environment due to the adoption of the new revenue standard on an ongoing basis. As previously discussed, the adoption of the new revenue standard did not impact the consolidated balance sheets or statements of cash flows but did result in changes to the presentation of the consolidated statements of comprehensive income for NW Holdings and NW Natural. Had the adoption of the new revenue standard not occurred, operating revenues for the year ended December 31, 2018 would have been $676.0 million for NW Holdings, compared to the reported amount of $706.1 million under the new revenue standard. Similarly, absent the impact of the new revenue standard, operating expenses would have been $543.9 million for NW Holdings, compared to the reported amount of $574.0 million under the new revenue standard for the year ended December 31, 2018. The effect of the change was an increase in both operating revenues and operating expenses of $30.1 million at NW Holdings and NW Natural for the year ended December 31, 2018; due to the change in presentation of revenue taxes. As part of the adoption of the new revenue standard, we evaluated the presentation of revenue taxes under the new guidance and across our peer group and concluded that the gross presentation of revenue taxes provides the greatest level of consistency and transparency. Prior to the adoption of the new revenue standard, a portion of revenue taxes was presented net in operating revenues and a portion was recorded directly on the balance sheet. During year ended December 31, 2018, $30.1 million in revenue taxes for NW Holdings and NW Natural was recognized in operating revenues and operating expenses. In comparison, for the years ended December 31, 2017 and 2016, $32.2 million and $28.3 million was recognized in revenue taxes for NW Holdings and NW Natural, of which $19.1 million and $17.1 million were recorded in operating revenues and $13.1 million and $11.2 million were recorded on the consolidated balance sheets, respectively. The change in presentation of revenue taxes had no impact on NGD margin, net income or earnings per share. Recently Issued Accounting Pronouncements CLOUD COMPUTING. On August 29, 2018, the FASB issued ASU 2018-15, "Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract." The purpose of the amendment is to align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted. The amended guidance can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's financial statements and disclosures. RETIREMENT BENEFITS. On August 28, 2018, the FASB issued ASU 2018-14, "Changes to the Disclosure Requirements for Defined Benefit Plans." The purpose of the amendment is to modify the disclosure requirements for defined benefit pension and other postretirement plans. The amendments in this update are effective for the year ended December 31, 2020. Early adoption is permitted. The amended presentation and disclosure guidance should be applied retrospectively. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's disclosures. FAIR VALUE MEASUREMENT. On August 28, 2018, the FASB issued ASU 2018-13, "Changes to the Disclosure Requirements for Fair Value Measurement." The purpose of the amendment is to modify the disclosure requirements for fair value measurements. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements and the narrative description of measurement uncertainty should be applied prospectively. All other amendments should be applied retrospectively. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's disclosures. ACCUMULATED OTHER COMPREHENSIVE INCOME. On February 14, 2018, the FASB issued ASU 2018-02, "Income Statement—Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." This update was issued in response to concerns from certain stakeholders regarding the current requirements under U.S. GAAP that deferred tax assets and liabilities are adjusted for a change in tax laws or rates, and the effect is to be included in income from continuing operations in the period of the enactment date. This requirement is also applicable to items in accumulated other comprehensive income where the related tax effects were originally recognized in other comprehensive income. The adjustment of deferred taxes due to the new corporate income tax rate enacted through the TCJA on December 22, 2017 recognized in income from continuing operations causes the tax effects of items within accumulated other comprehensive income (referred to as stranded tax effects) to not reflect the appropriate tax rate. The amendments in this update allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the TCJA and require certain disclosures about stranded tax effects. The amendments in this update are effective beginning January 1, 2019, and should be applied either in the period of adoption or retrospectively to each period in which the effect of the change in the federal corporate income tax rate in the TCJA is recognized. The reclassification allowed in this update is elective, and we are currently assessing whether we will make the reclassification. This update is not expected to have a material impact on the financial condition of NW Holdings or NW Natural. DERIVATIVES AND HEDGING. On August 28, 2017, the FASB issued ASU 2017-12, "Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities." The purpose of the amendment is to more closely align hedge accounting with companies’ risk management strategies. The ASU amends the accounting for risk component hedging, the hedged item in fair value hedges of interest rate risk, and amounts excluded from the assessment of hedge effectiveness. The guidance also amends the recognition and presentation of the effect of hedging instruments and includes other simplifications of hedge accounting. The amendments in this update are effective beginning January 1, 2019. Early adoption is permitted. The amended presentation and disclosure guidance is required prospectively. We do not anticipate the adoption of this standard to have a material impact on NW Holdings' and NW Natural's financial statements and disclosures. LEASES. On February 25, 2016, the FASB issued ASU 2016-02, "Leases," which revises the existing lease accounting guidance. Pursuant to the new standard, lessees will be required to recognize all leases, including operating leases that are greater than 12 months at lease commencement, on the balance sheet and record corresponding right-of-use assets and lease liabilities. Lessor accounting will remain substantially the same under the new standard. Quantitative and qualitative disclosures are also required for users of the financial statements to have a clear understanding of the nature of NW Natural's leasing activities. On July 30, 2018, the FASB approved an optional alternative transition approach that would allow entities to apply the transition requirements on the effective date of the standard. Additionally, on January 25, 2018, the FASB issued ASU 2018-01, "Land Easement Practical Expedient for Transition to Topic 842", to address the costs and complexity of applying the transition provisions of the new lease standard to land easements. This ASU provides an optional practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance. The standard and associated ASUs were effective for us beginning January 1, 2019. We elected the alternative prospective transition approach for adoption of ASC 842 beginning January 1, 2019. All comparative periods prior to January 1, 2019 will retain the financial reporting and disclosure requirements of ASC 840 “Leases” (“ASC 840”). We elected the land easement optional practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance. For the existing lease portfolio, we did not elect the optional practical expedient package to retain the legacy lease accounting conclusions upon adoption; rather, we re-assessed our existing contracts under the new leasing standard including whether the contract meets the definition of a lease and lease classification. As a result, we determined that most of our underground gas storage contracts no longer meet the definition of a lease under the new lease standard. Our lease portfolio under the new standard consists primarily of our leased headquarters, which expires in 2020. Upon adoption, NW Holdings expects to record a right-of-use lease asset and an associated lease liability of approximately $7.3 million , of which $7.0 million is expected to be recorded at NW Natural. In October 2017, NW Natural entered into a 20-year operating lease agreement commencing in 2020 for the new headquarters location in Portland, Oregon. Under the new lease standard, NW Natural is no longer considered the accounting owner of the asset during construction. As such, we expect to de-recognized the build-to-suit asset and liability balances of $26.0 million as of December 31, 2018 that were recorded under ASC 840 within property, plant and equipment and other non-current liabilities in the consolidated balance sheet. Refer to Note 16 for current lease commitments. CREDIT LOSSES. On June 16, 2016, the FASB issued ASU 2016-13, "Measurement of Credit Losses on Financial Instruments," which applies to financial assets subject to credit losses and measured at amortized cost. The new standard will require financial assets measured at amortized cost to be presented at the net amount expected to be collected and the allowance for credit losses is to be recorded as a valuation account that is deducted from the amortized cost basis. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted for fiscal years beginning after December 15, 2018. We are currently assessing the effect of this standard on the financial statements and disclosures of NW Holdings and NW Natural. Accounting Policies The accounting policies discussed below apply to both NW Holdings and NW Natural. Plant, Property, and Accrued Asset Removal Costs Plant and property are stated at cost, including capitalized labor, materials, and overhead. In accordance with regulatory accounting standards, the cost of acquiring and constructing long-lived plant and property generally includes an allowance for funds used during construction (AFUDC) or capitalized interest. AFUDC represents the regulatory financing cost incurred when debt and equity funds are used for construction (see “ AFUDC ” below). When constructed assets are subject to market-based rates rather than cost-based rates, the financing costs incurred during construction are included in capitalized interest in accordance with U.S. GAAP, not as regulatory financing costs under AFUDC. In accordance with long-standing regulatory treatment, our depreciation rates consist of three components: one based on the average service life of the asset, a second based on the estimated salvage value of the asset, and a third based on the asset’s estimated cost of removal. We collect, through rates, the estimated cost of removal on certain regulated properties through depreciation expense, with a corresponding offset to accumulated depreciation. These removal costs are non-legal obligations as defined by regulatory accounting guidance. Therefore, we have included these costs as non-current regulatory liabilities rather than as accumulated depreciation on our consolidated balance sheets. In the rate setting process, the liability for removal costs is treated as a reduction to the net rate base on which the NGD business has the opportunity to earn its allowed rate of return. The costs of NGD plant retired or otherwise disposed of are removed from NGD plant and charged to accumulated depreciation for recovery or refund through future rates. Gains from the sale of regulated assets are generally deferred and refunded to customers. For assets not related to NGD, we record a gain or loss upon the disposal of the property, and the gain or loss is recorded in operating income or loss in the consolidated statements of comprehensive income. The provision for depreciation of NGD property, plant, and equipment is recorded under the group method on a straight-line basis with rates computed in accordance with depreciation studies approved by regulatory authorities. The weighted-average depreciation rate for NGD assets in service was approximately 2.8 % for 2018 , 2017 , and 2016 , reflecting the approximate weighted-average economic life of the property. This includes 2018 weighted-average depreciation rates for the following asset categories: 2.7 % for transmission and distribution plant, 2.1 % for gas storage facilities, 4.5 % for general plant, and 3.1 % for intangible and other fixed assets. AFUDC. Certain additions to NGD plant include AFUDC, which represents the net cost of debt and equity funds used during construction. AFUDC is calculated using actual interest rates for debt and authorized rates for ROE, if applicable. If short-term debt balances are less than the total balance of construction work in progress, then a composite AFUDC rate is used to represent interest on all debt funds, shown as a reduction to interest charges, and on ROE funds, shown as other income. While cash is not immediately recognized from recording AFUDC, it is realized in future years through rate recovery resulting from the higher NGD cost of service. Our composite AFUDC rate was 5.2 % in 2018 , 5.5 % in 2017 , and 0.7 % in 2016 . IMPAIRMENT OF LONG-LIVED ASSETS. We review the carrying value of long-lived assets whenever events or changes in circumstances indicate the carrying amount of the assets may not be recoverable. Factors that would necessitate an impairment assessment of long-lived assets include a significant adverse change in the extent or manner in which the asset is used, a significant adverse change in legal factors or business climate that could affect the value of the asset, or a significant decline in the observable market value or expected future cash flows of the asset, among others. When such factors are present, we assess the recoverability by determining whether the carrying value of the asset will be recovered through expected future cash flows. An asset is determined to be impaired when the carrying value of the asset exceeds the expected undiscounted future cash flows from the use and eventual disposition of the asset. If an impairment is indicated, we record an impairment loss for the difference between the carrying value and the fair value of the long-lived assets. Fair value is estimated using appropriate valuation methodologies, which may include an estimate of discounted cash flows. In the fourth quarter of 2017, a non-cash pre-tax impairment of long-lived assets at the Gill Ranch Facility of $192.5 million was recognized. The income approach was used to estimate fair value, using the estimated future net cash flows. We also compared the results of the income approach to our own recent sale experience and recent market comparable transactions in order to estimate fair value. The Gill Ranch Facility was originally included in the gas storage segment, which has since been eliminated, and is now included in discontinued operations. We determined circumstances existed that indicated the carrying value of the assets may not be recoverable. Those circumstances included the completion of a comprehensive strategic review process that evaluated various alternatives including a potential sale, as well as contracting for available storage at lower than anticipated values for the coming storage year. Given these considerations, management re-evaluated the estimated cash flows from our interests in the Gill Ranch Facility, and determined that those estimated cash flows were no longer sufficient to cover the carrying value of the assets. The results of Gill Ranch have been presented as a discontinued operation for NW Holdings and NW Natural on the consolidated statements of comprehensive income and cash flows, and the assets and liabilities associated with Gill Ranch have been classified as discontinued operations assets and liabilities on the consolidated balance sheets. See Note 18 for additional information. Cash and Cash Equivalents For purposes of reporting cash flows, cash and cash equivalents include cash on hand plus highly liquid investment accounts with original maturity dates of three months or less. At December 31, 2018 and 2017 , NW Holdings had outstanding checks of approximately $ 2.7 million and $ 4.8 million , respectively, substantially all of which is recorded at NW Natural. These balances are included in accounts payable in the NW Holdings and NW Natural balance sheets. Revenue Recognition and Accrued Unbilled Revenue Revenues, derived primarily from the sale and transportation of natural gas, are recognized upon delivery of the gas commodity or service to customers. Revenues include accruals for gas or water delivered but not yet billed to customers based on estimates of deliveries from meter reading dates to month end (accrued unbilled revenue). Accrued unbilled revenue is dependent upon a number of factors that require management’s judgment, including total natural gas receipts and deliveries, customer use of natural gas or water by billing cycle, and weather factors. Accrued unbilled revenue is reversed the following month when actual billings occur. NW Holdings' accrued unbilled revenue at December 31, 2018 and 2017 was $ 57.8 million and $62.4 million , respectively, substantially all of which is accrued unbilled revenue at NW Natural. Revenues not related to NGD are derived primarily from Interstate Storage Services, asset management activities at the Mist gas storage facility, and other investments and business activities. At the Mist underground storage facility, revenues are primarily firm service revenues in the form of fixed monthly reservation charges. In addition, we also have asset management service revenue from an independent energy marketing company that optimizes commodity, storage, and pipeline capacity release transactions. Under this agreement, guaranteed asset management revenue is recognized using a straight-line, pro-rata methodology over the term of each contract. Revenues earned above the guaranteed amount are recognized as they are earned. Revenue Taxes Revenue-based taxes are primarily franchise taxes, which are collected from customers and remitted to taxing authorities. In 2018, revenue taxes are included in operating expenses in the statements of comprehensive income for NW Holdings and NW Natural. In 2017 and 2016, revenue taxes are included in operating revenues in the statements of comprehensive income for NW Holdings and NW Natural. All revenue taxes are recorded at NW Natural and were $30.1 million , $19.1 million , and $17.1 million for 2018 , 2017 , and 2016 , respectively. Accounts Receivable and Allowance for Uncollectible Accounts Accounts receivable consist primarily of amounts due for natural gas sales and transportation services to NGD customers, plus amounts due for gas storage services. At NW Holdings and NW Natural we establish allowances for uncollectible accounts (allowance) for trade receivables, including accrued unbilled revenue, based on the a

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Earnings Per Share [Text Block]	EARNINGS PER SHARE Basic earnings or loss per share are computed using NW Holdings' net income or loss and the weighted average number of common shares outstanding for each period presented. Diluted earnings per share are computed in the same manner, except it uses the weighted average number of common shares outstanding plus the effects of the assumed exercise of stock options and the payment of estimated stock awards from other stock-based compensation plans that are outstanding at the end of each period presented. Antidilutive stock awards are excluded from the calculation of diluted earnings or loss per common share. NW Holdings' diluted earnings or loss per share are calculated as follows: In thousands, except per share data 2018 2017 2016 Net income from continuing operations $ 67,311 $ 72,073 $ 62,419 Loss from discontinued operations, net of tax (2,742 ) (127,696 ) (3,524 ) Net income (loss) $ 64,569 $ (55,623 ) $ 58,895 Average common shares outstanding - basic 28,803 28,669 27,647 Additional shares for stock-based compensation plans (See Note 7) 70 84 132 Average common shares outstanding - diluted 28,873 28,753 27,779 Earnings (loss) from continuing operations per share of common stock: Basic $ 2.34 $ 2.51 $ 2.26 Diluted $ 2.33 $ 2.51 $ 2.25 Loss from discontinued operations per share of common stock: Basic $ (0.10 ) $ (4.45 ) $ (0.13 ) Diluted $ (0.09 ) $ (4.44 ) $ (0.13 ) Earnings (loss) per share of common stock: Basic $ 2.24 $ (1.94 ) $ 2.13 Diluted $ 2.24 $ (1.93 ) $ 2.12 Additional information: Antidilutive shares 2 13 5

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Information	SEGMENT INFORMATION We primarily operate in one reportable business segment, which is NW Natural's local gas distribution business and is referred to as the NGD segment. During the second quarter of 2018, we moved forward with our long-term strategic plans, which include a shift away from the California gas storage business, by entering into a Purchase and Sale Agreement that provides for the sale of all of the membership interests in Gill Ranch, subject to various regulatory approvals and closing conditions. As such, we reevaluated reportable segments and concluded that the remaining gas storage activities no longer meet the requirements of a reportable segment. Interstate Storage Services and asset management activities at the Mist gas storage facility are now reported as other under NW Natural. NW Natural and NW Holdings also have investments and business activities not specifically related to NGD, which are aggregated and reported as other and described below for each entity. No individual customer accounts for over 10% of NW Holdings' or NW Natural's operating revenues. Natural Gas Distribution The NGD segment is a regulated utility principally engaged in the purchase, sale, and delivery of natural gas and related services to customers in Oregon and southwest Washington. With regulated utility operations, NW Natural is responsible for building and maintaining a safe and reliable pipeline distribution system, purchasing sufficient gas supplies from producers and marketers, contracting for firm and interruptible transportation of gas over interstate pipelines to bring gas from the supply basins into its service territory, and re-selling the gas to customers subject to rates, terms, and conditions approved by the OPUC or WUTC. NGD also includes taking customer-owned gas and transporting it from interstate pipeline connections, or city gates, to the customers’ end-use facilities for a fee, which is approved by the OPUC or WUTC. Approximately 89 % of NGD customers are located in Oregon and 11 % in Washington. On an annual basis, residential and commercial customers typically account for around 60% of total NGD volumes delivered and around 90% of NGD margin. Industrial customers largely account for the remaining volumes and NGD margin. A small amount of the margin is also derived from miscellaneous services, gains or losses from an incentive gas cost sharing mechanism, and other service fees. Industrial sectors served by NW Natural include: pulp, paper, and other forest products; the manufacture of electronic, electrochemical and electrometallurgical products; the processing of farm and food products; the production of various mineral products; metal fabrication and casting; the production of machine tools, machinery, and textiles; the manufacture of asphalt, concrete, and rubber; printing and publishing; nurseries; and government and educational institutions. In addition to NW Natural's local gas distribution business, the NGD segment also includes the portion of the Mist underground storage facility used to serve NGD customers, the North Mist gas storage expansion in Oregon, and NWN Gas Reserves, which is a wholly-owned subsidiary of Energy Corp. NW Natural NW Natural activities included in Other includes Interstate Storage Services and third-party asset management services for the Mist facility in Oregon, appliance retail center operations, and corporate operating and non-operating revenues and expenses that cannot be allocated to NGD operations. Earnings from Interstate Storage Services assets are primarily related to firm storage capacity revenues. Earnings from the Mist facility also include revenue, net of amounts shared with NGD customers, from management of NGD assets at Mist and upstream pipeline capacity when not needed to serve NGD customers. Historically, under the Oregon sharing mechanism, NW Natural retained 80 % of the pre-tax income from these services when the costs of the capacity were not included in NGD rates, or 33 % of the pre-tax income when the costs have been included in these rates. The remaining 20 % and 67 %, respectively, were recorded to a deferred regulatory account for crediting back to NGD customers. After November 1, 2018 NW Natural retains 10% of the pre-tax income when the costs have been included in these rates, and the remaining 90% is recorded to a deferred regulatory account for crediting back to NGD customers. NW Holdings NW Holdings' activities included in Other includes all remaining activities not associated with NW Natural, specifically NWN Water, which consolidates the water operations and is pursuing other investments in the water sector itself and through its wholly-owned subsidiaries, NWN Gas Storage, a wholly-owned subsidiary of NWN Energy, NWN Energy's equity investment in TWH, which is pursuing development of a cross-Cascades transmission pipeline project (TWP), and other pipeline assets in NNG Financial. For more information on TWP, see Note 13 . Other also includes corporate revenues and expenses that cannot be allocated to other operations. All prior period amounts have been retrospectively adjusted to reflect the change in reportable segments and the designation of Gill Ranch as a discontinued operation for NW Holdings, and the designation of subsidiaries previously owned by NW Natural that are now owned by NW Holdings as discontinued operations for NW Natural. Segment Information Summary Inter-segment transactions were immaterial for the periods presented. The following table presents summary financial information concerning the reportable segments of continued operations. See Note 18 for information regarding discontinued operations for NW Holdings and NW Natural. In thousands NGD Other NW Natural Other NW Holdings 2018 Operating revenues $ 680,648 $ 24,923 $ 705,571 $ 572 $ 706,143 Depreciation and amortization 83,732 1,254 84,986 170 85,156 Income (loss) from operations 118,095 15,004 133,099 (937 ) 132,162 Net income (loss) from continuing operations 57,491 10,558 68,049 (738 ) 67,311 Capital expenditures 212,323 2,005 214,328 308 214,636 Total assets at December 31, 2018 (2) 3,141,969 50,767 3,192,736 36,657 3,229,393 2017 Operating revenues $ 732,942 $ 22,096 $ 755,038 $ — $ 755,038 Depreciation and amortization 79,734 1,290 81,024 29 81,053 Income (loss) from operations 138,450 12,472 150,922 (20 ) 150,902 Net income from continuing operations (1) 60,509 11,211 71,720 353 72,073 Capital expenditures 211,672 1,653 213,325 — 213,325 Total assets at December 31, 2017 (2) 2,961,326 50,471 3,011,797 14,075 3,025,872 2016 Operating revenues $ 650,477 $ 17,472 $ 667,949 $ 224 $ 668,173 Depreciation and amortization 76,289 1,286 77,575 29 77,604 Income (loss) from operations 137,178 14,109 151,287 (570 ) 150,717 Net income (loss) from continuing operations (3) 54,567 8,268 62,835 (416 ) 62,419 Capital expenditures 138,074 283 138,357 — 138,357 Total assets at December 31, 2016 (2) 2,806,627 48,719 2,855,346 14,040 2,869,386 (1) Includes $1.0 million of tax expense in NGD, $4.0 million of tax benefit in Other (NW Natural), and $0.4 million of tax benefit in Other (NW Holdings) from the TCJA remeasurement for the year ended December 31, 2017 . (2) Total assets for NW Holdings exclude assets related to discontinued operations of $13.3 million , $13.9 million and $210.4 million as of December 31, 2018 , 2017 , and 2016 , respectively. Total assets for NW Natural exclude assets related to discontinued operations of $31.9 million and $226.1 million as of December 31, 2017 , and 2016 , respectively. (3) Includes $2.0 million in 2016 of after-tax regulatory environmental disallowance charges in NGD. Natural Gas Distribution Margin NGD margin is a financial measure used by the CODM, consisting of NGD operating revenues, reduced by the associated cost of gas, environmental recovery revenues, and revenue taxes. The cost of gas purchased for NGD customers is generally a pass-through cost in the amount of revenues billed to regulated NGD customers. Environmental recovery revenues represent collections received from customers through the environmental recovery mechanism in Oregon. These collections are offset by the amortization of environmental liabilities, which is presented as environmental remediation expense in operating expenses. Revenue taxes are collected from NGD customers and remitted to taxing authorities. The collections from customers are offset by the expense recognition of the obligation to the taxing authority. By subtracting cost of gas, environmental remediation expense, and revenue taxes from NGD operating revenues, NGD margin provides a key metric used by the CODM in assessing the performance of the NGD segment. The following table presents additional segment information concerning NGD margin: In thousands 2018 2017 2016 NGD margin calculation: NGD operating revenues $ 680,648 $ 732,942 $ 650,477 Less: NGD cost of gas 255,743 325,019 260,588 Environmental remediation expense 11,127 15,291 13,298 Revenue taxes (1) 30,082 — — NGD margin $ 383,696 $ 392,632 $ 376,591 (1) The change in presentation of revenue taxes was a result of the adoption of ASU 2014-09 "Revenue From Contracts with Customers" and all related amendments on January 1, 2018. This change had no impact on NGD margin results as revenue taxes were previously presented net in NGD operating revenue. For additional information, see Note 2 .

Common Stock Common Stock

Common Stock Common Stock	12 Months Ended
	Dec. 31, 2018
Common Stock [Abstract]
Common Stock	COMMON STOCK As part of the reorganization of NW Holdings and NW Natural into a holding company structure effective October 1, 2018, NW Natural shareholders automatically became shareholders of NW Holdings on a one-for-one share basis with the same number of shares and same relative ownership percentage in NW Holdings as was held in NW Natural immediately prior to the reorganization. As of December 31, 2018 , NW Holdings had 100 million shares of common stock authorized. As of December 31, 2018 and 2017 , NW Natural had 100 million shares of common stock authorized. As of December 31, 2018 , NW Holdings had 24,339 shares reserved for issuance of common stock under the Employee Stock Purchase Plan (ESPP) and 394,204 shares reserved for issuance under the Dividend Reinvestment and Direct Stock Purchase Plan (DRPP). At NW Holdings' election, shares sold through the DRPP may be purchased in the open market or through original issuance of shares reserved for issuance under the DRPP. The Restated Stock Option Plan (SOP) was terminated with respect to new grants in 2012; however, options granted before the Restated SOP was terminated remain outstanding until the earlier of their expiration, forfeiture, or exercise. Options are now exercisable for shares of NW Holdings common stock. There were 55,938 options outstanding at December 31, 2018 , which were granted prior to termination of the plan. During November 2016, NW Natural completed an equity issuance consisting of an offering of 880,000 shares of common stock along with a 30 -day option for the underwriters to purchase an additional 132,000 shares. The offering closed on November 16, 2016 and resulted in a total issuance of 1,012,000 shares as both the initial offering and the underwriter option were fully executed. All shares were issued on November 16, 2016 at an offering price of $54.63 per share and resulted in total net proceeds of $52.8 million. Stock Repurchase Program NW Holdings has a share repurchase program under which it may purchase its common shares on the open market or through privately negotiated transactions. NW Holdings currently has Board authorization through May 2019 to repurchase up to an aggregate of the greater of 2.8 million shares or $100 million . No shares of common stock were repurchased pursuant to this program during the year ended December 31, 2018 . Since the plan’s inception in 2000 under NW Natural, a total of 2.1 million shares have been repurchased at a total cost of $83.3 million . The following table summarizes the changes in the number of shares of NW Holdings' common stock issued and outstanding: In thousands Shares Balance, December 31, 2015 27,427 Sales to employees under ESPP 18 Stock-based compensation 173 Equity Issuance 1,012 Balance, December 31, 2016 28,630 Sales to employees under ESPP 18 Stock-based compensation 88 Balance, December 31, 2017 28,736 Sales to employees under ESPP 19 Stock-based compensation 64 Sales to shareholders under DRPP 61 Balance, December 31, 2018 28,880

Revenue Revenue

Revenue Revenue	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue	REVENUE The following table presents disaggregated revenue from continuing operations: Year ended December 31, 2018 In thousands NGD Other (NW Natural) NW Natural Other (NW Holdings) NW Holdings Natural gas sales $ 670,662 $ — $ 670,662 $ — $ 670,662 Gas storage revenue, net — 10,780 10,780 — 10,780 Asset management revenue, net — 8,548 8,548 — 8,548 Appliance retail center revenue — 5,595 5,595 — 5,595 Other revenue — — — 572 572 Revenue from contracts with customers 670,662 24,923 695,585 572 696,157 Alternative revenue 8,989 — 8,989 — 8,989 Leasing revenue 997 — 997 — 997 Total operating revenues $ 680,648 $ 24,923 $ 705,571 $ 572 $ 706,143 NW Natural's revenue represents substantially all of NW Holdings' revenue and is recognized for both registrants when the obligation to customers is satisfied and in the amount expected to be received in exchange for transferring goods or providing services. Revenue from contracts with customers contain one performance obligation that is generally satisfied over time, using the output method based on time elapsed, due to the continuous nature of the service provided. The transaction price is determined per a set price agreed upon in the contract or dependent on regulatory tariffs. Customer accounts are settled on a monthly basis or paid at time of sale and based on historical experience. It is probable that we will collect substantially all of the consideration to which we are entitled. NW Holdings and NW Natural do not have any material contract assets, as net accounts receivable and accrued unbilled revenue balances are unconditional and only involve the passage of time until such balances are billed and collected. NW Holdings and NW Natural do not have any material contract liabilities. Revenue-based taxes are primarily franchise taxes, which are collected from NGD customers and remitted to taxing authorities. Beginning January 1, 2018, revenue taxes are included in operating revenues with an equal and offsetting expense recognized in operating expenses in the consolidated statements of comprehensive income. Natural Gas Distribution Natural gas sales. NW Natural's primary source of revenue is providing natural gas to customers in the NGD service territory, which includes residential, commercial, industrial and transportation customers. NGD revenue is generally recognized over time upon delivery of the gas commodity or service to the customer, and the amount of consideration received and recognized as revenue is dependent on the Oregon and Washington tariffs. Customer accounts are to be paid in full each month, and there is no right of return or warranty for services provided. Revenues include firm and interruptible sales and transportation services, franchise taxes recovered from the customer, late payment fees, service fees, and accruals for gas delivered but not yet billed (accrued unbilled revenue). The accrued unbilled revenue balance is based on estimates of deliveries during the period from the last meter reading and management judgment is required for a number of factors used in this calculation, including customer use and weather factors. We applied the significant financing practical expedient and have not adjusted the consideration NW Natural expects to receive from NGD customers for the effects of a significant financing component as all payment arrangements are settled annually. Due to the election of the right to invoice practical expedient, we do not disclose the value of unsatisfied performance obligations as of December 31, 2018 . Alternative revenue. Weather normalization (WARM) and decoupling mechanisms are considered to be alternative revenue programs. Alternative revenue programs are considered to be contracts between NW Natural and its regulator and are excluded from revenue from contracts with customers. Leasing revenue. Leasing revenue primarily consists of rental revenue for small leases of property owned for NGD to third parties. The majority of the transactions are accounted for as operating leases and the revenue is recognized on a straight-line basis over the term of the lease agreement. Lease revenue is excluded from revenue from contracts with customers. NW Natural Other Gas storage revenue. NW Natural's other revenue includes gas storage activity, which includes Interstate Storage Services used to store natural gas for customers. Gas storage revenue is generally recognized over time as the gas storage service is provided to the customer and the amount of consideration received and recognized as revenue is dependent on set rates defined per the storage agreements. Noncash consideration in the form of dekatherms of natural gas is received as consideration for providing gas injection services to gas storage customers. This noncash consideration is measured at fair value using the average spot rate. Customer accounts are generally paid in full each month, and there is no right of return or warranty for services provided. Revenues include firm and interruptible storage services, net of the profit sharing amount refunded to NGD customers. Asset management revenue. Asset management revenue is generally recognized over time using a straight-line approach over the term of each contract, and the amount of consideration received and recognized as revenue is dependent on a variable pricing model. Variable revenues earned above guaranteed amounts are estimated and recognized at the end of each period using the most likely amount approach. Revenues include the optimization of the storage assets and pipeline capacity provided, net of the profit sharing amount refunded to NGD customers. Asset management accounts are settled on a monthly basis. As of December 31, 2018 , unrecognized revenue for the fixed component of the transaction price related to gas storage and asset management revenue was approximately $56.0 million . Of this amount, approximately $14.1 million will be recognized in 2019 , $11.7 million in 2020 , $10.7 million in 2021 , $7.0 million in 2022 , $5.8 million in 2023 , and $6.7 million thereafter. The amounts presented here are calculated using current contracted rates. On October 12, 2018, NW Natural filed a rate petition with FERC for revised maximum cost-based rates, which incorporated the new federal corporate income tax rate. The revised rates became effective November 1, 2018. Appliance retail center revenue. NW Natural owns and operates an appliance store that is open to the public, where customers can purchase natural gas home appliances. Revenue from the sale of appliances is recognized at the point in time in which the appliance is transferred to the third party responsible for delivery and installation services and when the customer has legal title to the appliance. It is required that the sale be paid for in full prior to transfer of legal title. The amount of consideration received and recognized as revenue varies with changes in marketing incentives and discounts offered to customers. NW Holdings Other NW Holdings' primary source of other revenue is providing water distribution services to customers. Water distribution revenue is generally recognized over time upon delivery of the water commodity or service to the customer, and the amount of consideration received and recognized as revenue is dependent on the Oregon, Washington and Idaho tariffs. Customer accounts are to be paid in full each month, and there is no right of return or warranty for services provided. We applied the significant financing practical expedient and have not adjusted the consideration we expect to receive from water distribution customers for the effects of a significant financing component as all payment arrangements are settled annually. Due to the election of the right to invoice practical expedient, we do not disclose the value of unsatisfied performance obligations as of December 31, 2018 .

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Based Compensation	STOCK-BASED COMPENSATION Stock-based compensation plans are designed to promote stock ownership in NW Holdings by employees and officers. These compensation plans include a Long Term Incentive Plan (LTIP), an ESPP, and a Restated SOP. Long Term Incentive Plan The LTIP is intended to provide a flexible, competitive compensation program for eligible officers and key employees. Under the LTIP, shares of NW Holdings common stock are authorized for equity incentive grants in the form of stock, restricted stock, restricted stock units, stock options, or performance shares. An aggregate of 1,100,000 shares were authorized for issuance as of December 31, 2018 . Shares awarded under the LTIP may be purchased on the open market or issued as original shares. Of the 1,100,000 shares of common stock authorized for LTIP awards at December 31, 2018 , there were 574,787 shares available for issuance under any type of award. This assumes market, performance, and service-based grants currently outstanding are awarded at the target level. There were no outstanding grants of restricted stock or stock options under the LTIP at December 31, 2018 or 2017 . The LTIP stock awards are compensatory awards for which compensation expense is based on the fair value of stock awards, with expense being recognized over the performance and vesting period of the outstanding awards. Forfeitures are recognized as they occur. Performance Shares Since the LTIP’s inception in 2001, performance shares, which incorporate market, performance, and service-based factors, have been granted annually with three-year performance periods. The following table summarizes performance share expense information: Dollars in thousands Shares (1) Expense During Award Year (2) Total Expense for Award Estimated award: 2016-2018 grant (3) 28,218 $ 598 $ 1,413 Actual award: 2015-2017 grant 18,304 (346 ) 1,169 2014-2016 grant 31,388 168 1,685 (1) In addition to common stock shares, a participant also receives a dividend equivalent cash payment equal to the number of shares of common stock received on the award payout multiplied by the aggregate cash dividends paid per share during the performance period. (2) Amount represents the expense recognized in the third year of the vesting period noted above. For the 2015-2017 grant, targets were not met and expense was reversed during 2017 that had been previously recognized. (3) This represents the estimated number of shares to be awarded as of December 31, 2018 as certain performance share measures have been achieved. Amounts are subject to change with final payout amounts authorized by the Board of Directors in February 2019 . The aggregate number of performance shares granted and outstanding at the target and maximum levels were as follows: Dollars in thousands Performance Share Awards Outstanding 2018 Cumulative Expense Performance Period Target Maximum Expense December 31, 2018 2016-18 24,421 48,842 $ 598 $ 1,413 2017-19 31,372 62,744 458 1,400 2018-20 — — — — Total 55,793 111,586 $ 1,056 For the 2016-2018 performance period, performance share awards are based on EPS and Return on Invested Capital (ROIC) factors and a total shareholder return (TSR factor) relative to the Dow Jones U.S. Gas Distribution peer group over the three-year performance period. Additionally, these plans are based on performance results achieved relative to specific core and non-core strategies (strategic factor). For the 2017-2019 performance period, performance share awards are based on the achievement of EPS and ROIC factors, which can be modified by a TSR factor relative to the performance of the Russell 2500 Utilities Index over the three-year performance period and a growth modifier based on accumulative EBITDA measure. For the 2018-2020 performance period, performance share awards are based on the achievement of a three-year ROIC threshold that must be met and a cumulative EPS factor, which can be modified by a TSR factor relative to the performance of the Russell 2500 Utilities Index over the three-year performance period. The 2018-2020 performance period allows for one of the performance factors to remain variable until the first quarter of the third year of the award period. As the performance factor will not be approved until the first quarter of 2020, there is not a mutual understanding of the award’s key terms and conditions between NW Natural and the participants as of December 31, 2018, and therefore, no expense was recognized for the 2018-2020 performance period. NW Natural will calculate the grant date fair value and recognize expense once the final performance factor has been approved. If the target was achieved for the 2018 award, NW Holdings would grant 34,702 shares in the first quarter of 2020. Compensation expense is recognized in accordance with accounting standards for stock-based compensation and calculated based on performance levels achieved and an estimated fair value using the Monte-Carlo method. The weighted-average grant date fair value of nonvested shares at December 31, 2018 and 2017 was $57.05 and $ 56.40 per share, respectively. The weighted-average grant date fair value of shares vested during the year was $ 56.23 per share and there were no performance shares granted during the year for accounting purposes. As of December 31, 2018 , there was $ 1.1 million of unrecognized compensation expense related to the nonvested portion of performance awards expected to be recognized through 2019 . Restricted Stock Units In 2012, RSUs began being granted under the LTIP instead of stock options under the Restated SOP. Generally, the RSUs awarded are forfeitable and include a performance-based threshold as well as a vesting period of four years from the grant date. Upon vesting, the RSU holder is issued one share of common stock plus a cash payment equal to the total amount of dividends paid per share between the grant date and vesting date of that portion of the RSU. The fair value of an RSU is equal to the closing market price of NW Holdings' common stock on the grant date. During 2018 , total RSU expense was $ 1.8 million compared to $ 1.6 million in 2017 and $ 1.5 million in 2016 . As of December 31, 2018 , there was $ 3.1 million of unrecognized compensation cost from grants of RSUs, which is expected to be recognized over a period extending through 2023 . Information regarding the RSU activity is summarized as follows: Number of RSUs Weighted - Average Price Per RSU Nonvested, December 31, 2015 88,587 $ 44.78 Granted 40,271 54.36 Vested (29,488 ) 45.56 Forfeited (9,397 ) 44.59 Nonvested, December 31, 2016 89,973 48.85 Granted 32,168 60.51 Vested (35,341 ) 47.07 Forfeited (2,278 ) 53.78 Nonvested, December 31, 2017 84,522 53.90 Granted 32,450 57.59 Vested (32,689 ) 50.75 Forfeited (1,603 ) 59.95 Nonvested, December 31, 2018 82,680 $ 56.47 Restated Stock Option Plan The NW Natural Restated SOP was terminated for new option grants in 2012; however, options granted before the plan terminated remain outstanding until the earlier of their expiration, forfeiture, or exercise and are now exercisable for shares of NW Holdings common stock. Any new grants of stock options will be made under NW Holdings' LTIP, however, no option grants have been awarded since 2012 and all stock options were vested as of December 31, 2015. Options under the Restated SOP were granted to officers and key employees designated by a committee of the Board of Directors. All options were granted at an option price equal to the closing market price on the date of grant and may be exercised for a period of up to 10 years and seven days from the date of grant. Option holders may exchange shares they have owned for at least six months , valued at the current market price, to purchase shares at the option price. Information regarding the Restated SOP activity is summarized as follows: Option Shares Weighted - Average Price Per Share Intrinsic Value (In millions) Balance outstanding, December 31, 2015 352,688 $ 44.00 $ 2.3 Exercised (172,525 ) 43.61 2.0 Forfeited — n/a n/a Balance outstanding, December 31, 2016 180,163 44.38 2.8 Exercised (88,275 ) 44.33 1.8 Forfeited (200 ) 41.15 n/a Balance outstanding and exercisable, December 31, 2017 91,688 44.43 1.4 Exercised (35,450 ) 43.61 0.8 Expired (300 ) 43.29 n/a Balance outstanding and exercisable, December 31, 2018 55,938 $ 44.96 $ 0.9 During 2018 , cash of $ 1.5 million was received for stock options exercised and $0.2 million related tax benefit was recognized. The weighted-average remaining life of options exercisable and outstanding at December 31, 2018 was 1.69 years. Employee Stock Purchase Plan NW Holdings' ESPP allows employees of NW Holdings, NW Natural and certain designated subsidiaries to purchase common stock at 85% of the closing price on the trading day immediately preceding the initial offering date, which is set annually. Each eligible employee may purchase up to $ 21,205 worth of stock through payroll deductions over a period defined by the Board of Directors, with shares issued at the end of the subscription period. Stock-Based Compensation Expense Stock-based compensation expense is recognized as operations and maintenance expense or is capitalized as part of construction overhead at the entity at which the award recipient is employed. The following table summarizes the NW Holdings' financial statement impact, substantially all of which was recorded at NW Natural, of stock-based compensation under the LTIP, Restated SOP and ESPP: In thousands 2018 2017 2016 Operations and maintenance expense, for stock-based compensation $ 2,489 $ 2,354 $ 2,370 Income tax benefit (659 ) (930 ) (924 ) Net stock-based compensation effect on net income (loss) $ 1,830 $ 1,424 $ 1,446 Amounts capitalized for stock-based compensation $ 531 $ 528 $ 554

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt [Text Block]	DEBT Short-Term Debt The primary source of short-term liquidity for NW Holdings is cash balances, dividends from its operating subsidiaries, in particular NW Natural, available cash from a multi-year credit facility, and short-term credit facilities it may enter into from time to time. The primary source of short-term liquidity for NW Natural is from the sale of commercial paper and bank loans. NW Natural has a commercial paper program, and NW Holdings and NW Natural have separate bank facilities. In addition to issuing commercial paper or bank loans to meet working capital requirements, including seasonal requirements to finance gas purchases and accounts receivable, short-term debt may also be used to temporarily fund capital requirements. For NW Natural, commercial paper and bank loans are periodically refinanced through the sale of long-term debt or equity contributions from NW Holdings. NW Natural's commercial paper is sold through two commercial banks under an issuing and paying agency agreement and is supported by one or more unsecured revolving credit facilities. See “ Credit Agreements ” below. At December 31, 2018 and 2017 , NW Holdings had short-term debt outstanding of $217.6 million and $54.2 million , respectively, substantially all of which was recorded at NW Natural and was comprised primarily of NW Natural's commercial paper. The weighted average interest rate of commercial paper outstanding at December 31, 2018 and 2017 was 3.0 % and 1.9% , respectively. The carrying cost of commercial paper approximates fair value using Level 2 inputs, due to the short-term nature of the notes. See Note 2 for a description of the fair value hierarchy. At December 31, 2018 , NW Natural's commercial paper had a maximum remaining maturity of 46 days and an average remaining maturity of 22 days. Credit Agreements NW Holdings In October 2018, NW Holdings entered into a $100.0 million credit agreement, with a feature that allows it to request increases in the total commitment amount, up to a maximum of $150.0 million . The maturity date of the agreement is October 2, 2023 , with available extensions of commitments for two additional one-year periods, subject to lender approval. The NW Holdings credit agreement permits the issuance of letters of credit in an aggregate amount of up to $40.0 million . The principal amount of borrowings under the credit agreement is due and payable on the maturity date. The credit agreement requires NW Holdings 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. NW Holdings was in compliance with this covenant at December 31, 2018 . The agreement also requires NW Holdings to maintain debt ratings (which are defined by a formula using NW Natural's credit ratings in the event NW Holdings does not have a credit rating) with Standard & Poor's (S&P) and Moody's Investors Service, Inc. (Moody’s) and notify the lenders of any change in its senior unsecured debt ratings or senior secured debt ratings, as applicable, by such rating agencies. A change in NW Holdings' 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. NW Holdings does not currently maintain ratings with S&P or Moody's. There were no outstanding balances and no letters of credit issued or outstanding under the NW Holdings agreement at December 31, 2018 . NW Holdings had $2.8 million of letters of credit issued and outstanding, separate from the aforementioned credit agreement, at December 31, 2018 . NW Natural In October 2018, NW Natural entered into a new multi-year credit agreement for unsecured revolving loans totaling $300.0 million , with a feature that allows NW Natural to request increases in the total commitment amount, up to a maximum of $450.0 million . The maturity date of the agreement is October 2, 2023 with available extensions of commitments for two additional one-year periods, subject to lender approval. The new credit agreement is substantially similar to the prior credit agreement which was terminated upon the closing of the New Credit Agreement. The new credit agreement permits the issuance of letters of credit in an aggregate amount of up to $60.0 million . The principal amount of borrowings under the credit agreement is due and payable on the maturity date. There were no outstanding balances under NW Natural's prior credit agreement or the new credit agreement and no letters of credit issued or outstanding at December 31, 2018 and 2017 . NW Natural's prior credit agreement and the new credit agreement require 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. NW Natural was in compliance with this covenant at December 31, 2018 and 2017 . The new credit agreement also requires NW Natural to maintain credit ratings with S&P and Moody’s and notify the lenders of any change in NW Natural's senior unsecured debt ratings or senior secured debt ratings, as applicable, by such rating agencies. A change in NW Natural's 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 new credit agreement. Rather, interest rates on any loans outstanding under the new credit agreement are tied to debt ratings and therefore, a change in the debt rating would increase or decrease the cost of any loans under the new credit agreement when ratings are changed. Long-Term Debt NW Natural's issuance of FMBs, which includes NW Natural's medium-term notes, under the Mortgage and Deed of Trust (Mortgage) is limited by eligible property, adjusted net earnings, and other provisions of the Mortgage. The Mortgage constitutes a first mortgage lien on substantially all of NW Natural's NGD property. Maturities and Outstanding Long-Term Debt Retirement of long-term debt for each of the annual periods through December 31, 2023 and thereafter are as follows: In thousands Long-term debt maturities 2019 $ 30,000 2020 75,000 2021 60,000 2022 — 2023 90,000 Thereafter 484,700 The following table presents debt outstanding as of December 31: In thousands 2018 2017 NW Natural First Mortgage Bonds: 6.600% Series due 2018 $ — $ 22,000 1.545% Series due 2018 — 75,000 8.310% Series due 2019 10,000 10,000 7.630% Series due 2019 20,000 20,000 5.370% Series due 2020 75,000 75,000 9.050% Series due 2021 10,000 10,000 3.176% Series due 2021 50,000 50,000 3.542% Series due 2023 50,000 50,000 5.620% Series due 2023 40,000 40,000 7.720% Series due 2025 20,000 20,000 6.520% Series due 2025 10,000 10,000 7.050% Series due 2026 20,000 20,000 3.211% Series due 2026 35,000 35,000 7.000% Series due 2027 20,000 20,000 2.822% Series due 2027 25,000 25,000 6.650% Series due 2027 19,700 19,700 6.650% Series due 2028 10,000 10,000 7.740% Series due 2030 20,000 20,000 7.850% Series due 2030 10,000 10,000 5.820% Series due 2032 30,000 30,000 5.660% Series due 2033 40,000 40,000 5.250% Series due 2035 10,000 10,000 4.000% Series due 2042 50,000 50,000 4.136% Series due 2046 40,000 40,000 3.685% Series due 2047 75,000 75,000 4.110% Series due 2048 50,000 — 739,700 786,700 Less: current maturities 30,000 97,000 Total long-term debt $ 709,700 $ 689,700 Other NW Holdings Entities: Long-term debt obligations $ 2,113 $ — NW Holdings: Long-term debt, gross $ 741,813 $ 786,700 Less: current maturities 30,000 97,000 Total long-term debt $ 711,813 $ 689,700 First Mortgage Bonds In September 2018 , NW Natural issued $50.0 million of FMBs with a coupon rate of 4.110% , due in 2048 . In September 2017 , NW Natural issued $100.0 million of FMBs consisting of $25.0 million with a coupon rate of 2.822% and a maturity date in 2027 and $75.0 million with a coupon rate of 3.685% and a maturity date in 2047 . Retirements of Long-Term Debt In March 2018 , NW Natural retired $22.0 million of FMBs with a coupon rate of 6.600% , and retired $75.0 million of FMBs with a coupon rate of 1.545% in December 2018 . In August 2017 , NW Natural retired $40.0 million of FMBs with a coupon rate of 7.000% . Fair Value of Long-Term Debt NW Holdings' and NW Natural's outstanding debt does not trade in active markets. The fair value of debt is estimated using natural gas distribution companies with similar credit ratings, terms, and remaining maturities to NW Holdings' and NW Natural's debt that actively trade in public markets. Substantially all outstanding debt at NW Holdings is comprised of NW Natural debt. These valuations are based on Level 2 inputs as defined in the fair value hierarchy. See Note 2 . The following table provides an estimate of the fair value of NW Natural's long-term debt, including current maturities of long-term debt, using market prices in effect on the valuation date : December 31, In thousands 2018 2017 Gross long-term debt $ 739,700 $ 786,700 Unamortized debt issuance costs (5,577 ) (6,813 ) Carrying amount $ 734,123 $ 779,887 Estimated fair value $ 760,222 $ 853,339

Pension and Other Postretiremen

Pension and Other Postretirement Benefits Costs	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Pension and Other Postretirement Benefits Costs	PENSION AND OTHER POSTRETIREMENT BENEFIT COSTS NW Natural maintains a qualified non-contributory defined benefit pension plan, non-qualified supplemental pension plans for eligible executive officers and other key employees, and other postretirement employee benefit plans. NW Natural also has a qualified defined contribution plan (Retirement K Savings Plan) for all eligible employees. The qualified defined benefit pension plan and Retirement K Savings Plan have plan assets, which are held in qualified trusts to fund retirement benefits. Effective January 1, 2007 and 2010, the qualified defined benefit pension plans and postretirement benefits for non-union employees and union employees, respectively, were closed to new participants. Non-union and union employees hired or re-hired after December 31, 2006 and 2009, respectively, and employees of NW Natural subsidiaries are provided an enhanced Retirement K Savings Plan benefit. The following table provides a reconciliation of the changes in NW Natural's benefit obligations and fair value of plan assets, as applicable, for NW Natural's pension and other postretirement benefit plans, excluding the Retirement K Savings Plan, and a summary of the funded status and amounts recognized in NW Holdings' and NW Natural's consolidated balance sheets as of December 31: Postretirement Benefit Plans Pension Benefits Other Benefits In thousands 2018 2017 2018 2017 Reconciliation of change in benefit obligation: Obligation at January 1 $ 486,289 $ 457,839 $ 28,927 $ 29,395 Service cost 7,185 7,090 282 341 Interest cost 16,991 18,111 964 1,141 Net actuarial (gain) loss (32,979 ) 34,829 (327 ) (213 ) Benefits paid (1) (21,918 ) (31,580 ) (1,674 ) (1,737 ) Obligation at December 31 $ 455,568 $ 486,289 $ 28,172 $ 28,927 Reconciliation of change in plan assets: Fair value of plan assets at January 1 $ 287,925 $ 257,714 $ — $ — Actual return on plan assets (25,925 ) 40,308 — — Employer contributions 17,715 21,483 1,674 1,737 Benefits paid (1) (21,918 ) (31,580 ) (1,674 ) (1,737 ) Fair value of plan assets at December 31 $ 257,797 $ 287,925 $ — $ — Funded status at December 31 $ (197,771 ) $ (198,364 ) $ (28,172 ) $ (28,927 ) (1) In 2017, NW Natural completed a partial buy-out of its qualified defined benefit pension plan in which $9.3 million of plan assets and $8.7 million of liabilities were transferred to an insurer to provide annuities for buy-out plan participants. NW Natural's qualified defined benefit pension plan had a projected benefit obligation of $ 420.2 million and $ 449.7 million at December 31, 2018 and 2017 , respectively, and fair values of plan assets of $ 257.8 million and $ 287.9 million , respectively. The plan had an accumulated benefit obligation of $385.9 million and $410.3 million at December 31, 2018 and 2017 , respectively. The following table presents amounts realized through regulatory assets or in other comprehensive loss (income) for the years ended December 31: Regulatory Assets Other Comprehensive Loss (Income) Pension Benefits Other Postretirement Benefits Pension Benefits In thousands 2018 2017 2016 2018 2017 2016 2018 2017 2016 Net actuarial loss (gain) $ 14,261 $ 12,177 $ 14,005 $ (327 ) $ (214 ) $ (1,488 ) $ (677 ) $ 2,777 $ (1,196 ) Settlement Loss — — — — — — — — 193 Amortization of: Prior service cost (42 ) (127 ) (230 ) 468 468 468 — — — Actuarial loss (18,761 ) (14,802 ) (13,238 ) (448 ) (696 ) (705 ) (1,052 ) (946 ) 1,386 Total $ (4,542 ) $ (2,752 ) $ 537 $ (307 ) $ (442 ) $ (1,725 ) $ (1,729 ) $ 1,831 $ 383 The following table presents amounts recognized in regulatory assets and accumulated other comprehensive loss (AOCL) at December 31: Regulatory Assets AOCL Pension Benefits Other Postretirement Benefits Pension Benefits In thousands 2018 2017 2018 2017 2018 2017 Prior service cost (credit) $ 7 $ 49 $ (1,738 ) $ (2,206 ) $ — $ — Net actuarial loss 170,535 175,035 6,189 6,964 11,537 13,266 Total $ 170,542 $ 175,084 $ 4,451 $ 4,758 $ 11,537 $ 13,266 The following table presents amounts recognized by NW Holdings and NW Natural in AOCL and the changes in AOCL related to NW Natural's non-qualified employee benefit plans: Year Ended December 31, In thousands 2018 2017 Beginning balance $ (8,438 ) $ (6,951 ) Amounts reclassified to AOCL 642 (2,794 ) Amounts reclassified from AOCL: Amortization of actuarial losses 1,052 946 Total reclassifications before tax 1,694 (1,848 ) Tax expense (benefit) (444 ) 361 Total reclassifications for the period 1,250 (1,487 ) Ending balance $ (7,188 ) $ (8,438 ) In 2019 , NW Natural will amortize an estimated $ 13.7 million from regulatory assets to net periodic benefit costs, consisting of $ 14.2 million of actuarial losses offset by $0.5 million of prior service credits. A total of $ 0.6 million will be amortized from AOCL to earnings related to actuarial losses in 2019 . The assumed discount rate for NW Natural's pension plan and other postretirement benefit plans was determined independently based on the FTSE Above Median Curve (discount rate curve), which uses high quality corporate bonds rated AA- or higher by S&P or Aa3 or higher by Moody’s. The discount rate curve was applied to match the estimated cash flows in each of the plans to reflect the timing and amount of expected future benefit payments for these plans. The assumed expected long-term rate of return on plan assets for NW Natural's qualified pension plan was developed using a weighted-average of the expected returns for the target asset portfolio. In developing the expected long-term rate of return assumption, consideration was given to the historical performance of each asset class in which the plan’s assets are invested and the target asset allocation for plan assets. The investment strategy and policies for qualified pension plan assets held in the retirement trust fund were approved by the NW Natural Retirement Committee, which is composed of senior management with the assistance of an outside investment consultant. The policies set forth the guidelines and objectives governing the investment of plan assets. Plan assets are invested for total return with appropriate consideration for liquidity, portfolio risk, and return expectations. All investments are expected to satisfy the prudent investments rule under the Employee Retirement Income Security Act of 1974. The approved asset classes may include cash and short-term investments, fixed income, common stock and convertible securities, absolute and real return strategies, and real estate. Plan assets may be invested in separately managed accounts or in commingled or mutual funds. Investment re-balancing takes place periodically as needed, or when significant cash flows occur, in order to maintain the allocation of assets within the stated target ranges. The retirement trust fund is not currently invested in NW Holdings or NW Natural securities. The following table presents the NW Natural pension plan asset target allocation at December 31, 2018 : Asset Category Target Allocation Long government/credit 20 % U.S. large cap equity 18 Non-U.S. equity 18 Absolute return strategies 12 U.S. small/mid cap equity 10 Real estate funds 7 High yield bonds 5 Emerging markets equity 5 Emerging market debt 5 Non-qualified supplemental defined benefit plan obligations were $ 35.4 million and $ 36.6 million at December 31, 2018 and 2017 , respectively. These plans are not subject to regulatory deferral, and the changes in actuarial gains and losses, prior service costs, and transition assets or obligations are recognized in AOCL, net of tax until they are amortized as a component of net periodic benefit cost. These are unfunded, non-qualified plans with no plan assets; however, a significant portion of the obligations is indirectly funded with company and trust-owned life insurance and other assets. Other postretirement benefit plans are unfunded plans but are subject to regulatory deferral. The actuarial gains and losses, prior service costs, and transition assets or obligations for these plans are recognized as a regulatory asset. Net periodic benefit costs consist of service costs, interest costs, the expected returns on plan assets, and the amortization of gains and losses and prior service costs. The gains and losses are the sum of the actuarial and asset gains and losses throughout the year and are amortized over the average remaining service period of active participants. The asset gains and losses are based in part on a market-related valuation of assets. The market-related valuation reflects differences between expected returns and actual investment returns with the differences recognized over a two-year period from the year in which they occur, thereby reducing year-to-year net periodic benefit cost volatility. The service cost component of net periodic benefit cost for NW Natural pension and other postretirement benefit plans is recognized in operations and maintenance expense in the consolidated statements of comprehensive income. The other non-service cost components are recognized in other income (expense), net in the consolidated statements of comprehensive income. The following table provides the components of net periodic benefit cost for NW Natural's pension and other postretirement benefit plans for the years ended December 31: Pension Benefits Other Postretirement Benefits In thousands 2018 2017 2016 2018 2017 2016 Service cost $ 7,185 $ 7,090 $ 7,083 $ 282 $ 341 $ 391 Interest cost 16,991 18,111 18,399 964 1,141 1,175 Expected return on plan assets (20,639 ) (20,433 ) (20,054 ) — — — Amortization of prior service costs 43 127 231 (468 ) (468 ) (468 ) Amortization of net actuarial loss 19,813 15,748 14,624 448 696 705 Settlement expense — — 193 — — — Net periodic benefit cost 23,393 20,643 20,476 1,226 1,710 1,803 Amount allocated to construction (2,764 ) (6,597 ) (5,746 ) (98 ) (587 ) (600 ) Amount deferred to regulatory balancing account (10,314 ) (6,542 ) (6,252 ) — — — Net amount charged to expense $ 10,315 $ 7,504 $ 8,478 $ 1,128 $ 1,123 $ 1,203 Net periodic benefit costs are reduced by amounts capitalized to NGD plant based on approximately 25% to 35% payroll overhead charge. In addition, a certain amount of net periodic benefit costs were recorded to the regulatory balancing account, representing net periodic pension expense for the qualified plan above the amount set in rates, as approved by the OPUC, from 2011 through October 31, 2018. On October 26, 2018 the OPUC ordered that the balancing account be frozen as of October 31, 2018, with recovery subject to future proceedings. Effective November 1, 2018 the OPUC authorized an additional $8.1 million to be included in rates for defined benefit pension plan expenses. The following table provides the assumptions used in measuring periodic benefit costs and benefit obligations for the years ended December 31: Pension Benefits Other Postretirement Benefits 2018 2017 2016 2018 2017 2016 Assumptions for net periodic benefit cost: Weighted-average discount rate 3.51 % 3.99 % 4.17 % 3.44 % 3.85 % 4.00 % Rate of increase in compensation 3.25-4.5% 3.25-4.5% 3.25-4.5% n/a n/a n/a Expected long-term rate of return 7.50 % 7.50 % 7.50 % n/a n/a n/a Assumptions for year-end funded status: Weighted-average discount rate 4.20 % 3.52 % 4.00 % 4.13 % 3.44 % 3.85 % Rate of increase in compensation 3.25-3.5% 3.25-4.5% 3.25-4.5% n/a n/a n/a Expected long-term rate of return 7.50 % 7.50 % 7.50 % n/a n/a n/a The assumed annual increase in health care cost trend rates used in measuring other postretirement benefits as of December 31, 2018 was 6.75% . These trend rates apply to both medical and prescription drugs. Medical costs and prescription drugs are assumed to decrease gradually each year to a rate of 4.75% by 2025 . Assumed health care cost trend rates can have a significant effect on the amounts reported for the health care plans; however, other postretirement benefit plans have a cap on the amount of costs reimbursable by NW Natural. A one percentage point change in assumed health care cost trend rates would have the following effects: In thousands 1% Increase 1% Decrease Effect on net periodic postretirement health care benefit cost $ 43 $ (39 ) Effect on the accumulated postretirement benefit obligation 622 (560 ) Mortality assumptions are reviewed annually and are updated for material changes as necessary. In 2018, mortality rate assumptions were updated from RP-2006 mortality tables for employees and healthy annuitants with a fully generational projection using scale MP-2017 to RP-2014 mortality tables using scale MP-2018, which partially offset increases of the projected benefit obligation. The following table provides information regarding employer contributions and benefit payments for NW Natural's qualified pension plan, non-qualified pension plans, and other postretirement benefit plans for the years ended December 31, and estimated future contributions and payments: In thousands Pension Benefits Other Benefits Employer Contributions: 2017 $ 21,483 $ 1,737 2018 17,715 1,674 2019 (estimated) 13,318 1,787 Benefit Payments: 2016 20,959 1,732 2017 31,580 1,737 2018 21,918 1,674 Estimated Future Benefit Payments: 2019 22,699 1,787 2020 23,622 1,846 2021 24,516 1,930 2022 25,316 1,941 2023 26,074 1,993 2024-2028 145,917 9,628 Employer Contributions to Company-Sponsored Defined Benefit Pension Plans NW Natural makes contributions to its qualified defined benefit pension plans based on actuarial assumptions and estimates, tax regulations, and funding requirements under federal law. The Pension Protection Act of 2006 (the Act) established funding requirements for defined benefit plans. The Act establishes a 100% funding target over seven years for plan years beginning after December 31, 2008. In 2012 the Moving Ahead for Progress in the 21st Century Act (MAP-21) legislation changed several provisions affecting pension plans, including temporary funding relief and Pension Benefit Guaranty Corporation (PBGC) premium increases, which reduces the level of minimum required contributions in the near-term but generally increases contributions in the long-run and increases the operational costs of running a pension plan. In 2014, the Highway and Transportation Funding Act (HATFA) was signed and extended funding relief for an additional five years. The qualified defined benefit pension plan was underfunded by $ 162.4 million at December 31, 2018 . Including the impacts of MAP-21 and HATFA, NW Natural made cash contributions totaling $ 15.5 million to its qualified defined benefit pension plan for 2018 . During 2019 , NW Natural expects to make contributions of approximately $11.0 million to this plan. Multiemployer Pension Plan In addition to the NW Natural-sponsored defined benefit plans presented above, prior to 2014 NW Natural contributed to a multiemployer pension plan for its NGD union employees known as the Western States Office and Professional Employees International Union Pension Fund (Western States Plan). The plan's employer identification number is 94-6076144. Effective December 22, 2013, NW Natural withdrew from the plan, which was a noncash transaction. Vested participants will receive all benefits accrued through the date of withdrawal. As the plan was underfunded at the time of withdrawal, NW Natural was assessed a withdrawal liability of $8.3 million , plus interest, which requires NW Natural to pay $0.6 million each year to the plan for 20 years beginning in July 2014. The cost of the withdrawal liability was deferred to a regulatory account on the balance sheet. Payments were $0.6 million for 2018 , and as of December 31, 2018 the liability balance was $6.8 million . For 2017 and 2016 , contributions to the plan were $0.6 million and $0.6 million , respectively, which was approximately 4% to 6% of the total contributions to the plan by all employer participants in those years. Defined Contribution Plan NW Natural's Retirement K Savings Plan is a qualified defined contribution plan under Internal Revenue Code Sections 401(a) and 401(k). NW Natural contributions totaled $ 6.5 million , $5.4 million , and $4.6 million for 2018 , 2017 , and 2016 , respectively. The Retirement K Savings Plan includes an Employee Stock Ownership Plan. Deferred Compensation Plans NW Natural's supplemental deferred compensation plans for eligible officers and senior managers are non-qualified plans. These plans are designed to enhance the retirement savings of employees and to assist them in strengthening their financial security by providing an incentive to save and invest regularly. Fair Value Below is a description of the valuation methodologies used for assets measured at fair value. In cases where NW Natural's pension plan is invested through a collective trust fund or mutual fund, the fund's market value is utilized. Market values for investments directly owned are also utilized. U.S. EQUITY. These are non-published net asset value (NAV) assets. The non-published NAV assets consist of commingled trusts where NAV is not published but the investment can be readily disposed of at NAV or market value. The underlying investments in this asset class includes investments primarily in U.S. common stocks. INTERNATIONAL/GLOBAL EQUITY. These are Level 1 and non-published NAV assets. The Level 1 asset is a mutual fund, and the non-published NAV assets consist of commingled trusts where the NAV/unit price is not published but the investment can be readily disposed of at the NAV/unit price. The mutual funds has a readily determinable fair value, including a published NAV, and the commingled trusts are valued at unit price. This asset class includes investments primarily in foreign equity common stocks. LIABILITY HEDGING. These are non-published NAV assets. The non-published NAV assets consist of commingled trusts where NAV is not published but the investment can be readily disposed of at NAV or market value. The underlying investments in this asset class include long duration fixed income investments primarily in U.S. treasuries, U.S. government agencies, municipal securities, mortgage-backed securities, asset-backed securities, as well as U.S. and international investment-grade corporate bonds. OPPORTUNISTIC. These are non-published NAV assets consisting of commingled trusts where the investments can be readily disposed of at unit price, and a hedge fund of funds where the valuation is not published. This hedge fund of funds is winding down. Based on recent dispositions, NW Natural believes the remaining investment is fairly valued. The hedge fund of funds is valued at the weighted average value of investments in various hedge funds, which in turn are valued at the closing price of the underlying securities. This asset class includes investments in emerging market debt, leveraged loans, REITs, high yield bonds, a commodities fund, and a hedge fund of funds . ABSOLUTE RETURN STRATEGY. This is a non-published NAV asset consisting of a hedge fund of funds where the valuation is not published. This hedge fund of funds is winding down. Based on recent dispositions, NW Natural believes the remaining investment is fairly valued. The hedge fund of funds is valued at the weighted average value of investments in various hedge funds, which in turn are valued at the closing price of the underlying securities. This asset class primarily includes investments in common stocks and fixed income securities. CASH AND CASH EQUIVALENTS. These are Level 1 and non-published NAV assets. The Level 1 assets consist of cash in U.S. dollars, which can be readily disposed of at face value. The non-published NAV assets represent mutual funds without published NAV's but the investment can be readily disposed of at the NAV. The mutual funds are valued at the NAV of the shares held by the plan at the valuation date. The preceding valuation methods may produce a fair value calculation that is not indicative of net realizable value or reflective of future fair values. Although we believe these valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain investments could result in a different fair value measurement at the reporting date. Investment securities are exposed to various financial risks including interest rate, market, and credit risks. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of NW Natural's investment securities will occur in the near term and such changes could materially affect NW Natural's investment account balances and the amounts reported as plan assets available for benefit payments. The following tables present the fair value of NW Natural's plan assets, including outstanding receivables and liabilities, of NW Natural's retirement trust fund: In thousands December 31, 2018 Investments Level 1 Level 2 Level 3 Non-Published NAV (1) Total US equity $ — $ — $ — $ 85,233 $ 85,233 International / Global equity 24,994 — — 70,017 95,011 Liability hedging — — — 45,659 45,659 Opportunistic — — — 23,186 23,186 Cash and cash equivalents — — — 8,707 8,707 Total investments $ 24,994 $ — $ — $ 232,802 $ 257,796 December 31, 2017 Investments Level 1 Level 2 Level 3 Non-Published NAV (1) Total US equity $ — $ — $ — $ 98,375 $ 98,375 International / Global equity 21,211 — — 84,818 106,029 Liability hedging — — — 53,981 53,981 Opportunistic — — — 23,895 23,895 Cash and cash equivalents 82 — — 5,533 5,615 Total investments $ 21,293 $ — $ — $ 266,602 $ 287,895 December 31, 2018 2017 Receivables: Accrued interest and dividend income $ 1 $ 30 Due from broker for securities sold — — Total receivables $ 1 $ 30 Liabilities: Due to broker for securities purchased $ — $ — Total investment in retirement trust $ 257,797 $ 287,925 (1) The fair value for these investments is determined using Net Asset Value per share (NAV) as of December 31, as a practical expedient, and therefore they are not classified within the fair value hierarchy. These investments primarily consist of institutional investment products, for which the NAV is generally not publicly available.

Income Tax

Income Tax	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax [Text Block]	INCOME TAX The following table provides a reconciliation between income taxes calculated at the statutory federal tax rate and the provision for income taxes reflected in the NW Holdings and NW Natural statements of comprehensive income or loss for December 31: NW Holdings NW Natural Dollars in thousands 2018 2017 2016 2018 2017 2016 Income taxes at federal statutory rate $ 19,222 $ 39,578 $ 36,901 $ 19,434 $ 39,624 $ 37,137 Increase (decrease): State income tax, net of federal 4,927 5,066 4,844 4,982 5,072 4,858 Differences required to be flowed-through by regulatory commissions 1,302 2,357 2,357 1,302 2,357 2,357 Effect of the TCJA — (3,376 ) — — (2,956 ) — Deferred tax rate differential post-TCJA (76 ) — — (75 ) — — Other, net (1,184 ) (2,617 ) (1,091 ) (1,184 ) (2,619 ) (1,077 ) Total provision for income taxes $ 24,191 $ 41,008 $ 43,011 $ 24,459 $ 41,478 $ 43,275 Effective tax rate 26.4 % 36.3 % 40.8 % 26.4 % 36.6 % 40.8 % The NW Holdings and NW Natural effective income tax rates for 2018 compared to 2017 changed primarily as a result of the lower federal corporate income tax rate provided for by the TCJA. The effective tax rates for 2017 compared to 2016 changed primarily as a result of the lower federal corporate income tax rate provided for by the TCJA and NW Natural's increased benefits from the equity portion of AFUDC and excess tax benefits related to stock based compensation. The provision for current and deferred income taxes consists of the following at December 31: NW Holdings NW Natural In thousands 2018 2017 2016 2018 2017 2016 Current Federal $ 8,953 $ 19,345 $ 10,042 $ 9,127 $ 19,304 $ 10,158 State 3,785 5,963 3,116 3,846 5,956 3,131 12,738 25,308 13,158 12,973 25,260 13,289 Deferred Federal 9,001 13,869 25,473 9,025 14,371 25,581 State 2,452 1,831 4,380 2,461 1,847 4,405 11,453 15,700 29,853 11,486 16,218 29,986 Income tax provision $ 24,191 $ 41,008 $ 43,011 $ 24,459 $ 41,478 $ 43,275 The following table summarizes the tax effect of significant items comprising NW Holdings and NW Natural's deferred income tax balances recorded at December 31: NW Holdings NW Natural In thousands 2018 2017 (1) 2018 2017 Deferred tax liabilities: Plant and property $ 288,385 $ 278,735 $ 303,186 $ 296,113 Pension and postretirement obligations 27,135 23,352 27,135 23,352 Income tax regulatory asset 21,403 22,209 21,402 22,209 Other 1,061 2,766 537 2,250 Total deferred income tax liabilities $ 337,984 $ 327,062 $ 352,260 $ 343,924 Deferred income tax assets: Income tax regulatory liability $ 57,469 $ 56,470 $ 57,469 $ 56,470 Alternative minimum tax credit carryforward 52 66 52 66 Total deferred income tax assets $ 57,521 $ 56,536 $ 57,521 $ 56,536 Total net deferred income tax liabilities $ 280,463 $ 270,526 $ 294,739 $ 287,388 (1) Amounts have been reclassified among categories to conform to current period presentation. At December 31, 2018 and 2017 , regulatory income tax assets of $19.1 million and $21.3 million , respectively, were recorded by NW Natural, a portion of which is recorded in current assets. These regulatory income tax assets primarily represent future rate recovery of deferred tax liabilities, resulting from differences in NGD plant financial statement and tax bases and NGD plant removal costs, which were previously flowed through for rate making purposes and to take into account the additional future taxes, which will be generated by that recovery. These deferred tax liabilities, and the associated regulatory income tax assets, are currently being recovered through customer rates. At December 31, 2018 and 2017 , regulatory income tax assets of $2.3 million and $0.9 million , respectively, were recorded by NW Natural, representing future recovery of deferred tax liabilities resulting from the equity portion of AFUDC. At December 31, 2018 and 2017 , deferred tax assets of $57.5 million and $56.5 million , respectively, were recorded by NW Natural representing the future income tax benefit associated with the excess deferred income tax regulatory liability recorded as a result of the lower federal corporate income tax rate provided for by the TCJA. At December 31, 2018 and 2017 , regulatory liability balances representing the net tax benefit of the change in deferred taxes as a result of the TCJA of $217.1 million and $213.3 million , respectively, were recorded by NW Natural. NW Natural’s natural gas utility rates include an allowance to provide for the recovery of the anticipated provision for income taxes incurred as a result of providing regulated services. As a result of the 21 percent federal corporate income tax rate enacted in 2017, NW Natural recorded an additional regulatory liability in 2018 reflecting the estimated net reduction in the provision for income taxes. This revenue deferral is based on the estimated net benefit to customers and includes a gross-up for income taxes. As of December 31, 2018, a regulatory liability of $8.2 million , including accrued interest, was recorded to reflect this estimated revenue deferral. NW Holdings and NW Natural assess the available positive and negative evidence to estimate if sufficient taxable income will be generated to utilize their respective existing deferred tax assets. Based upon this assessment, NW Holdings and NW Natural determined that it is more likely than not that all of their respective deferred tax assets recorded as of December 31, 2018 will be realized. Uncertain tax positions are accounted for in accordance with accounting standards that require an assessment of the anticipated settlement outcome of material uncertain tax positions taken in a prior year, or planned to be taken in the current year. Until such positions are sustained, the uncertain tax benefits resulting from such positions would not be recognized. No reserves for uncertain tax positions were recorded as of December 31, 2018, 2017, or 2016. NW Holdings files a consolidated U.S. federal income tax return that includes NW Natural. Income tax expense is allocated on a separate company basis. The federal income tax returns for tax years 2014 and earlier are closed by statute. The IRS Compliance Assurance Process (CAP) examination of the 2015, and 2016 tax years have been completed. There were no material changes to these returns as filed. The 2017 and 2018 tax years are currently under IRS CAP examination. Our 2019 CAP application has been accepted by the IRS. Under the CAP program, NW Holdings and NW Natural work with the IRS to identify and resolve material tax matters before the tax return is filed each year. As of December 31, 2018, income tax years 2015 through 2018 remain open for state examination. The State of Oregon is currently examining the Oregon corporate income tax returns for tax years 2015, 2016, and 2017. No material changes are anticipated as a result of this examination. U.S. Federal TCJA Matters On December 22, 2017, the TCJA was enacted and lowered the U.S. federal corporate income tax rate to 21% from the existing maximum rate of 35% , effective for the tax year beginning January 1, 2018. The TCJA included specific provisions related to regulated public utilities that provided for the continued deductibility of interest expense and the elimination of bonus tax depreciation for property both acquired and placed in service on or after January 1, 2018. Under pre-TCJA law, business interest was generally deductible in the determination of taxable income. The TCJA imposed a new limitation on the deductibility of net business interest expense in excess of approximately 30 percent of adjusted taxable income. Taxpayers operating in the trade or business of a regulated utility are excluded from these new interest expense limitations. Proposed U.S. Treasury Regulations were published in November of 2018 which provide a de minimis rule whereby if 90 percent or more of a taxpayer's adjusted asset basis is allocable to regulated utility activities, then all of the business interest expense of that taxpayer is deemed to be excepted business interest of the regulated utility activity and is thereby not limited under the TCJA. As a result of the de minimis rule, NW Holdings and NW Natural anticipate that business interest expense will not be limited under the TCJA. The TCJA generally provides for immediate full expensing for qualified property both acquired and placed in service after September 27, 2017 and before January 1, 2023. This would generally provide for accelerated cost recovery for capital investments. However, the definition of qualified property excludes property used in the trade or business of a regulated utility. Proposed U.S. Treasury Regulations were published in August of 2018 which indicated that bonus tax depreciation would not be available for regulated utility activity assets acquired and placed in service by NW Holdings or NW Natural on or after January 1, 2018, but bonus tax depreciation would be available for regulated utility activity assets acquired and placed in service by NW Holdings or NW Natural before January 1, 2018. The SEC staff previously issued Staff Accounting Bulletin 118, which provided guidance on accounting for the tax effects of the TCJA. SAB 118 provided a measurement period that should not extend beyond one year from the TCJA enactment date for companies to complete the accounting for the TCJA under ASC 740. To the extent that a company’s accounting for certain income tax effects of the TCJA was incomplete but a reasonable estimate could be made, a company would record a provisional estimate in the financial statements. NW Natural previously disclosed that due to uncertainties with respect to the availability of bonus tax depreciation for regulated utility activity assets under the TCJA that the effects of bonus tax depreciation for assets placed in service after September 27, 2017 but before January 1, 2018 had not been recorded. The determination to exclude all assets placed in service after September 27, 2017 but before January 1, 2018 from bonus tax depreciation was provisional as provided for under SAB 118. As a result of the Proposed Regulations on bonus tax depreciation published in August of 2018, NW Natural revised the provisional estimate of deferred taxes and income taxes payable to reflect the effects of bonus tax depreciation for assets placed in service after September 27, 2017 but before January 1, 2018. In the third quarter, NW Natural recognized increases to prepaid income tax of $7.4 million , deferred income tax liability of $4.1 million , and regulatory liability of $3.3 million . In the fourth quarter, NW Natural recognized additional increases to prepaid income tax of $0.5 million , deferred income tax liability of $0.3 million , and regulatory liability of $0.2 million . The accounting for income tax effects of the TCJA is now complete.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2018
Public Utilities, Property, Plant and Equipment [Abstract]
Property, Plant and Equipment [Text Block]	PROPERTY, PLANT, AND EQUIPMENT The following table sets forth the major classifications of property, plant, and equipment and accumulated depreciation at December 31: In thousands 2018 2017 NW Natural: NGD plant in service $ 3,134,122 $ 2,975,217 NGD work in progress 204,978 159,924 Less: Accumulated depreciation 974,252 942,879 NGD plant, net 2,364,848 2,192,262 Other plant in service 66,009 64,997 Other construction work in progress 5,330 4,122 Less: Accumulated depreciation 18,603 17,406 Other plant, net (1) 52,736 51,713 Total property, plant, and equipment $ 2,417,584 $ 2,243,975 Other (NW Holdings): Other plant in service $ 4,051 $ 375 Less: Accumulated depreciation 263 192 Other plant, net (1) 3,788 183 NW Holdings: Total property, plant, and equipment $ 2,421,372 $ 2,244,158 NW Natural and NW Holdings: Capital expenditures in accrued liabilities $ 23,676 $ 34,761 (1) NW Natural's previously reported other balances were restated due to certain assets and liabilities now being classified as discontinued operations assets and liabilities in its balance sheets. See Note 18 for further discussion. Accumulated depreciation does not include the accumulated provision for asset removal costs of $380.5 million and $360.9 million at December 31, 2018 and 2017 , respectively. These accrued asset removal costs are reflected on the balance sheet as regulatory liabilities. See Note 2 . During 2018 and 2017 , no equipment was acquired under capital leases. NW Holdings Other plant balances include long-lived assets associated with water operations and non-regulated activities not held by NW Natural or its subsidiaries. NW Natural Other plant balances include long-lived assets not related to NGD. The weighted average depreciation rate for NGD assets was 2.8% during 2018 , 2017 , and 2016 . The weighted average depreciation rate for assets not related to NGD was 2.2% in 2018 , 1.9% in 2017 , and 2.0% in 2016 .

Gas Reserves

Gas Reserves	12 Months Ended
	Dec. 31, 2018
Gas Reserves [Abstract]
Gas Reserves [Text Block]	GAS RESERVES NW Natural has invested $188 million through the gas reserves program in the Jonah Field located in Wyoming as of December 31, 2018 . Gas reserves are stated at cost, net of regulatory amortization, with the associated deferred tax benefits recorded as liabilities in the consolidated balance sheets. The investment in gas reserves provides long-term price protection for NGD customers through the original agreement with Encana Oil & Gas (USA) Inc. under which NW Natural invested $178 million and the amended agreement with Jonah Energy LLC under which an additional $10 million was invested. NW Natural entered into the original agreements with Encana in 2011 under which NW Natural holds working interests in certain sections of the Jonah Field. Gas produced in these sections is sold at prevailing market prices, and revenues from such sales, net of associated operating and production costs and amortization, are credited to the NGD cost of gas. The cost of gas, including a carrying cost for the rate base investment, is included in the annual Oregon PGA filing, which allows NW Natural to recover these costs through customer rates. The investment under the original agreement, less accumulated amortization and deferred taxes, earns a rate of return. In March 2014, NW Natural amended the original gas reserves agreement in order to facilitate Encana's proposed sale of its interest in the Jonah field to Jonah Energy. Under the amendment, NW Natural ended the drilling program with Encana, but increased its working interests in its assigned sections of the Jonah field. NW Natural also retained the right to invest in new wells with Jonah Energy. Under the amended agreement there is still the option to invest in additional wells on a well-by-well basis with drilling costs and resulting gas volumes shared at NW Natural's amended proportionate working interest for each well in which it invests. NW Natural elected to participate in some of the additional wells drilled in 2014, but did not participate in additional wells since 2014. However, there may be the opportunity to participate in more wells in the future. Gas produced from the additional wells is included in the Oregon PGA at a fixed rate of $0.4725 per therm, which approximates the 10-year hedge rate plus financing costs at the inception of the investment. Gas reserves acted to hedge the cost of gas for approximately 6% , 6% and 8% of NGD gas supplies for the years ended December 31, 2018 , 2017 , and 2016 respectively. The following table outlines NW Natural's net gas reserves investment at December 31: In thousands 2018 2017 Gas reserves, current $ 16,647 $ 15,704 Gas reserves, non-current 170,660 171,832 Less: Accumulated amortization 104,463 87,779 Total gas reserves (1) 82,844 99,757 Less: Deferred taxes on gas reserves 20,071 22,712 Net investment in gas reserves $ 62,773 $ 77,045 (1) The net investment in additional wells included in total gas reserves was $4.8 million and $5.8 million at December 31, 2018 and 2017 , respectively. NW Natural's investment is included in NW Holdings' and NW Natural's consolidated balance sheets under gas reserves with the maximum loss exposure limited to the investment balance.

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments [Abstract]
Investments	13. INVESTMENTS Investments include financial investments in life insurance policies, and equity method investments in certain partnerships and limited liability companies. The following table summarizes other investments at December 31: NW Holdings NW Natural In thousands 2018 2017 2018 2017 Investments in life insurance policies $ 49,922 $ 50,792 $ 49,922 $ 50,792 Investments in gas pipeline 13,571 13,669 — — Other 65 1,902 — 1,862 Total other investments $ 63,558 $ 66,363 $ 49,922 $ 52,654 Investment in Life Insurance Policies NW Natural has invested in key person life insurance contracts to provide an indirect funding vehicle for certain long-term employee and director benefit plan liabilities. The amount in the above table is reported at cash surrender value, net of policy loans. Investments in Gas Pipeline TWP, a wholly-owned subsidiary of TWH, is pursuing the development of a new gas transmission pipeline that would provide an interconnection with NW Natural's NGD system. NWN Energy, a wholly-owned subsidiary of NW Holdings, owns 50% of TWH, and 50% is owned by TransCanada American Investments Ltd., an indirect wholly-owned subsidiary of TransCanada Corporation. Variable Interest Entity (VIE) Analysis TWH is a VIE, with NW Holdings' investment in TWP reported under equity method accounting. It has been determined that NW Holdings is not the primary beneficiary of TWH’s activities as it only has a 50% share of the entity, and there are no stipulations that allow NW Holdings a disproportionate influence over it. Investments in TWH and TWP are included in other investments on NW Holdings' balance sheet. If this investment is not developed, then the maximum loss exposure related to TWH is limited to NW Holdings' equity investment balance, less its share of any cash or other assets available to NW Holdings as a 50% owner. The investment balance in TWH was $13.4 million at December 31, 2018 and 2017 . Impairment Analysis Investments in nonconsolidated entities accounted for under the equity method are reviewed for impairment at each reporting period and following updates to our corporate planning assumptions. If it is determined a loss in value is other than temporary, a charge is recognized for the difference between the investment’s carrying value and its estimated fair value. Fair value is based on quoted market prices when available or on the present value of expected future cash flows. Differing assumptions could affect the timing and amount of a charge recorded in any period. In 2011, TWP withdrew its original application with the FERC for a proposed natural gas pipeline in Oregon and informed FERC that it intended to re-file an application to reflect changes in the project scope aligning the project with the region’s current and future gas infrastructure needs. TWP continues working with customers in the Pacific Northwest to further understand their gas transportation needs and determine the commercial support for a revised pipeline proposal. A new FERC certificate application is expected to be filed to reflect a revised scope based on these regional needs. NW Holdings' equity investment was not impaired at December 31, 2018 as the fair value of expected cash flows from planned development exceeded NW Holdings' remaining equity investment of $ 13.4 million at December 31, 2018 . However, if NW Holdings learns that the project is not viable or will not go forward, it could be required to recognize a maximum charge of up to approximately $ 13.4 million based on the current amount of the equity investment, net of cash and working capital at TWP. NW Holdings will continue to monitor and update the impairment analysis as required.

Business Combinations Business

Business Combinations Business Combinations	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Business Combinations	BUSINESS COMBINATIONS Falls Water On September 13, 2018 , NWN Water, then a wholly-owned subsidiary of NW Natural and now a wholly-owned subsidiary of NW Holdings, completed the acquisition of Falls Water Co., Inc. (Falls Water), a privately-owned water utility in the Pacific Northwest for preliminary non-cash consideration of $8.5 million , subject to closing adjustments, in the form of 125,000 shares of NW Natural common stock. Falls Water became a wholly-owned subsidiary of NWN Water and marked its first acquisition in the water services sector. This acquisition aligns with NW Holdings' water sector strategy as the acquisition provides NWN Water entry into Idaho, expands service area, and opens further opportunity for growth. Falls Water is based in Idaho Falls, Idaho and serves approximately 5,300 connections . Through the purchase of all of the outstanding shares of Falls Water, NWN Water acquired the net assets and 100% control of Falls Water. We determined that the Falls Water acquisition met the criteria of a business combination, and as such performed a preliminary allocation of the consideration to the acquired assets and assumed liabilities based on their fair value as of the acquisition date, the majority of which was allocated to goodwill. The allocation is considered preliminary as of December 31, 2018 , and is primarily associated with certain tax positions and goodwill. Subsequent adjustments are not expected to be significant, and any such adjustments are expected to be completed within a one-year measurement period. The acquisition costs were insignificant and were expensed as incurred. The results of Falls Water are not material to the consolidated financial results of NW Holdings. Preliminary goodwill of $6.4 million was recognized from this acquisition and is attributable to Falls Water's regulated service territory and experienced workforce as well as the strategic benefits expected from this high-growth service territory. NW Holdings has included this goodwill in other for segment reporting purposes, and it is not deductible for income tax purposes. No intangible assets aside from goodwill were acquired. See Note 2 for goodwill impairment information. Other Acquisitions During 2018, in addition to the Falls Water acquisition, NWN Water completed three acquisitions qualifying as business combinations. The aggregate fair value of the preliminary consideration transferred for these acquisitions was approximately $2.8 million . These business combinations, both individually and in aggregate, were not significant to NW Holdings' results of operations. As a result of all acquisitions completed, total goodwill was $9.0 million as of December 31, 2018 .

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	DERIVATIVE INSTRUMENTS NW Natural enters into financial derivative contracts to hedge a portion of the NGD segment’s natural gas sales requirements. These contracts include swaps, options, and combinations of option contracts. These derivative financial instruments are primarily used to manage commodity price variability. A small portion of NW Natural's derivative hedging strategy involves foreign currency exchange contracts. NW Natural enters into these financial derivatives, up to prescribed limits, primarily to hedge price variability related to physical gas supply contracts as well as to hedge spot purchases of natural gas. The foreign currency forward contracts are used to hedge the fluctuation in foreign currency exchange rates for pipeline demand charges paid in Canadian dollars. In the normal course of business, NW Natural also enters into indexed-price physical forward natural gas commodity purchase contracts and options to meet the requirements of NGD customers. These contracts qualify for regulatory deferral accounting treatment. NW Natural also enters into exchange contracts related to the third-party asset management of its gas portfolio, some of which are derivatives that do not qualify for hedge accounting or regulatory deferral, but are subject to NW Natural's regulatory sharing agreement. These derivatives are recognized in operating revenues, net of amounts shared with NGD customers. Notional Amounts The following table presents the absolute notional amounts related to open positions on NW Natural derivative instruments: At December 31, In thousands 2018 2017 Natural gas (in therms): Financial 408,850 429,100 Physical 472,275 520,268 Foreign exchange $ 6,936 $ 7,669 Purchased Gas Adjustment (PGA) Derivatives entered into by NW Natural for the procurement or hedging of natural gas for future gas years generally receive regulatory deferral accounting treatment. In general, commodity hedging for the current gas year is completed prior to the start of the gas year, and hedge prices are reflected in the weighted-average cost of gas in the PGA filing. Hedge contracts entered into after the start of the PGA period are subject to the PGA incentive sharing mechanism in Oregon. NW Natural entered the 2018-19 and 2017-18 gas year with forecasted sales volumes hedged at 48% and 49% in financial swap and option contracts, and 24% and 26% in physical gas supplies, respectively. Hedge contracts entered into prior to the PGA filing, in September 2018 , were included in the PGA for the 2018-19 gas year. Hedge contracts entered into after the PGA filing, and related to subsequent gas years, may be included in future PGA filings and qualify for regulatory deferral. Unrealized and Realized Gain/Loss The following table reflects the income statement presentation for the unrealized gains and losses from NW Natural's derivative instruments, which also represents all derivative instruments at NW Holdings: December 31, 2018 December 31, 2017 In thousands Natural gas commodity Foreign exchange Natural gas commodity Foreign exchange Benefit (expense) to cost of gas $ (1,239 ) $ (284 ) $ (26,000 ) $ 107 Operating revenues 1,660 — (1,021 ) — Amounts deferred to regulatory accounts on balance sheet (211 ) 284 26,665 (107 ) Total gain (loss) in pre-tax earnings $ 210 $ — $ (356 ) $ — UNREALIZED GAIN/LOSS. Outstanding derivative instruments related to regulated NGD operations are deferred in accordance with regulatory accounting standards. The cost of foreign currency forward and natural gas derivative contracts are recognized immediately in the cost of gas; however, costs above or below the amount embedded in the current year PGA are subject to a regulatory deferral tariff and therefore, are recorded as a regulatory asset or liability. REALIZED GAIN/LOSS. Net gains of $ 7.4 million and net losses of $7.8 million were realized for the years ended December 31, 2018 and 2017 , respectively, from the settlement of natural gas financial derivative contracts. Realized gains and losses are recorded in cost of gas, deferred through regulatory accounts, and amortized through customer rates in the following year. Credit Risk Management of Financial Derivatives Instruments No collateral was posted with or by NW Natural counterparties as of December 31, 2018 or 2017 . NW Natural attempts to minimize the potential exposure to collateral calls by counterparties to manage liquidity risk. Counterparties generally allow a certain credit limit threshold before requiring NW Natural to post collateral against loss positions. Given NW Natural's counterparty credit limits and portfolio diversification, it was not subject to collateral calls in 2018 or 2017 . The collateral call exposure is set forth under credit support agreements, which generally contain credit limits. NW Natural could also be subject to collateral call exposure where it has agreed to provide adequate assurance, which is not specific as to the amount of credit limit allowed, but could potentially require additional collateral in the event of a material adverse change. Based on current commodity financial swap and option contracts outstanding, which reflect unrealized losses of $ 7.8 million at December 31, 2018 , we have estimated the level of collateral demands, with and without potential adequate assurance calls, using current gas prices and various credit downgrade rating scenarios for NW Natural as follows: Credit Rating Downgrade Scenarios In thousands (Current Ratings) A+/A3 BBB+/Baa1 BBB/Baa2 BBB-/Baa3 Specu-lative With Adequate Assurance Calls $ — $ — $ — $ (3,940 ) $ (6,059 ) Without Adequate Assurance Calls — — — (3,940 ) (4,452 ) NW Natural's financial derivative instruments are subject to master netting arrangements; however, they are presented on a gross basis in NW Natural's consolidated balance sheets. NW Natural and its counterparties have the ability to set-off obligations to each other under specified circumstances. Such circumstances may include a defaulting party, a credit change due to a merger affecting either party, or any other termination event. If netted by counterparty, NW Natural's derivative position would result in an asset of $3.6 million and a liability of $9.3 million as of December 31, 2018 , and an asset of $ 2.9 million and a liability of $ 23.3 million as of December 31, 2017 . NW Natural is exposed to derivative credit and liquidity risk primarily through securing fixed price natural gas commodity swaps to hedge the risk of price increases for natural gas purchases made on behalf of customers. NW Natural utilizes master netting arrangements through International Swaps and Derivatives Association contracts to minimize this risk along with collateral support agreements with counterparties based on their credit ratings. In certain cases, NW Natural requires guarantees or letters of credit from counterparties to meet its minimum credit requirement standards. NW Natural's financial derivatives policy requires counterparties to have a certain investment-grade credit rating at the time the derivative instrument is entered into, and the policy specifies limits on the contract amount and duration based on each counterparty’s credit rating. NW Natural does not speculate with derivatives; instead, derivatives are used to hedge exposure above risk tolerance limits. Any increase in market risk created by the use of derivatives should be offset by the exposures they modify. We actively monitor NW Natural's derivative credit exposure and place counterparties on hold for trading purposes or require other forms of credit assurance, such as letters of credit, cash collateral, or guarantees as circumstances warrant. The ongoing assessment of counterparty credit risk includes consideration of credit ratings, credit default swap spreads, bond market credit spreads, financial condition, government actions, and market news. A Monte-Carlo simulation model is used to estimate the change in credit and liquidity risk from the volatility of natural gas prices. The results of the model are used to establish earnings-at-risk trading limits. NW Natural's credit risk for all outstanding financial derivatives at December 31, 2018 extends to October 31, 2021 . We could become materially exposed to credit risk with one or more of our counterparties if natural gas prices experience a significant increase. If a counterparty were to become insolvent or fail to perform on its obligations, we could suffer a material loss; however, we would expect such a loss to be eligible for regulatory deferral and rate recovery, subject to a prudence review. All of our existing counterparties currently have investment-grade credit ratings. Fair Value In accordance with fair value accounting, non-performance risk is included in calculating fair value adjustments. This includes a credit risk adjustment based on the credit spreads of NW Natural's counterparties when it is in an unrealized gain position, or on NW Natural's own credit spread when it is in an unrealized loss position. The inputs in the valuation models include natural gas futures, volatility, credit default swap spreads, and interest rates. Additionally, the assessment of non-performance risk is generally derived from the credit default swap market and from bond market credit spreads. The impact of the credit risk adjustments for all outstanding derivatives was immaterial to the fair value calculation at December 31, 2018 . As of December 31, 2018 and 2017 , the net fair value was a liability of $ 5.7 million and a liability of $ 20.3 million , respectively, using significant other observable, or Level 2, inputs. No Level 3 inputs were used in the derivative valuations, and there were no transfers between Level 1 or Level 2 during the years ended December 31, 2018 and 2017 .

Commitments and Contigencies

Commitments and Contigencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Leases Land, buildings, and equipment are leased under agreements that expire in various years, including a 99-year land lease that extends through 2108 . Rental costs for continuing operations were $5.9 million , $7.3 million , and $5.9 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively, a portion of which was capitalized. The following table reflects NW Natural's future minimum lease payments due under non-cancelable operating leases for continuing operations at December 31, 2018 . These commitments relate principally to the lease of the office headquarters and underground gas storage facilities. In thousands Minimum lease payments 2019 $ 5,368 2020 4,812 2021 7,077 2022 7,223 2023 7,304 Thereafter 149,881 Total $ 181,665 In October 2017, NW Natural entered into a 20 -year operating lease agreement for a new headquarters in Portland, Oregon in anticipation of the expiration of the current lease in 2020 . Payments under the new lease are expected to commence in 2020 . Total estimated base rent payments over the life of the lease are approximately $160 million and have been included in the table above. There is an option to extend the term of the lease for two additional seven -year periods. Additionally, the lease was analyzed under the lease standard in effect at the time of signing in consideration of build-to-suit lease accounting implications, and NW Natural concluded that it was the accounting owner of the asset during construction. As a result, NW Natural recognized $26.0 million and $0.5 million in property, plant and equipment and an obligation in other non-current liabilities for the same amount in its consolidated balance sheet at December 31, 2018 and 2017 , respectively. Gas Purchase and Pipeline Capacity Purchase and Release Commitments NW Natural has signed agreements providing for the reservation of firm pipeline capacity under which it is required to make fixed monthly payments for contracted capacity. The pricing component of the monthly payment is established, subject to change, by U.S. or Canadian regulatory bodies. In addition, NW Natural has entered into long-term sale agreements to release firm pipeline capacity. NW Natural also enters into short-term and long-term gas purchase agreements. The aggregate amounts of these agreements were as follows at December 31, 2018 : In thousands Gas Pipeline Pipeline 2019 $ 144,500 $ 78,449 $ 4,272 2020 2,776 76,613 3,560 2021 2,313 66,656 — 2022 — 61,075 — 2023 — 60,619 — Thereafter — 580,022 — Total 149,589 923,434 7,832 Less: Amount representing interest 1,314 201,224 183 Total at present value $ 148,275 $ 722,210 $ 7,649 Total payments for fixed charges under capacity purchase agreements were $ 82.6 million for 2018 , $85.3 million for 2017 , and $ 85.0 million for 2016 . Included in the amounts were reductions for capacity release sales of $ 4.3 million for 2018 , $4.5 million for 2017 , and $ 4.5 million for 2016 . In addition, per-unit charges are required to be paid based on the actual quantities shipped under the agreements. In certain take-or-pay purchase commitments, annual deficiencies may be offset by prepayments subject to recovery over a longer term if future purchases exceed the minimum annual requirements. Environmental Matters Refer to Note 17 for a discussion of environmental commitments and contingencies.

Environmental Matters

Environmental Matters	12 Months Ended
	Dec. 31, 2018
Environmental Remediation Obligations [Abstract]
Environmental Matters	ENVIRONMENTAL MATTERS NW Natural owns, or previously owned, properties that may require environmental remediation or action. The range of loss for environmental liabilities is estimated based on current remediation technology, enacted laws and regulations, industry experience gained at similar sites, and an assessment of the probable level of involvement and financial condition of other potentially responsible parties (PRPs). When amounts are prudently expended related to site remediation of those sites described herein, NW Natural has a recovery mechanism in place to collect 96.68% of remediation costs from Oregon customers, and NW Natural is allowed to defer environmental remediation costs allocated to customers in Washington annually until they are reviewed for prudence at a subsequent proceeding. These sites are subject to the remediation process prescribed by the Environmental Protection Agency (EPA) and the Oregon Department of Environmental Quality (ODEQ). The process begins with a remedial investigation (RI) to determine the nature and extent of contamination and then a risk assessment (RA) to establish whether the contamination at the site poses unacceptable risks to humans and the environment. Next, a feasibility study (FS) or an engineering evaluation/cost analysis (EE/CA) evaluates various remedial alternatives. It is at this point in the process when NW Natural is able to estimate a range of remediation costs and record a reasonable potential remediation liability, or make an adjustment to the existing liability. From this study, the regulatory agency selects a remedy and issues a Record of Decision (ROD). After a ROD is issued, NW Natural would seek to negotiate a consent decree or consent judgment for designing and implementing the remedy. NW Natural would have the ability to further refine estimates of remediation liabilities at that time. Remediation may include treatment of contaminated media such as sediment, soil and groundwater, removal and disposal of media, institutional controls such as legal restrictions on future property use, or natural recovery. Following construction of the remedy, the EPA and ODEQ also have requirements for ongoing maintenance, monitoring, and other post-remediation care that may continue for many years. Where appropriate and reasonably known, NW Natural will provide for these costs in the remediation liabilities described below. Due to the numerous uncertainties surrounding the course of environmental remediation and the preliminary nature of several site investigations, in some cases, NW Natural may not be able to reasonably estimate the high end of the range of possible loss. In those cases, the nature of the possible loss has been disclosed, as has the fact that the high end of the range cannot be reasonably estimated where a range of potential loss is available. Unless there is an estimate within the range of possible losses that is more likely than other cost estimates within that range, NW Natural records the liability at the low end of this range. It is likely changes in these estimates and ranges will occur throughout the remediation process for each of these sites due to the continued evaluation and clarification concerning responsibility, the complexity of environmental laws and regulations, and the determination by regulators of remediation alternatives. In addition to remediation costs, NW Natural could also be subject to Natural Resource Damages (NRD) claims. NW Natural will assess the likelihood and probability of each claim and recognize a liability if deemed appropriate. Refer to " Other Portland Harbor " below. Environmental Sites The following table summarizes information regarding liabilities related to environmental sites, which are recorded in other current liabilities and other noncurrent liabilities in NW Natural's balance sheet at December 31: Current Liabilities Non-Current Liabilities In thousands 2018 2017 2018 2017 Portland Harbor site: Gasco/Siltronic Sediments $ 5,117 $ 2,683 $ 44,351 $ 45,346 Other Portland Harbor 2,600 1,949 6,273 4,163 Gasco/Siltronic Upland site 13,983 13,422 44,830 47,835 Central Service Center site 10 25 — — Front Street site 11,402 1,009 3 10,757 Oregon Steel Mills — — 179 179 Total $ 33,112 $ 19,088 $ 95,636 $ 108,280 PORTLAND HARBOR SITE. The Portland Harbor is an EPA listed Superfund site that is approximately 10 miles long on the Willamette River and is adjacent to NW Natural's Gasco uplands sites. NW Natural is one of over one hundred PRPs to the Superfund site. In January 2017, the EPA issued its Record of Decision, which selects the remedy for the clean-up of the Portland Harbor site (Portland Harbor ROD). The Portland Harbor ROD estimates the present value total cost at approximately $1.05 billion with an accuracy between -30% and +50% of actual costs. NW Natural's potential liability is a portion of the costs of the remedy for the entire Portland Harbor Superfund site. The cost of that remedy is expected to be allocated among more than 100 PRPs. In addition, NW Natural is actively pursuing clarification and flexibility under the ROD in order to better understand its obligation under the clean-up. NW Natural is also participating in a non-binding allocation process with the other PRPs in an effort to resolve its potential liability. The Portland Harbor ROD does not provide any additional clarification around allocation of costs among PRPs and, as a result of the issuance of the Portland Harbor ROD, NW Natural has not modified any of the recorded liabilities at this time. NW Natural manages its liability related to the Superfund site as two distinct remediation projects, the Gasco/Siltronic Sediments and Other Portland Harbor projects. Gasco/Siltronic Sediments. In 2009, NW Natural and Siltronic Corporation entered into a separate Administrative Order on Consent with the EPA to evaluate and design specific remedies for sediments adjacent to the Gasco uplands and Siltronic uplands sites. NW Natural submitted a draft EE/CA to the EPA in May 2012 to provide the estimated cost of potential remedial alternatives for this site. At this time, the estimated costs for the various sediment remedy alternatives in the draft EE/CA, for the additional studies and design work needed before the cleanup can occur, and for regulatory oversight throughout the clean-up range from $49.5 million to $350 million . NW Natural has recorded a liability of $ 49.5 million for the sediment clean-up, which reflects the low end of the range. At this time, we believe sediments at this site represent the largest portion of NW Natural's liability related to the Portland Harbor site discussed above. Other Portland Harbor. While we believe liabilities associated with the Gasco/Siltronic sediments site represent NW Natural's largest exposure, there are other potential exposures associated with the Portland Harbor ROD, including NRD costs and harborwide clean-up costs (including downstream petroleum contamination), for which allocations among the PRPs have not yet been determined. NW Natural and other parties have signed a cooperative agreement with the Portland Harbor Natural Resource Trustee council to participate in a phased NRD assessment to estimate liabilities to support an early restoration-based settlement of NRD claims. One member of this Trustee council, the Yakama Nation, withdrew from the council in 2009, and in 2017, filed suit against NW Natural and 29 other parties seeking remedial costs and NRD assessment costs associated with the Portland Harbor, set forth in the complaint. The complaint seeks recovery of alleged costs totaling $0.3 million in connection with the selection of a remedial action for the Portland Harbor as well as declaratory judgment for unspecified future remedial action costs and for costs to assess the injury, loss, or destruction of natural resources resulting from the release of hazardous substances at and from the Portland Harbor site. The Magistrate Judge has recommended granting NW Natural and certain other defendants' motion to stay the case. NW Natural has recorded a liability for NRD claims which is at the low end of the range of the potential liability; the high end of the range cannot be reasonably estimated at this time. The NRD liability is not included in the aforementioned range of costs provided in the Portland Harbor ROD. GASCO UPLANDS SITE. A predecessor of NW Natural, Portland Gas and Coke Company, owned a former gas manufacturing plant that was closed in 1958 (Gasco site) and is adjacent to the Portland Harbor site described above. The Gasco site has been under investigation by NW Natural for environmental contamination under the ODEQ Voluntary Clean-Up Program (VCP). It is not included in the range of remedial costs for the Portland Harbor site noted above. The Gasco site is managed in two parts, the uplands portion and the groundwater source control action. NW Natural submitted a revised Remedial Investigation Report for the uplands to ODEQ in May 2007. In March 2015, ODEQ approved the RA, enabling commencement of work on the FS in 2016. NW Natural has recognized a liability for the remediation of the uplands portion of the site which is at the low end of the range of potential liability; the high end of the range cannot be reasonably estimated at this time. In October 2016, ODEQ and NW Natural agreed to amend their VCP agreement to incorporate a portion of the Siltronic property adjacent to the Gasco site formerly owned by Portland Gas & Coke between 1939 and 1960 into the Gasco RA and FS, excluding the uplands for Siltronic. Previously, NW Natural was conducting an investigation of manufactured gas plant constituents on the entire Siltronic uplands for ODEQ. Siltronic will be working with ODEQ directly on environmental impacts to the remainder of its property. In September 2013, NW Natural completed construction of a groundwater source control system, including a water treatment station, at the Gasco site. NW Natural has estimated the cost associated with the ongoing operation of the system and has recognized a liability which is at the low end of the range of potential costs. NW Natural cannot estimate the high end of the range at this time due to the uncertainty associated with the duration of the operation of the water treatment station, which is highly dependent on the remedy determined for both the upland portion as well as the final remedy for Gasco sediment exposure. OTHER SITES. In addition to those sites above, NW Natural has environmental exposures at three other sites: Central Service Center, Front Street, and Oregon Steel Mills. NW Natural may have exposure at other sites that have not been identified at this time. Due to the uncertainty of the design of remediation, regulation, timing of the remediation, and in the case of the Oregon Steel Mills site, pending litigation, liabilities for each of these sites have been recognized at their respective low end of the range of potential liability; the high end of the range cannot be reasonably estimated at this time. Central Service Center site. NW Natural is currently performing an environmental investigation of the property under ODEQ's Independent Cleanup Pathway. This site is on ODEQ's list of sites with confirmed releases of hazardous substances, and cleanup is necessary. Front Street site. The Front Street site was the former location of a gas manufacturing plant NW Natural operated (the former Portland Gas Manufacturing site, or PGM). At ODEQ’s request, NW Natural conducted a sediment and source control investigation and provided findings to ODEQ. In December 2015, a FS on the former Portland Gas Manufacturing site was completed. In July 2017, ODEQ issued the PGM ROD. The ROD specifies the selected remedy, which requires a combination of dredging, capping, treatment, and natural recovery. In addition, the selected remedy also requires institutional controls and long-term inspection and maintenance. NW Natural revised the liability in the second quarter of 2017 to incorporate the estimated undiscounted cost of approximately $10.5 million for the selected remedy. Further, NW Natural has recognized an additional liability of $0.9 million for additional studies and design costs as well as regulatory oversight throughout the clean-up. NW Natural plans to complete the remedial design in early 2019 and expects to construct the remedy during 2019. Oregon Steel Mills site. Refer to the “Legal Proceedings,” below. Site Remediation and Recovery Mechanism (SRRM) NW Natural has an SRRM through which it tracks and has the ability to recover past deferred and future prudently incurred environmental remediation costs allocable to Oregon, subject to an earnings test, for those sites identified therein. In the February 2015 Order establishing the SRRM (2015 Order), the OPUC addressed outstanding issues related to the SRRM, which required NW Natural to forego the collection of $15 million out of approximately $95 million in total environmental remediation expenses and associated carrying costs. As a follow-up to the 2015 Order, the OPUC issued an additional Order in January 2016 (2016 Order) regarding the SRRM implementation in which the OPUC: (1) disallowed the recovery of $2.8 million of interest earned on the previously disallowed environmental expenditure amounts; (2) clarified the state allocation of 96.68% of environmental remediation costs for all environmental sites allocable to Oregon; and (3) confirmed NW Natural's treatment of $13.8 million of expenses put into the SRRM amortization account was correct and in compliance with prior OPUC orders. As a result of the 2016 Order, NW Natural recognized a $3.3 million non-cash charge in the first quarter, of which $2.8 million is reflected in other income and expense, net and $0.5 million is included in operations and maintenance expense. COLLECTIONS FROM OREGON CUSTOMERS. Under the SRRM collection process there are three types of deferred environmental remediation expense: • Pre-review - This class of costs represents remediation spend that has not yet been deemed prudent by the OPUC. Carrying costs on these remediation expenses are recorded at NW Natural's authorized cost of capital. NW Natural anticipates the prudence review for annual costs and approval of the earnings test prescribed by the OPUC to occur by the third quarter of the following year. • Post-review - This class of costs represents remediation spend that has been deemed prudent and allowed after applying the earnings test, but is not yet included in amortization. NW Natural earns a carrying cost on these amounts at a rate equal to the five-year treasury rate plus 100 basis points. • Amortization - This class of costs represents amounts included in current customer rates for collection and is generally calculated as one-fifth of the post-review deferred balance. NW Natural earns a carrying cost equal to the amortization rate determined annually by the OPUC, which approximates a short-term borrowing rate. In addition to the collection amount noted above, the Order also provides for the annual collection of $5.0 million from Oregon customers through a tariff rider. As NW Natural collects amounts from customers, it recognizes these collections as revenue and separately amortizes an equal and offsetting amount of its deferred regulatory asset balance through the environmental remediation operating expense line shown separately in the operating expense section of the income statement. NW Natural received total environmental insurance proceeds of approximately $150.0 million as a result of settlements from litigation that was dismissed in July 2014. Under the 2015 OPUC Order, one-third of the Oregon allocated proceeds were applied to costs deferred through 2012 with the remaining two-thirds applied to costs at a rate of $5.0 million per year plus interest over the following 20 years. NW Natural accrues interest on the insurance proceeds in the customer’s favor at a rate equal to the five-year treasury rate plus 100 basis points. As of December 31, 2018 , NW Natural has applied $73.2 million of insurance proceeds to prudently incurred remediation costs allocated to Oregon. The following table presents information regarding the total regulatory asset deferred as of December 31: In thousands 2018 2017 Deferred costs and interest (1) $ 41,883 $ 45,546 Accrued site liabilities (2) 128,369 126,950 Insurance proceeds and interest (88,502 ) (94,170 ) Total regulatory asset deferral (1) $ 81,750 $ 78,326 Current regulatory assets (3) 5,601 6,198 Long-term regulatory assets (3) 76,149 72,128 (1) Includes pre-review and post-review deferred costs, amounts currently in amortization, and interest, net of amounts collected from customers. (2) Excludes 3.32% of the Front Street site liability, or $0.4 million in 2018 and $0.4 million in 2017 , as the OPUC only allows recovery of 96.68% of costs for those sites allocable to Oregon, including those that historically served only Oregon customers. (3) Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, NW Natural earns a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. NW Natural also accrues a carrying charge on insurance proceeds for amounts owed to customers. In Washington, a carrying charge related to deferred amounts will be determined in a future proceeding. Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5.0 million tariff rider. The amounts allocable to Oregon are recoverable through NGD rates, subject to an earnings test. ENVIRONMENTAL EARNINGS TEST. To the extent NW Natural earns at or below its authorized Return on Equity (ROE), remediation expenses and interest in excess of the $5.0 million tariff rider and $5.0 million insurance proceeds are recoverable through the SRRM. To the extent NW Natural earns more than its authorized ROE in a year, it is required to cover environmental expenses and interest on expenses greater than the $10.0 million with those earnings that exceed its authorized ROE. Under the 2015 Order, the OPUC stated they would revisit the deferral and amortization of future remediation expenses, as well as the treatment of remaining insurance proceeds three years from the original Order, or earlier if NW Natural gains greater certainty about its future remediation costs, to consider whether adjustments to the mechanism may be appropriate. NW Natural filed an update with the OPUC in March 2018 and recommended no changes. WASHINGTON DEFERRAL. In Washington, cost recovery and carrying charges on amounts deferred for costs associated with services provided to Washington customers will be determined in a future proceeding. Legal Proceedings NW Holdings is not currently party to any direct claims or litigation, though in the future it may be subject to claims and litigation arising in the ordinary course of business. NW Natural is subject to claims and litigation arising in the ordinary course of business. Although the final outcome of any of these legal proceedings cannot be predicted with certainty, including the matter described below, NW Natural and NW Holdings do not expect that the ultimate disposition of any of these matters will have a material effect on financial condition, results of operations, or cash flows. OREGON STEEL MILLS SITE. In 2004, NW Natural was served with a third-party complaint by the Port of Portland (the Port) in a Multnomah County Circuit Court case, Oregon Steel Mills, Inc. v. The Port of Portland. The Port alleges that in the 1940s and 1950s petroleum wastes generated by NW Natural's predecessor, Portland Gas & Coke Company, and 10 other third-party defendants, were disposed of in a waste oil disposal facility operated by the United States or Shaver Transportation Company on property then owned by the Port and now owned by Evraz Oregon Steel Mills. The complaint seeks contribution for unspecified past remedial action costs incurred by the Port regarding the former waste oil disposal facility as well as a declaratory judgment allocating liability for future remedial action costs. No date has been set for trial. In August 2017, the case was stayed pending outcome of the Portland Harbor allocation process or other mediation. Although the final outcome of this proceeding cannot be predicted with certainty, NW Natural and NW Holdings do not expect the ultimate disposition of this matter will have a material effect on NW Natural's or NW Holdings' financial condition, results of operations, or cash flows. For additional information regarding other commitments and contingencies, see Note 16 .

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2018
Northwest Natural Gas Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operations	DISCONTINUED OPERATIONS NW Holdings On June 20, 2018, NWN Gas Storage, then a wholly-owned subsidiary of NW Natural, entered into a Purchase and Sale Agreement (the Agreement) that provides for the sale by NWN Gas Storage of all of the membership interests in Gill Ranch. Gill Ranch owns a 75% interest in the natural gas storage facility located near Fresno, California known as the Gill Ranch Gas Storage Facility. PG&E owns the remaining 25% interest in the Gill Ranch Gas Storage Facility. The CPUC regulates Gill Ranch under a market-based rate model which allows for the price of storage services to be set by the marketplace. The CPUC also regulates the issuance of securities, system of accounts, and regulates intrastate storage services. The Agreement provides for an initial cash purchase price of $25.0 million (subject to a working capital adjustment), plus potential additional payments to NWN Gas Storage of up to $26.5 million in the aggregate if Gill Ranch achieves certain economic performance levels for the first three full gas storage years (April 1 of one year through March 31 of the following year) occurring after the closing and the remaining portion of the gas storage year during which the closing occurs. We expect the transaction to close in 2019. The closing of the transaction is subject to approval by the CPUC, satisfaction of representations, warranties and covenants of the Agreement, and other customary closing conditions. In July 2018, Gill Ranch filed an application with the CPUC for approval of this transaction. On February 14, 2019, the active parties to the CPUC proceeding filed a settlement agreement with the CPUC. The CPUC is expected to rule on the settlement agreement within 90 days of its filing, but may grant further time for public comment. We expect an order on this matter by the end of June. As a result of the strategic shift away from the California gas storage market and the significance of Gill Ranch's financial results in 2017, we concluded that the pending sale of Gill Ranch qualified it as assets and liabilities held for sale and discontinued operations. As such, the assets and liabilities associated with Gill Ranch have been classified as discontinued operations assets and discontinued operations liabilities, respectively, and, the results of Gill Ranch are presented, net of tax, as discontinued operations separate from the results of continuing operations for all periods presented. The expenses included in the results of discontinued operations are the direct operating expenses incurred by Gill Ranch that may be reasonably segregated from the costs of our continuing operations. The following table presents the carrying amounts of the major components of Gill Ranch that are classified as discontinued operations assets and liabilities on NW Holdings' consolidated balance sheets: NW Holdings Discontinued Operations In thousands 2018 2017 Assets: Accounts receivable $ 390 $ 2,126 Inventories 685 396 Other current assets 333 535 Property, plant, and equipment 11,621 10,816 Less: Accumulated depreciation 7 — Other non-current assets 247 1 Discontinued operations - current assets 13,269 3,057 Discontinued operations - non-current assets — 10,817 Total discontinued operations assets $ 13,269 $ 13,874 Liabilities: Accounts payable $ 873 $ 1,287 Other current liabilities 307 306 Other non-current liabilities 11,779 12,043 Discontinued operations - current liabilities 12,959 1,593 Discontinued operations - non-current liabilities — 12,043 Total discontinued operations liabilities $ 12,959 $ 13,636 (1) The total assets and liabilities of Gill Ranch are classified as current as of December 31, 2018 because it is probable that the sale will be completed within one year. The following table presents the operating results of Gill Ranch, which was historically reported within the gas storage segment, and is presented net of tax on NW Holdings' consolidated statements of comprehensive income: NW Holdings Discontinued Operations In thousands, except per share data 2018 2017 2016 Revenues $ 3,579 $ 7,135 $ 7,794 Expenses Operations and maintenance 5,771 7,245 6,643 General taxes 479 1,373 1,295 Depreciation and amortization 430 4,525 4,685 Other expenses and interest 609 975 992 Impairment expense — 192,478 — Total expenses 7,289 206,596 13,615 Loss from discontinued operations before income tax (3,710 ) (199,461 ) (5,821 ) Income tax benefit (1) (968 ) (71,765 ) (2,297 ) Loss from discontinued operations, net of tax $ (2,742 ) $ (127,696 ) $ (3,524 ) Loss from discontinued operations per share of common stock: Basic $ (0.10 ) $ (4.45 ) $ (0.13 ) Diluted $ (0.09 ) $ (4.44 ) $ (0.13 ) (1) 2017 income tax benefit includes approximately $18 million of tax benefit from the enactment of the TCJA. The TCJA was enacted December 22, 2017 and resulted in the federal tax rate changing from 35% to 21% . NW Natural As part of the holding company reorganization in October 2018, NWN Energy, NWN Gas Storage, Gill Ranch, NNG Financial, NWN Water, and NW Holdings, which were direct and indirect subsidiaries of NW Natural prior to the reorganization, are no longer subsidiaries of NW Natural. See Note 1 for additional information. As a result, NW Natural's financial statements reflect amounts related to these entities as discontinued operations for all periods presented. The expenses included in the results of discontinued operations are the direct operating expenses incurred by the entities that may be reasonably segregated from the costs of NW Natural's continuing operations. The following table presents the carrying amounts of the major components of NWN Energy, NWN Gas Storage, Gill Ranch, NNG Financial, NWN Water, and NW Holdings that are classified as discontinued operations assets and liabilities on NW Natural's consolidated balance sheets: NW Natural Discontinued Operations In thousands 2017 Assets: Cash $ 362 Accounts receivable 2,126 Intercompany receivables 3,664 Inventories 396 Other current assets 622 Property, plant, and equipment 11,191 Less: Accumulated depreciation 192 Other investments 13,710 Other non-current assets — Discontinued operations - current assets 7,170 Discontinued operations - non-current assets 24,709 Total discontinued operations assets $ 31,879 Liabilities: Accounts payable $ 1,954 Intercompany payables 266 Other current liabilities 345 Deferred tax liabilities (16,862 ) Other non-current liabilities 12,130 Discontinued operations - current liabilities 2,565 Discontinued operations - non-current liabilities (4,732 ) Total discontinued operations liabilities $ (2,167 ) The following table presents the operating results prior to the holding company reorganization effective October 1, 2018 of NWN Energy, NWN Gas Storage, Gill Ranch, NNG Financial, NWN Water, and NW Holdings, which were historically reported within the gas storage segment and other, and is presented net of tax on NW Natural's consolidated statements of comprehensive income: NW Natural Discontinued Operations In thousands, except per share data 2018 2017 2016 Revenues $ 3,016 $ 7,360 $ 8,018 Expenses Operations and maintenance 4,151 7,423 7,387 General taxes 448 1,410 1,317 Depreciation and amortization 420 4,555 4,714 Other expenses and interest 342 650 1,097 Impairment expense — 192,478 — Total expenses 5,361 206,516 14,515 Loss from discontinued operations before income tax (2,345 ) (199,156 ) (6,497 ) Income tax benefit (1) (622 ) (71,813 ) (2,557 ) Loss from discontinued operations, net of tax $ (1,723 ) $ (127,343 ) $ (3,940 ) (1) 2017 income tax benefit includes approximately $18 million of tax benefit from the enactment of the TCJA. The TCJA was enacted December 22, 2017 and resulted in the federal tax rate changing from 35% to 21% .

Quarterly Financial Information

Quarterly Financial Information	12 Months Ended
	Dec. 31, 2018
Schedule Of Quarterly Financial Information [Abstract]
Quarterly Financial Information [Text Block]	NORTHWEST NATURAL HOLDING COMPANY QUARTERLY FINANCIAL INFORMATION (UNAUDITED) NW Holdings Quarter ended (1) In thousands, except per share data March 31 June 30 September 30 December 31 2018 Operating revenues $ 263,635 $ 124,567 $ 91,239 $ 226,702 Net income (loss) from continuing operations 42,011 (339 ) (11,144 ) 36,783 Loss from discontinued operations, net of tax (474 ) (659 ) (650 ) (959 ) Net income (loss) 41,537 (998 ) (11,794 ) 35,824 Average common shares outstanding: Basic 28,753 28,791 28,815 28,851 Diluted 28,803 28,791 28,815 28,940 Earnings (loss) from continuing operations per share of common stock: Basic 1.46 (0.01 ) (0.39 ) 1.27 Diluted 1.46 (0.01 ) (0.39 ) 1.27 Loss from discontinued operations per share of common stock: Basic (0.02 ) (0.02 ) (0.02 ) (0.03 ) Diluted (0.02 ) (0.02 ) (0.02 ) (0.03 ) Earnings (loss) per share of common stock: Basic 1.44 (0.03 ) (0.41 ) 1.24 Diluted 1.44 (0.03 ) (0.41 ) 1.24 2017 Operating revenues $ 295,724 $ 134,476 $ 86,212 $ 238,626 Net income (loss) from continuing operations 41,397 4,075 (7,887 ) 34,488 Loss from discontinued operations, net of tax (1,087 ) (1,346 ) (608 ) (124,655 ) Net income (loss) 40,310 2,729 (8,495 ) (90,167 ) Average common shares outstanding: Basic 28,633 28,648 28,678 28,716 Diluted 28,723 28,717 28,678 28,797 Earnings (loss) from continuing operations per share of common stock: Basic 1.45 0.14 (0.28 ) 1.20 Diluted 1.44 0.14 (0.28 ) 1.20 Loss from discontinued operations per share of common stock: Basic (0.04 ) (0.04 ) (0.02 ) (4.34 ) Diluted (0.04 ) (0.04 ) (0.02 ) (4.33 ) Earnings (loss) per share of common stock: Basic 1.41 0.10 (0.30 ) (3.14 ) Diluted 1.40 0.10 (0.30 ) (3.13 ) (1) Quarterly earnings (loss) per share are based upon the average number of common shares outstanding during each quarter. Variations in earnings between quarterly periods are due primarily to the seasonal nature of our business. NORTHWEST NATURAL GAS COMPANY QUARTERLY FINANCIAL INFORMATION (UNAUDITED) NW Natural Quarter ended In thousands March 31 June 30 September 30 December 31 2018 Operating revenues $ 263,635 $ 124,563 $ 91,227 $ 226,146 Net income (loss) from continuing operations 42,014 (271 ) (11,275 ) 37,581 Loss from discontinued operations, net of tax (477 ) (727 ) (519 ) — Net income (loss) 41,537 (998 ) (11,794 ) 37,581 2017 Operating revenues $ 295,668 $ 134,420 $ 86,157 $ 238,793 Net income (loss) from continuing operations 41,438 4,072 (7,876 ) 34,086 Loss from discontinued operations, net of tax (1,128 ) (1,343 ) (619 ) (124,253 ) Net income (loss) 40,310 2,729 (8,495 ) (90,167 )

Condensed Financials Standalone

Condensed Financials Standalone Holdco Condensed Financials Standalone Holdco (Notes)	12 Months Ended
	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Condensed Financial Information of Parent Company Only Disclosure [Text Block]	SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF NORTHWEST NATURAL HOLDING COMPANY NORTHWEST NATURAL HOLDING COMPANY CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (PARENT COMPANY ONLY) In thousands Inception through December 31, 2018 Operating expenses: Operations and maintenance $ 838 Total operating expenses 838 Loss from operations (838 ) Earnings from investment in subsidiaries, net of tax 36,469 Other income (expense), net 36 Interest expense, net 53 Income before income taxes 35,614 Income tax expense (benefit) (225 ) Net income $ 35,839 See Notes to Condensed Financial Statements NORTHWEST NATURAL HOLDING COMPANY CONDENSED BALANCE SHEETS (PARENT COMPANY ONLY) As of December 31, In thousands 2018 Assets: Current assets: Cash and cash equivalents $ 4,011 Receivables from affiliates 2,796 Income taxes receivable 6,000 Other current assets 3,078 Total current assets 15,885 Non-current assets: Investments in subsidiaries 754,971 Other investments 65 Other non-current assets 310 Total non-current assets 755,346 Total assets $ 771,231 Liabilities and equity: Current liabilities: Accounts payable 168 Payables to affiliates 9,166 Interest accrued 32 Total current liabilities 9,366 Long-term debt (1 ) Deferred credits and other non-current liabilities: Deferred tax liabilities 7 Total deferred credits and other non-current liabilities 7 Equity: Common stock 739,722 Retained earnings 22,137 Total equity 761,859 Total liabilities and equity $ 771,231 See Notes to Condensed Financial Statements NORTHWEST NATURAL HOLDING COMPANY CONDENSED STATEMENTS OF CASH FLOWS (PARENT COMPANY ONLY) In thousands Inception through December 31, 2018 Operating activities: Net income $ 35,839 Adjustments to reconcile net income to cash used in operations: Equity in earnings of subsidiaries, net of tax (36,469 ) Deferred income taxes 7 Other 15 Changes in assets and liabilities: Receivables, net (585 ) Income and other taxes (9,034 ) Accounts payable 9,304 Interest accrued 32 Other, net (44 ) Cash used in operating activities (935 ) Investing activities: Contributions to subsidiaries (1,804 ) Cash used in investing activities (1,804 ) Financing activities: Cash dividend payments on common stock (12,923 ) Capital contributions 20,000 Other (327 ) Cash provided by financing activities 6,750 Increase in cash and cash equivalents 4,011 Cash and cash equivalents, beginning of period — Cash and cash equivalents, end of period $ 4,011 See Notes to Condensed Financial Statements NOTES TO CONDENSED FINANCIAL STATEMENTS 1. BASIS OF PRESENTATION NW Holdings is an energy services holding company that conducts substantially all of its business operations through its subsidiaries, particularly NW Natural. These condensed financial statements and related footnotes have been prepared in accordance with Rule 12-04, Schedule I of Regulation S-X. These financial statements, in which NW Holdings' subsidiaries have been included using the equity method, should be read in conjunction with the consolidated financial statements and notes thereto of NW Holdings included in Item 8 of this Form 10-K. Equity earnings of subsidiaries included earnings from NW Natural of $36.5 million for the year ended December 31, 2018. 2. DEBT For information concerning NW Holdings' debt obligations, see Note 8 to the consolidated financial statements included in Item 8 of this report.

Valuation Allowances and Reserv

Valuation Allowances and Reserves	12 Months Ended
	Dec. 31, 2018
Entity Information [Line Items]
Schedule of Valuation and Qualifying Accounts	NORTHWEST NATURAL HOLDING COMPANY SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES COLUMN A COLUMN B COLUMN C COLUMN D COLUMN E Additions Deductions In thousands (year ended December 31) Balance at beginning of period Charged to costs and expenses Charged to other accounts Net write-offs Balance at end of period 2018 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 956 $ 680 $ — $ 659 $ 977 2017 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 1,290 $ 865 $ — $ 1,199 $ 956 2016 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 870 $ 1,246 $ — $ 826 $ 1,290 NORTHWEST NATURAL GAS COMPANY SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES COLUMN A COLUMN B COLUMN C COLUMN D COLUMN E Additions Deductions In thousands (year ended December 31) Balance at beginning of period Charged to costs and expenses Charged to other accounts Net write-offs Balance at end of period 2018 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 956 $ 678 $ — $ 659 $ 975 2017 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 1,290 $ 865 $ — $ 1,199 $ 956 2016 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 870 $ 1,246 $ — $ 826 $ 1,290

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America (U.S. GAAP) requires management to make estimates and assumptions that affect reported amounts in the consolidated financial statements and accompanying notes. Actual amounts could differ from those estimates, and changes would most likely be reported in future periods. Management believes the estimates and assumptions used are reasonable .
Industry Regulation	Industry Regulation NW Holdings' principal business is to operate as a holding company for NW Natural, NWN Water and its other subsidiaries. NW Natural's principal business is the distribution of natural gas, which is regulated by the OPUC and WUTC. NW Natural also has natural gas storage services, which are regulated by the FERC, and to a certain extent by the OPUC and WUTC. Additionally, certain NW Holdings' subsidiaries own water businesses, which are regulated by the OPUC, WUTC, or IPUC. Accounting records and practices of the regulated businesses conform to the requirements and uniform system of accounts prescribed by these regulatory authorities in accordance with U.S. GAAP. The businesses in which customer rates are regulated by the OPUC, WUTC, IPUC, and FERC have approved cost-based rates which are intended to allow such businesses to earn a reasonable return on invested capital. In applying regulatory accounting principles, we capitalize or defer certain costs and revenues as regulatory assets and liabilities pursuant to orders of the OPUC, WUTC, or IPUC, which provide for the recovery of revenues or expenses from, or refunds to, utility customers in future periods, including a return or a carrying charge in certain cases. At December 31, NW Natural deferred the following amounts as regulatory assets and liabilities: Regulatory Assets In thousands 2018 2017 Current: Unrealized loss on derivatives (1) $ 12,381 $ 18,712 Gas costs 2,873 154 Environmental costs (2) 5,601 6,198 Decoupling (3) 9,140 11,227 Income taxes 2,218 2,218 Other (4) 9,717 7,272 Total current $ 41,930 $ 45,781 Non-current: Unrealized loss on derivatives (1) $ 3,025 $ 4,649 Pension balancing (5) 74,173 60,383 Income taxes 19,185 19,991 Pension and other postretirement benefit liabilities 174,993 179,824 Environmental costs (2) 76,149 72,128 Gas costs 9,978 84 Decoupling (3) 2,545 3,970 Other (4) 11,738 15,579 Total non-current $ 371,786 $ 356,608 Regulatory Liabilities In thousands 2018 2017 Current: Gas costs $ 17,182 $ 14,886 Unrealized gain on derivatives (1) 8,740 1,674 Decoupling (3) 2,264 322 Other (4) 19,250 17,131 Total current $ 47,436 $ 34,013 Non-current: Gas costs $ 552 $ 4,630 Unrealized gain on derivatives (1) 725 1,306 Decoupling (3) — 957 Income taxes (6) 225,408 213,306 Accrued asset removal costs (7) 380,464 360,929 Other (4) 4,411 4,965 Total non-current $ 611,560 $ 586,093 (1) Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through natural gas distribution rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement. (2) Refer to footnote (3) of the Deferred Regulatory Asset table in Note 17 for a description of environmental costs. (3) This deferral represents the margin adjustment resulting from differences between actual and expected volumes. (4) Balances consist of deferrals and amortizations under approved regulatory mechanisms and typically earn a rate of return or carrying charge. (5) Refer to footnote (1) of the Net Periodic Benefit Cost table in Note 9 for information regarding the deferral of pension expenses. (6) This balance represents estimated amounts associated with the Tax Cuts and Jobs Act. See Note 10 . (7) Estimated costs of removal on certain regulated properties are collected through rates. See "Accounting Policies— Plant, Property, and Accrued Asset Removal Costs " below. The amortization period for NW Natural's regulatory assets and liabilities ranges from less than one year to an indeterminable period. Regulatory deferrals for gas costs payable are generally amortized over 12 months beginning each November 1 following the gas contract year during which the deferred gas costs are recorded. Similarly, most other regulatory deferred accounts are amortized over 12 months . However, certain regulatory account balances, such as income taxes, environmental costs, pension liabilities, and accrued asset removal costs, are large and tend to be amortized over longer periods once NW Natural has agreed upon an amortization period with the respective regulatory agency. We believe all costs incurred and deferred at December 31, 2018 are prudent. All regulatory assets and liability are reviewed annually for recoverability, or more often if circumstances warrant. If it is determined that all or a portion of these regulatory assets or liabilities no longer meet the criteria for continued application of regulatory accounting, then NW Natural would be required to write-off the net unrecoverable balances in the period such determination is made. Environmental Regulatory Accounting See Note 17 for information about the SRRM and OPUC orders regarding implementation.
New Accounting Standards	New Accounting Standards NW Holdings and NW Natural consider the applicability and impact of all accounting standards updates (ASUs) issued by the Financial Accounting Standards Board (FASB). ASUs not listed below were assessed and determined to be either not applicable or are expected to have minimal impact on the consolidated financial position or results of operations. Recently Adopted Accounting Pronouncements STOCK COMPENSATION. On May 10, 2017, the FASB issued ASU 2017-09, "Stock Compensation - Scope of Modification Accounting." The purpose of the amendment is to provide clarity, reduce diversity in practice, and reduce the cost and complexity when applying the guidance in Topic 718, related to a change to the terms or conditions of a share-based payment award. Specifically, an entity would not apply modification accounting if the fair value, vesting conditions, and classification of the awards are the same immediately before and after the modification. The amendments in this update were effective beginning January 1, 2018, and will be applied prospectively to any award modified on or after the adoption date. The adoption did not have a material impact to the financial statements or disclosures of NW Holdings or NW Natural. RETIREMENT BENEFITS. On March 10, 2017, the FASB issued ASU 2017-07, "Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost." The ASU requires entities to disaggregate current service cost from the other components of net periodic benefit cost and present it with other current compensation costs for related employees in the income statement. Additionally, the other components of net periodic benefit costs are to be presented elsewhere in the income statement and outside of income from operations if that subtotal is presented. Only the service cost component of the net periodic benefit cost is eligible for capitalization. The amendments in this update were effective beginning January 1, 2018. Upon adoption, the ASU required that changes to the income statement presentation of net periodic benefit cost be applied retrospectively, while changes to amounts capitalized must be applied prospectively. As such, the interest cost, expected return on assets, amortization of prior service costs, and other costs have been reclassified from operations and maintenance expense to other income (expense), net on the consolidated statements of comprehensive income for the years ended December 31, 2017 and 2016. We did not elect the practical expedient which would have allowed for the reclassification of amounts disclosed previously in the pension and other postretirement benefits footnote disclosure as the basis for applying retrospective presentation. As mentioned above, on a prospective basis, the other components of net periodic benefit cost will not be eligible for capitalization. The retrospective presentation requirement related to the other components of net periodic benefit cost affected the operations and maintenance expense and other income (expense), net lines on the NW Natural consolidated statements of comprehensive income. For the years ended December 31, 2017 and 2016, $5.6 million and $6.6 million of expense was reclassified from operations and maintenance expense and included in other income (expense), net, respectively. GOODWILL. On January 26, 2017, the FASB issued ASU 2017-04, "Simplifying the Test for Goodwill Impairment." The ASU removes Step 2 from the goodwill impairment test and under the amended guidance an entity should perform its annual goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the amount in which the carrying amounts exceed the fair value of the reporting unit. The amendments in this standard are effective beginning January 1, 2020 and early adoption is permitted for interim or annual goodwill impairment tests performed after January 1, 2017. ASU 2017-04 was early adopted in the third quarter ended September 30, 2018. The adoption of this ASU did not materially affect the financial statements or disclosures of NW Holdings or NW Natural and is currently not applicable to NW Natural. STATEMENT OF CASH FLOWS. On August 26, 2016, the FASB issued ASU 2016-15, "Classification of Certain Cash Receipts and Cash Payments." The ASU adds guidance pertaining to the classification of certain cash receipts and payments on the statement of cash flows. The purpose of the amendment is to clarify issues that have been creating diversity in practice. The amendments in this standard were effective beginning January 1, 2018, and the adoption did not have a material impact to financial statements or disclosures as our historical practices and presentation were consistent with the directives of this ASU for NW Holdings and NW Natural. FINANCIAL INSTRUMENTS. On January 5, 2016, the FASB issued ASU 2016-01, "Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities." The ASU enhances the reporting model for financial instruments, which includes amendments to address aspects of recognition, measurement, presentation, and disclosure. The new standard was effective beginning January 1, 2018, and the adoption did not materially impact the financial statements or disclosures of NW Holdings or NW Natural. REVENUE RECOGNITION. On May 28, 2014, the FASB issued ASU 2014-09 "Revenue From Contracts with Customers." The underlying principle of the guidance requires entities to recognize revenue depicting the transfer of goods or services to customers at amounts the entity is expected to be entitled to in exchange for those goods or services. The ASU also prescribes a five-step approach to revenue recognition: (1) identify the contract(s) with the customer; (2) identify the separate performance obligations in the contract(s); (3) determine the transaction price; (4) allocate the transaction price to separate performance obligations; and (5) recognize revenue when, or as, each performance obligation is satisfied. The guidance also requires additional disclosures, both qualitative and quantitative, regarding the nature, amount, timing and uncertainty of revenue and cash flows. The new accounting standard and all related amendments were effective beginning January 1, 2018. The accounting standard was applied to all contracts using the modified retrospective method. The new standard is primarily reflected in the consolidated statements of comprehensive income and Note 6 . The implementation of the new revenue standard did not result in changes to how NW Holdings and NW Natural currently recognize revenue, and therefore, no cumulative effect or adjustment to the opening balances of retained earnings was required. The implementation did result in changes to the disclosures and presentation of revenues and expenses. The comparative information for prior years has not been restated. There is no material impact to the financial results of NW Holdings or NW Natural and no significant changes to our control environment due to the adoption of the new revenue standard on an ongoing basis. As previously discussed, the adoption of the new revenue standard did not impact the consolidated balance sheets or statements of cash flows but did result in changes to the presentation of the consolidated statements of comprehensive income for NW Holdings and NW Natural. Had the adoption of the new revenue standard not occurred, operating revenues for the year ended December 31, 2018 would have been $676.0 million for NW Holdings, compared to the reported amount of $706.1 million under the new revenue standard. Similarly, absent the impact of the new revenue standard, operating expenses would have been $543.9 million for NW Holdings, compared to the reported amount of $574.0 million under the new revenue standard for the year ended December 31, 2018. The effect of the change was an increase in both operating revenues and operating expenses of $30.1 million at NW Holdings and NW Natural for the year ended December 31, 2018; due to the change in presentation of revenue taxes. As part of the adoption of the new revenue standard, we evaluated the presentation of revenue taxes under the new guidance and across our peer group and concluded that the gross presentation of revenue taxes provides the greatest level of consistency and transparency. Prior to the adoption of the new revenue standard, a portion of revenue taxes was presented net in operating revenues and a portion was recorded directly on the balance sheet. During year ended December 31, 2018, $30.1 million in revenue taxes for NW Holdings and NW Natural was recognized in operating revenues and operating expenses. In comparison, for the years ended December 31, 2017 and 2016, $32.2 million and $28.3 million was recognized in revenue taxes for NW Holdings and NW Natural, of which $19.1 million and $17.1 million were recorded in operating revenues and $13.1 million and $11.2 million were recorded on the consolidated balance sheets, respectively. The change in presentation of revenue taxes had no impact on NGD margin, net income or earnings per share. Recently Issued Accounting Pronouncements CLOUD COMPUTING. On August 29, 2018, the FASB issued ASU 2018-15, "Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract." The purpose of the amendment is to align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted. The amended guidance can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's financial statements and disclosures. RETIREMENT BENEFITS. On August 28, 2018, the FASB issued ASU 2018-14, "Changes to the Disclosure Requirements for Defined Benefit Plans." The purpose of the amendment is to modify the disclosure requirements for defined benefit pension and other postretirement plans. The amendments in this update are effective for the year ended December 31, 2020. Early adoption is permitted. The amended presentation and disclosure guidance should be applied retrospectively. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's disclosures. FAIR VALUE MEASUREMENT. On August 28, 2018, the FASB issued ASU 2018-13, "Changes to the Disclosure Requirements for Fair Value Measurement." The purpose of the amendment is to modify the disclosure requirements for fair value measurements. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements and the narrative description of measurement uncertainty should be applied prospectively. All other amendments should be applied retrospectively. We are currently assessing the effect of this standard on NW Holdings' and NW Natural's disclosures. ACCUMULATED OTHER COMPREHENSIVE INCOME. On February 14, 2018, the FASB issued ASU 2018-02, "Income Statement—Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." This update was issued in response to concerns from certain stakeholders regarding the current requirements under U.S. GAAP that deferred tax assets and liabilities are adjusted for a change in tax laws or rates, and the effect is to be included in income from continuing operations in the period of the enactment date. This requirement is also applicable to items in accumulated other comprehensive income where the related tax effects were originally recognized in other comprehensive income. The adjustment of deferred taxes due to the new corporate income tax rate enacted through the TCJA on December 22, 2017 recognized in income from continuing operations causes the tax effects of items within accumulated other comprehensive income (referred to as stranded tax effects) to not reflect the appropriate tax rate. The amendments in this update allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the TCJA and require certain disclosures about stranded tax effects. The amendments in this update are effective beginning January 1, 2019, and should be applied either in the period of adoption or retrospectively to each period in which the effect of the change in the federal corporate income tax rate in the TCJA is recognized. The reclassification allowed in this update is elective, and we are currently assessing whether we will make the reclassification. This update is not expected to have a material impact on the financial condition of NW Holdings or NW Natural. DERIVATIVES AND HEDGING. On August 28, 2017, the FASB issued ASU 2017-12, "Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities." The purpose of the amendment is to more closely align hedge accounting with companies’ risk management strategies. The ASU amends the accounting for risk component hedging, the hedged item in fair value hedges of interest rate risk, and amounts excluded from the assessment of hedge effectiveness. The guidance also amends the recognition and presentation of the effect of hedging instruments and includes other simplifications of hedge accounting. The amendments in this update are effective beginning January 1, 2019. Early adoption is permitted. The amended presentation and disclosure guidance is required prospectively. We do not anticipate the adoption of this standard to have a material impact on NW Holdings' and NW Natural's financial statements and disclosures. LEASES. On February 25, 2016, the FASB issued ASU 2016-02, "Leases," which revises the existing lease accounting guidance. Pursuant to the new standard, lessees will be required to recognize all leases, including operating leases that are greater than 12 months at lease commencement, on the balance sheet and record corresponding right-of-use assets and lease liabilities. Lessor accounting will remain substantially the same under the new standard. Quantitative and qualitative disclosures are also required for users of the financial statements to have a clear understanding of the nature of NW Natural's leasing activities. On July 30, 2018, the FASB approved an optional alternative transition approach that would allow entities to apply the transition requirements on the effective date of the standard. Additionally, on January 25, 2018, the FASB issued ASU 2018-01, "Land Easement Practical Expedient for Transition to Topic 842", to address the costs and complexity of applying the transition provisions of the new lease standard to land easements. This ASU provides an optional practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance. The standard and associated ASUs were effective for us beginning January 1, 2019. We elected the alternative prospective transition approach for adoption of ASC 842 beginning January 1, 2019. All comparative periods prior to January 1, 2019 will retain the financial reporting and disclosure requirements of ASC 840 “Leases” (“ASC 840”). We elected the land easement optional practical expedient to not evaluate existing or expired land easements that were not previously accounted for as leases under the current lease guidance. For the existing lease portfolio, we did not elect the optional practical expedient package to retain the legacy lease accounting conclusions upon adoption; rather, we re-assessed our existing contracts under the new leasing standard including whether the contract meets the definition of a lease and lease classification. As a result, we determined that most of our underground gas storage contracts no longer meet the definition of a lease under the new lease standard. Our lease portfolio under the new standard consists primarily of our leased headquarters, which expires in 2020. Upon adoption, NW Holdings expects to record a right-of-use lease asset and an associated lease liability of approximately $7.3 million , of which $7.0 million is expected to be recorded at NW Natural. In October 2017, NW Natural entered into a 20-year operating lease agreement commencing in 2020 for the new headquarters location in Portland, Oregon. Under the new lease standard, NW Natural is no longer considered the accounting owner of the asset during construction. As such, we expect to de-recognized the build-to-suit asset and liability balances of $26.0 million as of December 31, 2018 that were recorded under ASC 840 within property, plant and equipment and other non-current liabilities in the consolidated balance sheet. Refer to Note 16 for current lease commitments. CREDIT LOSSES. On June 16, 2016, the FASB issued ASU 2016-13, "Measurement of Credit Losses on Financial Instruments," which applies to financial assets subject to credit losses and measured at amortized cost. The new standard will require financial assets measured at amortized cost to be presented at the net amount expected to be collected and the allowance for credit losses is to be recorded as a valuation account that is deducted from the amortized cost basis. The amendments in this update are effective beginning January 1, 2020. Early adoption is permitted for fiscal years beginning after December 15, 2018. We are currently assessing the effect of this standard on the financial statements and disclosures of NW Holdings and NW Natural.
Property, Plant and Accrued Asset Removal Costs	Plant, Property, and Accrued Asset Removal Costs Plant and property are stated at cost, including capitalized labor, materials, and overhead. In accordance with regulatory accounting standards, the cost of acquiring and constructing long-lived plant and property generally includes an allowance for funds used during construction (AFUDC) or capitalized interest. AFUDC represents the regulatory financing cost incurred when debt and equity funds are used for construction (see “ AFUDC ” below). When constructed assets are subject to market-based rates rather than cost-based rates, the financing costs incurred during construction are included in capitalized interest in accordance with U.S. GAAP, not as regulatory financing costs under AFUDC. In accordance with long-standing regulatory treatment, our depreciation rates consist of three components: one based on the average service life of the asset, a second based on the estimated salvage value of the asset, and a third based on the asset’s estimated cost of removal. We collect, through rates, the estimated cost of removal on certain regulated properties through depreciation expense, with a corresponding offset to accumulated depreciation. These removal costs are non-legal obligations as defined by regulatory accounting guidance. Therefore, we have included these costs as non-current regulatory liabilities rather than as accumulated depreciation on our consolidated balance sheets. In the rate setting process, the liability for removal costs is treated as a reduction to the net rate base on which the NGD business has the opportunity to earn its allowed rate of return. The costs of NGD plant retired or otherwise disposed of are removed from NGD plant and charged to accumulated depreciation for recovery or refund through future rates. Gains from the sale of regulated assets are generally deferred and refunded to customers. For assets not related to NGD, we record a gain or loss upon the disposal of the property, and the gain or loss is recorded in operating income or loss in the consolidated statements of comprehensive income. The provision for depreciation of NGD property, plant, and equipment is recorded under the group method on a straight-line basis with rates computed in accordance with depreciation studies approved by regulatory authorities. The weighted-average depreciation rate for NGD assets in service was approximately 2.8 % for 2018 , 2017 , and 2016 , reflecting the approximate weighted-average economic life of the property. This includes 2018 weighted-average depreciation rates for the following asset categories: 2.7 % for transmission and distribution plant, 2.1 % for gas storage facilities, 4.5 % for general plant, and 3.1 % for intangible and other fixed assets. AFUDC. Certain additions to NGD plant include AFUDC, which represents the net cost of debt and equity funds used during construction. AFUDC is calculated using actual interest rates for debt and authorized rates for ROE, if applicable. If short-term debt balances are less than the total balance of construction work in progress, then a composite AFUDC rate is used to represent interest on all debt funds, shown as a reduction to interest charges, and on ROE funds, shown as other income. While cash is not immediately recognized from recording AFUDC, it is realized in future years through rate recovery resulting from the higher NGD cost of service. Our composite AFUDC rate was 5.2 % in 2018 , 5.5 % in 2017 , and 0.7 % in 2016 . IMPAIRMENT OF LONG-LIVED ASSETS. We review the carrying value of long-lived assets whenever events or changes in circumstances indicate the carrying amount of the assets may not be recoverable. Factors that would necessitate an impairment assessment of long-lived assets include a significant adverse change in the extent or manner in which the asset is used, a significant adverse change in legal factors or business climate that could affect the value of the asset, or a significant decline in the observable market value or expected future cash flows of the asset, among others. When such factors are present, we assess the recoverability by determining whether the carrying value of the asset will be recovered through expected future cash flows. An asset is determined to be impaired when the carrying value of the asset exceeds the expected undiscounted future cash flows from the use and eventual disposition of the asset. If an impairment is indicated, we record an impairment loss for the difference between the carrying value and the fair value of the long-lived assets. Fair value is estimated using appropriate valuation methodologies, which may include an estimate of discounted cash flows. In the fourth quarter of 2017, a non-cash pre-tax impairment of long-lived assets at the Gill Ranch Facility of $192.5 million was recognized. The income approach was used to estimate fair value, using the estimated future net cash flows. We also compared the results of the income approach to our own recent sale experience and recent market comparable transactions in order to estimate fair value. The Gill Ranch Facility was originally included in the gas storage segment, which has since been eliminated, and is now included in discontinued operations. We determined circumstances existed that indicated the carrying value of the assets may not be recoverable. Those circumstances included the completion of a comprehensive strategic review process that evaluated various alternatives including a potential sale, as well as contracting for available storage at lower than anticipated values for the coming storage year. Given these considerations, management re-evaluated the estimated cash flows from our interests in the Gill Ranch Facility, and determined that those estimated cash flows were no longer sufficient to cover the carrying value of the assets. The results of Gill Ranch have been presented as a discontinued operation for NW Holdings and NW Natural on the consolidated statements of comprehensive income and cash flows, and the assets and liabilities associated with Gill Ranch have been classified as discontinued operations assets and liabilities on the consolidated balance sheets. See Note 18 for additional information.
Cash and Cash Equivalents	Cash and Cash Equivalents For purposes of reporting cash flows, cash and cash equivalents include cash on hand plus highly liquid investment accounts with original maturity dates of three months or less. At December 31, 2018 and 2017 , NW Holdings had outstanding checks of approximately $ 2.7 million and $ 4.8 million , respectively, substantially all of which is recorded at NW Natural. These balances are included in accounts payable in the NW Holdings and NW Natural balance sheets.
Revenue Recognition and Accrued Unbilled Revenue	Revenue Recognition and Accrued Unbilled Revenue Revenues, derived primarily from the sale and transportation of natural gas, are recognized upon delivery of the gas commodity or service to customers. Revenues include accruals for gas or water delivered but not yet billed to customers based on estimates of deliveries from meter reading dates to month end (accrued unbilled revenue). Accrued unbilled revenue is dependent upon a number of factors that require management’s judgment, including total natural gas receipts and deliveries, customer use of natural gas or water by billing cycle, and weather factors. Accrued unbilled revenue is reversed the following month when actual billings occur. NW Holdings' accrued unbilled revenue at December 31, 2018 and 2017 was $ 57.8 million and $62.4 million , respectively, substantially all of which is accrued unbilled revenue at NW Natural. Revenues not related to NGD are derived primarily from Interstate Storage Services, asset management activities at the Mist gas storage facility, and other investments and business activities. At the Mist underground storage facility, revenues are primarily firm service revenues in the form of fixed monthly reservation charges. In addition, we also have asset management service revenue from an independent energy marketing company that optimizes commodity, storage, and pipeline capacity release transactions. Under this agreement, guaranteed asset management revenue is recognized using a straight-line, pro-rata methodology over the term of each contract. Revenues earned above the guaranteed amount are recognized as they are earned.
Revenue Taxes	Revenue Taxes Revenue-based taxes are primarily franchise taxes, which are collected from customers and remitted to taxing authorities. In 2018, revenue taxes are included in operating expenses in the statements of comprehensive income for NW Holdings and NW Natural. In 2017 and 2016, revenue taxes are included in operating revenues in the statements of comprehensive income for NW Holdings and NW Natural. All revenue taxes are recorded at NW Natural and were $30.1 million , $19.1 million , and $17.1 million for 2018 , 2017 , and 2016 , respectively.
Accounts Receivable and Allowance for Uncollectible Accounts	Accounts Receivable and Allowance for Uncollectible Accounts Accounts receivable consist primarily of amounts due for natural gas sales and transportation services to NGD customers, plus amounts due for gas storage services. At NW Holdings and NW Natural we establish allowances for uncollectible accounts (allowance) for trade receivables, including accrued unbilled revenue, based on the aging of receivables, collection experience of past due account balances including payment plans, and historical trends of write-offs as a percent of revenues. A specific allowance is established and recorded for large individual customer receivables when amounts are identified as unlikely to be partially or fully recovered. Inactive accounts are written-off against the allowance after they are 120 days past due or when deemed uncollectible. Differences between the estimated allowance and actual write-offs will occur based on a number of factors, including changes in economic conditions, customer creditworthiness, and natural gas prices. The allowance for uncollectible accounts is adjusted quarterly, as necessary, based on information currently available.
Inventories	Inventories NGD gas inventories, which consist of natural gas in storage for NGD, are stated at the lower of average cost or net realizable value. The regulatory treatment of these inventories provides for cost recovery in customer rates. NGD gas inventories injected into storage are priced in inventory based on actual purchase costs, and those withdrawn from storage are charged to cost of gas during the current period they are withdrawn at the weighted-average inventory cost. Gas storage inventories, which primarily represent inventories at the Gill Ranch Facility and are included in Discontinued operations - current assets on the consolidated balance sheets, mainly consist of natural gas received as fuel-in-kind from storage customers. Gas storage inventories are valued at the lower of average cost or net realizable value. Cushion gas is not included in inventory balances, is recorded at original cost, and is classified as a long-term plant asset. Materials and supplies inventories consist of inventories both related to and unrelated to NGD and are stated at the lower of average cost or net realizable value. NW Natural's NGD and gas storage inventories totaled $ 29.9 million and $ 36.7 million at December 31, 2018 and 2017 , respectively. At December 31, 2018 and 2017 , NW Holdings' materials and supplies inventories, which are comprised primarily of NW Natural's materials and supplies, totaled $ 14.2 million and $ 10.9 million , respectively.
Gas Reserves	Gas Reserves Gas reserves are payments to acquire and produce natural gas reserves. Gas reserves are stated at cost, adjusted for regulatory amortization, with the associated deferred tax benefits recorded as liabilities on the balance sheet. The current portion is calculated based on expected gas deliveries within the next fiscal year. NW Natural recognizes regulatory amortization of this asset on a volumetric basis calculated using the estimated gas reserves and the estimated therms extracted and sold each month. The amortization of gas reserves is recorded to cost of gas along with gas production revenues and production costs. See Note 12 .
Derivatives	Derivatives NW Natural's derivatives are measured at fair value and recognized as either assets or liabilities on the balance sheet. Changes in the fair value of the derivatives are recognized currently in earnings unless specific regulatory or hedge accounting criteria are met. Accounting for derivatives and hedges provides an exception for contracts intended for normal purchases and normal sales for which physical delivery is probable. In addition, certain derivative contracts are approved by regulatory authorities for recovery or refund through customer rates. Accordingly, the changes in fair value of these approved contracts are deferred as regulatory assets or liabilities pursuant to regulatory accounting principles. NW Natural's financial derivatives generally qualify for deferral under regulatory accounting. NW Natural's index-priced physical derivative contracts also qualify for regulatory deferral accounting treatment. Derivative contracts entered into for NGD requirements after the annual PGA rate has been set and maturing during the PGA year are subject to the PGA incentive sharing mechanism. In Oregon, NW Natural participates in a PGA sharing mechanism under which it is required to select either an 80% or 90% deferral of higher or lower gas costs such that the impact on current earnings from the gas cost sharing is either 20% or 10% of gas cost differences compared to PGA prices, respectively. For the PGA years in Oregon beginning November 1, 2018 , 2017 , and 2016 , NW Natural selected the 90% , 90% , and 90% deferral of gas cost differences, respectively. In Washington, 100% of the differences between the PGA prices and actual gas costs are deferred. See Note 15 . NW Natural's financial derivatives policy sets forth the guidelines for using selected derivative products to support prudent risk management strategies within designated parameters. NW Natural's objective for using derivatives is to decrease the volatility of gas prices, earnings, and cash flows without speculative risk. The use of derivatives is permitted only after the risk exposures have been identified, are determined not to exceed acceptable tolerance levels, and are determined necessary to support normal business activities. NW Natural does not enter into derivative instruments for trading purposes. All derivatives for NW Holdings are currently held at NW Natural.
Fair Value	Fair Value In accordance with fair value accounting, we use the following fair value hierarchy for determining inputs for our debt, pension plan assets, and derivative fair value measurements: • Level 1: Valuation is based on quoted prices for identical instruments traded in active markets; • Level 2: Valuation is based on quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant assumptions are observable in the market; and • Level 3: Valuation is generated from model-based techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect our own estimates of assumptions market participants would use in valuing the asset or liability. When developing fair value measurements, it is our policy to use quoted market prices whenever available or to maximize the use of observable inputs and minimize the use of unobservable inputs when quoted market prices are not available. Fair values are primarily developed using industry-standard models that consider various inputs including: (a) quoted future prices for commodities; (b) forward currency prices; (c) time value; (d) volatility factors; (e) current market and contractual prices for underlying instruments; (f) market interest rates and yield curves; (g) credit spreads; and (h) other relevant economic measures. NW Natural considers liquid points for natural gas hedging to be those points for which there are regularly published prices in a nationally recognized publication or where the instruments are traded on an exchange.
Goodwill	Goodwill NW Holdings, through its wholly-owned subsidiary NWN Water and NW Water's wholly-owned subsidiaries, has completed various acquisitions that resulted in the recognition of goodwill. Goodwill is measured as the excess of the acquisition-date fair value of the consideration transferred over the acquisition-date fair value of the net identifiable assets assumed. Adjustments are recorded during the measurement period to finalize the allocation of the purchase price. The carrying value of goodwill is reviewed annually during the fourth quarter using balances as of October 1, or whenever events or changes in circumstance indicate that such carrying values may not be recoverable. The goodwill assessment policy begins with a qualitative analysis in which events and circumstances are evaluated, including macroeconomic conditions, industry and market conditions, regulatory environments, and overall financial performance of the reporting unit. If the qualitative assessment indicates that the carrying value may be at risk of recoverability, a quantitative evaluation is performed to measure the carrying value of the goodwill against the fair value of the reporting unit. The reporting unit is determined primarily based on current operating segments and the level of review provided by the Chief Operating Decision Maker (CODM) and/or segment management on the operating segment's financial results. Reporting units are evaluated periodically for changes in the corporate environment. As of December 31, 2018 , NW Holdings had goodwill of $9.0 million . All of NW Holdings' goodwill was acquired in 2018 through the business combinations completed by NWN Water and its wholly-owned subsidiaries. No impairment charges were recorded as a result of the fourth quarter goodwill impairment assessment.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the enactment date period unless, for NW Natural, a regulatory Order specifies deferral of the effect of the change in tax rates over a longer period of time. For NW Natural, deferred income tax assets and liabilities are also recognized for temporary differences where the deferred income tax benefits or expenses have previously been flowed through in the ratemaking process of the NGD business. Regulatory tax assets and liabilities are recorded on these deferred tax assets and liabilities to the extent it is believed they will be recoverable from or refunded to customers in future rates. Deferred investment tax credits on NGD plant additions, which reduce income taxes payable, are deferred for financial statement purposes and amortized over the life of the related plant. Interest and penalties related to unrecognized tax benefits, if any, are recognized within income tax expense and accrued interest and penalties are recognized within the related tax liability line in the consolidated balance sheets. No accrued interest or penalties for uncertain tax benefits have been recorded. See Note 10 .
Environmental Contingencies	Environmental Contingencies Loss contingencies are recorded as liabilities when it is probable a liability has been incurred and the amount of the loss is reasonably estimable in accordance with accounting standards for contingencies. Estimating probable losses requires an analysis of uncertainties that often depend upon judgments about potential actions by third parties. Accruals for loss contingencies are recorded based on an analysis of potential results. With respect to environmental liabilities and related costs, estimates are developed based on a review of information available from numerous sources, including completed studies and site specific negotiations. NW Natural's policy is to accrue the full amount of such liability when information is sufficient to reasonably estimate the amount of probable liability. When information is not available to reasonably estimate the probable liability, or when only the range of probable liabilities can be estimated and no amount within the range is more likely than another, it is our policy to accrue at the low end of the range. Accordingly, due to numerous uncertainties surrounding the course of environmental remediation and the preliminary nature of several site investigations, in some cases, it may not be possible to reasonably estimate the high end of the range of possible loss. In those cases, the nature of the potential loss and the fact that the high end of the range cannot be reasonably estimated is disclosed. See Note 17 .
Subsequent Events	Subsequent Events We monitor significant events occurring after the balance sheet date and prior to the issuance of the financial statements to determine the impacts, if any, of events on the financial statements to be issued. We do not have any subsequent events to report.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Regulatory Assets	At December 31, NW Natural deferred the following amounts as regulatory assets and liabilities: Regulatory Assets In thousands 2018 2017 Current: Unrealized loss on derivatives (1) $ 12,381 $ 18,712 Gas costs 2,873 154 Environmental costs (2) 5,601 6,198 Decoupling (3) 9,140 11,227 Income taxes 2,218 2,218 Other (4) 9,717 7,272 Total current $ 41,930 $ 45,781 Non-current: Unrealized loss on derivatives (1) $ 3,025 $ 4,649 Pension balancing (5) 74,173 60,383 Income taxes 19,185 19,991 Pension and other postretirement benefit liabilities 174,993 179,824 Environmental costs (2) 76,149 72,128 Gas costs 9,978 84 Decoupling (3) 2,545 3,970 Other (4) 11,738 15,579 Total non-current $ 371,786 $ 356,608
Schedule of Regulatory Liabilities	Regulatory Liabilities In thousands 2018 2017 Current: Gas costs $ 17,182 $ 14,886 Unrealized gain on derivatives (1) 8,740 1,674 Decoupling (3) 2,264 322 Other (4) 19,250 17,131 Total current $ 47,436 $ 34,013 Non-current: Gas costs $ 552 $ 4,630 Unrealized gain on derivatives (1) 725 1,306 Decoupling (3) — 957 Income taxes (6) 225,408 213,306 Accrued asset removal costs (7) 380,464 360,929 Other (4) 4,411 4,965 Total non-current $ 611,560 $ 586,093 (1) Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through natural gas distribution rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement. (2) Refer to footnote (3) of the Deferred Regulatory Asset table in Note 17 for a description of environmental costs. (3) This deferral represents the margin adjustment resulting from differences between actual and expected volumes. (4) Balances consist of deferrals and amortizations under approved regulatory mechanisms and typically earn a rate of return or carrying charge. (5) Refer to footnote (1) of the Net Periodic Benefit Cost table in Note 9 for information regarding the deferral of pension expenses. (6) This balance represents estimated amounts associated with the Tax Cuts and Jobs Act. See Note 10 . (7) Estimated costs of removal on certain regulated properties are collected through rates. See "Accounting Policies— Plant, Property, and Accrued Asset Removal Costs " below.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
EPS Calculation [Table Text Block]	NW Holdings' diluted earnings or loss per share are calculated as follows: In thousands, except per share data 2018 2017 2016 Net income from continuing operations $ 67,311 $ 72,073 $ 62,419 Loss from discontinued operations, net of tax (2,742 ) (127,696 ) (3,524 ) Net income (loss) $ 64,569 $ (55,623 ) $ 58,895 Average common shares outstanding - basic 28,803 28,669 27,647 Additional shares for stock-based compensation plans (See Note 7) 70 84 132 Average common shares outstanding - diluted 28,873 28,753 27,779 Earnings (loss) from continuing operations per share of common stock: Basic $ 2.34 $ 2.51 $ 2.26 Diluted $ 2.33 $ 2.51 $ 2.25 Loss from discontinued operations per share of common stock: Basic $ (0.10 ) $ (4.45 ) $ (0.13 ) Diluted $ (0.09 ) $ (4.44 ) $ (0.13 ) Earnings (loss) per share of common stock: Basic $ 2.24 $ (1.94 ) $ 2.13 Diluted $ 2.24 $ (1.93 ) $ 2.12 Additional information: Antidilutive shares 2 13 5

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment	The following table presents summary financial information concerning the reportable segments of continued operations. See Note 18 for information regarding discontinued operations for NW Holdings and NW Natural. In thousands NGD Other NW Natural Other NW Holdings 2018 Operating revenues $ 680,648 $ 24,923 $ 705,571 $ 572 $ 706,143 Depreciation and amortization 83,732 1,254 84,986 170 85,156 Income (loss) from operations 118,095 15,004 133,099 (937 ) 132,162 Net income (loss) from continuing operations 57,491 10,558 68,049 (738 ) 67,311 Capital expenditures 212,323 2,005 214,328 308 214,636 Total assets at December 31, 2018 (2) 3,141,969 50,767 3,192,736 36,657 3,229,393 2017 Operating revenues $ 732,942 $ 22,096 $ 755,038 $ — $ 755,038 Depreciation and amortization 79,734 1,290 81,024 29 81,053 Income (loss) from operations 138,450 12,472 150,922 (20 ) 150,902 Net income from continuing operations (1) 60,509 11,211 71,720 353 72,073 Capital expenditures 211,672 1,653 213,325 — 213,325 Total assets at December 31, 2017 (2) 2,961,326 50,471 3,011,797 14,075 3,025,872 2016 Operating revenues $ 650,477 $ 17,472 $ 667,949 $ 224 $ 668,173 Depreciation and amortization 76,289 1,286 77,575 29 77,604 Income (loss) from operations 137,178 14,109 151,287 (570 ) 150,717 Net income (loss) from continuing operations (3) 54,567 8,268 62,835 (416 ) 62,419 Capital expenditures 138,074 283 138,357 — 138,357 Total assets at December 31, 2016 (2) 2,806,627 48,719 2,855,346 14,040 2,869,386 (1) Includes $1.0 million of tax expense in NGD, $4.0 million of tax benefit in Other (NW Natural), and $0.4 million of tax benefit in Other (NW Holdings) from the TCJA remeasurement for the year ended December 31, 2017 . (2) Total assets for NW Holdings exclude assets related to discontinued operations of $13.3 million , $13.9 million and $210.4 million as of December 31, 2018 , 2017 , and 2016 , respectively. Total assets for NW Natural exclude assets related to discontinued operations of $31.9 million and $226.1 million as of December 31, 2017 , and 2016 , respectively. (3) Includes $2.0 million in 2016 of after-tax regulatory environmental disallowance charges in NGD.
Utility Margin	The following table presents additional segment information concerning NGD margin: In thousands 2018 2017 2016 NGD margin calculation: NGD operating revenues $ 680,648 $ 732,942 $ 650,477 Less: NGD cost of gas 255,743 325,019 260,588 Environmental remediation expense 11,127 15,291 13,298 Revenue taxes (1) 30,082 — — NGD margin $ 383,696 $ 392,632 $ 376,591 (1) The change in presentation of revenue taxes was a result of the adoption of ASU 2014-09 "Revenue From Contracts with Customers" and all related amendments on January 1, 2018. This change had no impact on NGD margin results as revenue taxes were previously presented net in NGD operating revenue. For additional information, see Note 2 .

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Common Stock [Abstract]
Summary Of Common Stock Shares Issued And Outstanding Table	The following table summarizes the changes in the number of shares of NW Holdings' common stock issued and outstanding: In thousands Shares Balance, December 31, 2015 27,427 Sales to employees under ESPP 18 Stock-based compensation 173 Equity Issuance 1,012 Balance, December 31, 2016 28,630 Sales to employees under ESPP 18 Stock-based compensation 88 Balance, December 31, 2017 28,736 Sales to employees under ESPP 19 Stock-based compensation 64 Sales to shareholders under DRPP 61 Balance, December 31, 2018 28,880

Revenue (Tables)

Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The following table presents disaggregated revenue from continuing operations: Year ended December 31, 2018 In thousands NGD Other (NW Natural) NW Natural Other (NW Holdings) NW Holdings Natural gas sales $ 670,662 $ — $ 670,662 $ — $ 670,662 Gas storage revenue, net — 10,780 10,780 — 10,780 Asset management revenue, net — 8,548 8,548 — 8,548 Appliance retail center revenue — 5,595 5,595 — 5,595 Other revenue — — — 572 572 Revenue from contracts with customers 670,662 24,923 695,585 572 696,157 Alternative revenue 8,989 — 8,989 — 8,989 Leasing revenue 997 — 997 — 997 Total operating revenues $ 680,648 $ 24,923 $ 705,571 $ 572 $ 706,143

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule Of Performance Based Award [Table Text Block]	The aggregate number of performance shares granted and outstanding at the target and maximum levels were as follows: Dollars in thousands Performance Share Awards Outstanding 2018 Cumulative Expense Performance Period Target Maximum Expense December 31, 2018 2016-18 24,421 48,842 $ 598 $ 1,413 2017-19 31,372 62,744 458 1,400 2018-20 — — — — Total 55,793 111,586 $ 1,056
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]	Number of RSUs Weighted - Average Price Per RSU Nonvested, December 31, 2015 88,587 $ 44.78 Granted 40,271 54.36 Vested (29,488 ) 45.56 Forfeited (9,397 ) 44.59 Nonvested, December 31, 2016 89,973 48.85 Granted 32,168 60.51 Vested (35,341 ) 47.07 Forfeited (2,278 ) 53.78 Nonvested, December 31, 2017 84,522 53.90 Granted 32,450 57.59 Vested (32,689 ) 50.75 Forfeited (1,603 ) 59.95 Nonvested, December 31, 2018 82,680 $ 56.47
Share-based Compensation, Stock Options, Activity [Table Text Block]	Option Shares Weighted - Average Price Per Share Intrinsic Value (In millions) Balance outstanding, December 31, 2015 352,688 $ 44.00 $ 2.3 Exercised (172,525 ) 43.61 2.0 Forfeited — n/a n/a Balance outstanding, December 31, 2016 180,163 44.38 2.8 Exercised (88,275 ) 44.33 1.8 Forfeited (200 ) 41.15 n/a Balance outstanding and exercisable, December 31, 2017 91,688 44.43 1.4 Exercised (35,450 ) 43.61 0.8 Expired (300 ) 43.29 n/a Balance outstanding and exercisable, December 31, 2018 55,938 $ 44.96 $ 0.9
Schedule Of Stock Based Compensation Capitalized Under LTIP SOP and ESPP [Table Text Block]	In thousands 2018 2017 2016 Operations and maintenance expense, for stock-based compensation $ 2,489 $ 2,354 $ 2,370 Income tax benefit (659 ) (930 ) (924 ) Net stock-based compensation effect on net income (loss) $ 1,830 $ 1,424 $ 1,446 Amounts capitalized for stock-based compensation $ 531 $ 528 $ 554
Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Share-based Payment Awards [Table Text Block]	The following table summarizes performance share expense information: Dollars in thousands Shares (1) Expense During Award Year (2) Total Expense for Award Estimated award: 2016-2018 grant (3) 28,218 $ 598 $ 1,413 Actual award: 2015-2017 grant 18,304 (346 ) 1,169 2014-2016 grant 31,388 168 1,685 (1) In addition to common stock shares, a participant also receives a dividend equivalent cash payment equal to the number of shares of common stock received on the award payout multiplied by the aggregate cash dividends paid per share during the performance period. (2) Amount represents the expense recognized in the third year of the vesting period noted above. For the 2015-2017 grant, targets were not met and expense was reversed during 2017 that had been previously recognized. (3) This represents the estimated number of shares to be awarded as of December 31, 2018 as certain performance share measures have been achieved. Amounts are subject to change with final payout amounts authorized by the Board of Directors in February 2019 .

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Maturities of Long-term Debt [Table Text Block]	Retirement of long-term debt for each of the annual periods through December 31, 2023 and thereafter are as follows: In thousands Long-term debt maturities 2019 $ 30,000 2020 75,000 2021 60,000 2022 — 2023 90,000 Thereafter 484,700
Schedule of Debt [Table Text Block]	The following table presents debt outstanding as of December 31: In thousands 2018 2017 NW Natural First Mortgage Bonds: 6.600% Series due 2018 $ — $ 22,000 1.545% Series due 2018 — 75,000 8.310% Series due 2019 10,000 10,000 7.630% Series due 2019 20,000 20,000 5.370% Series due 2020 75,000 75,000 9.050% Series due 2021 10,000 10,000 3.176% Series due 2021 50,000 50,000 3.542% Series due 2023 50,000 50,000 5.620% Series due 2023 40,000 40,000 7.720% Series due 2025 20,000 20,000 6.520% Series due 2025 10,000 10,000 7.050% Series due 2026 20,000 20,000 3.211% Series due 2026 35,000 35,000 7.000% Series due 2027 20,000 20,000 2.822% Series due 2027 25,000 25,000 6.650% Series due 2027 19,700 19,700 6.650% Series due 2028 10,000 10,000 7.740% Series due 2030 20,000 20,000 7.850% Series due 2030 10,000 10,000 5.820% Series due 2032 30,000 30,000 5.660% Series due 2033 40,000 40,000 5.250% Series due 2035 10,000 10,000 4.000% Series due 2042 50,000 50,000 4.136% Series due 2046 40,000 40,000 3.685% Series due 2047 75,000 75,000 4.110% Series due 2048 50,000 — 739,700 786,700 Less: current maturities 30,000 97,000 Total long-term debt $ 709,700 $ 689,700 Other NW Holdings Entities: Long-term debt obligations $ 2,113 $ — NW Holdings: Long-term debt, gross $ 741,813 $ 786,700 Less: current maturities 30,000 97,000 Total long-term debt $ 711,813 $ 689,700
Fair Value Of Long Term Debt Table [Text Block]	The following table provides an estimate of the fair value of NW Natural's long-term debt, including current maturities of long-term debt, using market prices in effect on the valuation date : December 31, In thousands 2018 2017 Gross long-term debt $ 739,700 $ 786,700 Unamortized debt issuance costs (5,577 ) (6,813 ) Carrying amount $ 734,123 $ 779,887 Estimated fair value $ 760,222 $ 853,339

Pension and Other Postretirem_2

Pension and Other Postretirement Benefits Costs (Tables)	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Schedule of Defined Benefit Plans Disclosures	The following table provides a reconciliation of the changes in NW Natural's benefit obligations and fair value of plan assets, as applicable, for NW Natural's pension and other postretirement benefit plans, excluding the Retirement K Savings Plan, and a summary of the funded status and amounts recognized in NW Holdings' and NW Natural's consolidated balance sheets as of December 31: Postretirement Benefit Plans Pension Benefits Other Benefits In thousands 2018 2017 2018 2017 Reconciliation of change in benefit obligation: Obligation at January 1 $ 486,289 $ 457,839 $ 28,927 $ 29,395 Service cost 7,185 7,090 282 341 Interest cost 16,991 18,111 964 1,141 Net actuarial (gain) loss (32,979 ) 34,829 (327 ) (213 ) Benefits paid (1) (21,918 ) (31,580 ) (1,674 ) (1,737 ) Obligation at December 31 $ 455,568 $ 486,289 $ 28,172 $ 28,927 Reconciliation of change in plan assets: Fair value of plan assets at January 1 $ 287,925 $ 257,714 $ — $ — Actual return on plan assets (25,925 ) 40,308 — — Employer contributions 17,715 21,483 1,674 1,737 Benefits paid (1) (21,918 ) (31,580 ) (1,674 ) (1,737 ) Fair value of plan assets at December 31 $ 257,797 $ 287,925 $ — $ — Funded status at December 31 $ (197,771 ) $ (198,364 ) $ (28,172 ) $ (28,927 ) (1) In 2017, NW Natural completed a partial buy-out of its qualified defined benefit pension plan in which $9.3 million of plan assets and $8.7 million of liabilities were transferred to an insurer to provide annuities for buy-out plan participants.
Schedule Of Amounts Recognized In Regulatory Assets Or Other Comprehensive Income Loss	The following table presents amounts realized through regulatory assets or in other comprehensive loss (income) for the years ended December 31: Regulatory Assets Other Comprehensive Loss (Income) Pension Benefits Other Postretirement Benefits Pension Benefits In thousands 2018 2017 2016 2018 2017 2016 2018 2017 2016 Net actuarial loss (gain) $ 14,261 $ 12,177 $ 14,005 $ (327 ) $ (214 ) $ (1,488 ) $ (677 ) $ 2,777 $ (1,196 ) Settlement Loss — — — — — — — — 193 Amortization of: Prior service cost (42 ) (127 ) (230 ) 468 468 468 — — — Actuarial loss (18,761 ) (14,802 ) (13,238 ) (448 ) (696 ) (705 ) (1,052 ) (946 ) 1,386 Total $ (4,542 ) $ (2,752 ) $ 537 $ (307 ) $ (442 ) $ (1,725 ) $ (1,729 ) $ 1,831 $ 383
Schedule Of Defined Benefit Plan Amounts In AOCL And Regulatory Assets	The following table presents amounts recognized in regulatory assets and accumulated other comprehensive loss (AOCL) at December 31: Regulatory Assets AOCL Pension Benefits Other Postretirement Benefits Pension Benefits In thousands 2018 2017 2018 2017 2018 2017 Prior service cost (credit) $ 7 $ 49 $ (1,738 ) $ (2,206 ) $ — $ — Net actuarial loss 170,535 175,035 6,189 6,964 11,537 13,266 Total $ 170,542 $ 175,084 $ 4,451 $ 4,758 $ 11,537 $ 13,266
Schedule of Amounts Recognized in Other Comprehensive Income (Loss)	The following table presents amounts recognized by NW Holdings and NW Natural in AOCL and the changes in AOCL related to NW Natural's non-qualified employee benefit plans: Year Ended December 31, In thousands 2018 2017 Beginning balance $ (8,438 ) $ (6,951 ) Amounts reclassified to AOCL 642 (2,794 ) Amounts reclassified from AOCL: Amortization of actuarial losses 1,052 946 Total reclassifications before tax 1,694 (1,848 ) Tax expense (benefit) (444 ) 361 Total reclassifications for the period 1,250 (1,487 ) Ending balance $ (7,188 ) $ (8,438 )
Schedule of Allocation of Plan Assets	The following table presents the NW Natural pension plan asset target allocation at December 31, 2018 : Asset Category Target Allocation Long government/credit 20 % U.S. large cap equity 18 Non-U.S. equity 18 Absolute return strategies 12 U.S. small/mid cap equity 10 Real estate funds 7 High yield bonds 5 Emerging markets equity 5 Emerging market debt 5
Schedule of Net Benefit Costs	following table provides the components of net periodic benefit cost for NW Natural's pension and other postretirement benefit plans for the years ended December 31: Pension Benefits Other Postretirement Benefits In thousands 2018 2017 2016 2018 2017 2016 Service cost $ 7,185 $ 7,090 $ 7,083 $ 282 $ 341 $ 391 Interest cost 16,991 18,111 18,399 964 1,141 1,175 Expected return on plan assets (20,639 ) (20,433 ) (20,054 ) — — — Amortization of prior service costs 43 127 231 (468 ) (468 ) (468 ) Amortization of net actuarial loss 19,813 15,748 14,624 448 696 705 Settlement expense — — 193 — — — Net periodic benefit cost 23,393 20,643 20,476 1,226 1,710 1,803 Amount allocated to construction (2,764 ) (6,597 ) (5,746 ) (98 ) (587 ) (600 ) Amount deferred to regulatory balancing account (10,314 ) (6,542 ) (6,252 ) — — — Net amount charged to expense $ 10,315 $ 7,504 $ 8,478 $ 1,128 $ 1,123 $ 1,203
Sensitivity Analysis Of Retirement Benefit Costs and Benefit Obligations	The following table provides the assumptions used in measuring periodic benefit costs and benefit obligations for the years ended December 31: Pension Benefits Other Postretirement Benefits 2018 2017 2016 2018 2017 2016 Assumptions for net periodic benefit cost: Weighted-average discount rate 3.51 % 3.99 % 4.17 % 3.44 % 3.85 % 4.00 % Rate of increase in compensation 3.25-4.5% 3.25-4.5% 3.25-4.5% n/a n/a n/a Expected long-term rate of return 7.50 % 7.50 % 7.50 % n/a n/a n/a Assumptions for year-end funded status: Weighted-average discount rate 4.20 % 3.52 % 4.00 % 4.13 % 3.44 % 3.85 % Rate of increase in compensation 3.25-3.5% 3.25-4.5% 3.25-4.5% n/a n/a n/a Expected long-term rate of return 7.50 % 7.50 % 7.50 % n/a n/a n/a
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates	A one percentage point change in assumed health care cost trend rates would have the following effects: In thousands 1% Increase 1% Decrease Effect on net periodic postretirement health care benefit cost $ 43 $ (39 ) Effect on the accumulated postretirement benefit obligation 622 (560 )
Defined Benefit Plan Estimated Future Employer Contributions Benefit Payments and Estimated Future Payments	The following table provides information regarding employer contributions and benefit payments for NW Natural's qualified pension plan, non-qualified pension plans, and other postretirement benefit plans for the years ended December 31, and estimated future contributions and payments: In thousands Pension Benefits Other Benefits Employer Contributions: 2017 $ 21,483 $ 1,737 2018 17,715 1,674 2019 (estimated) 13,318 1,787 Benefit Payments: 2016 20,959 1,732 2017 31,580 1,737 2018 21,918 1,674 Estimated Future Benefit Payments: 2019 22,699 1,787 2020 23,622 1,846 2021 24,516 1,930 2022 25,316 1,941 2023 26,074 1,993 2024-2028 145,917 9,628
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following tables present the fair value of NW Natural's plan assets, including outstanding receivables and liabilities, of NW Natural's retirement trust fund: In thousands December 31, 2018 Investments Level 1 Level 2 Level 3 Non-Published NAV (1) Total US equity $ — $ — $ — $ 85,233 $ 85,233 International / Global equity 24,994 — — 70,017 95,011 Liability hedging — — — 45,659 45,659 Opportunistic — — — 23,186 23,186 Cash and cash equivalents — — — 8,707 8,707 Total investments $ 24,994 $ — $ — $ 232,802 $ 257,796 December 31, 2017 Investments Level 1 Level 2 Level 3 Non-Published NAV (1) Total US equity $ — $ — $ — $ 98,375 $ 98,375 International / Global equity 21,211 — — 84,818 106,029 Liability hedging — — — 53,981 53,981 Opportunistic — — — 23,895 23,895 Cash and cash equivalents 82 — — 5,533 5,615 Total investments $ 21,293 $ — $ — $ 266,602 $ 287,895 December 31, 2018 2017 Receivables: Accrued interest and dividend income $ 1 $ 30 Due from broker for securities sold — — Total receivables $ 1 $ 30 Liabilities: Due to broker for securities purchased $ — $ — Total investment in retirement trust $ 257,797 $ 287,925 (1) The fair value for these investments is determined using Net Asset Value per share (NAV) as of December 31, as a practical expedient, and therefore they are not classified within the fair value hierarchy. These investments primarily consist of institutional investment products, for which the NAV is generally not publicly available.

Income Tax (Tables)

Income Tax (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	The following table provides a reconciliation between income taxes calculated at the statutory federal tax rate and the provision for income taxes reflected in the NW Holdings and NW Natural statements of comprehensive income or loss for December 31: NW Holdings NW Natural Dollars in thousands 2018 2017 2016 2018 2017 2016 Income taxes at federal statutory rate $ 19,222 $ 39,578 $ 36,901 $ 19,434 $ 39,624 $ 37,137 Increase (decrease): State income tax, net of federal 4,927 5,066 4,844 4,982 5,072 4,858 Differences required to be flowed-through by regulatory commissions 1,302 2,357 2,357 1,302 2,357 2,357 Effect of the TCJA — (3,376 ) — — (2,956 ) — Deferred tax rate differential post-TCJA (76 ) — — (75 ) — — Other, net (1,184 ) (2,617 ) (1,091 ) (1,184 ) (2,619 ) (1,077 ) Total provision for income taxes $ 24,191 $ 41,008 $ 43,011 $ 24,459 $ 41,478 $ 43,275 Effective tax rate 26.4 % 36.3 % 40.8 % 26.4 % 36.6 % 40.8 %
Schedule of Components of Income Tax Expense (Benefit) [Table Text Block]	The provision for current and deferred income taxes consists of the following at December 31: NW Holdings NW Natural In thousands 2018 2017 2016 2018 2017 2016 Current Federal $ 8,953 $ 19,345 $ 10,042 $ 9,127 $ 19,304 $ 10,158 State 3,785 5,963 3,116 3,846 5,956 3,131 12,738 25,308 13,158 12,973 25,260 13,289 Deferred Federal 9,001 13,869 25,473 9,025 14,371 25,581 State 2,452 1,831 4,380 2,461 1,847 4,405 11,453 15,700 29,853 11,486 16,218 29,986 Income tax provision $ 24,191 $ 41,008 $ 43,011 $ 24,459 $ 41,478 $ 43,275
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	The following table summarizes the tax effect of significant items comprising NW Holdings and NW Natural's deferred income tax balances recorded at December 31: NW Holdings NW Natural In thousands 2018 2017 (1) 2018 2017 Deferred tax liabilities: Plant and property $ 288,385 $ 278,735 $ 303,186 $ 296,113 Pension and postretirement obligations 27,135 23,352 27,135 23,352 Income tax regulatory asset 21,403 22,209 21,402 22,209 Other 1,061 2,766 537 2,250 Total deferred income tax liabilities $ 337,984 $ 327,062 $ 352,260 $ 343,924 Deferred income tax assets: Income tax regulatory liability $ 57,469 $ 56,470 $ 57,469 $ 56,470 Alternative minimum tax credit carryforward 52 66 52 66 Total deferred income tax assets $ 57,521 $ 56,536 $ 57,521 $ 56,536 Total net deferred income tax liabilities $ 280,463 $ 270,526 $ 294,739 $ 287,388

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Public Utilities, Property, Plant and Equipment [Abstract]
Major Classifications Of Property, Plant And Equipment And Accumulated Depreciation [Text Block]	The following table sets forth the major classifications of property, plant, and equipment and accumulated depreciation at December 31: In thousands 2018 2017 NW Natural: NGD plant in service $ 3,134,122 $ 2,975,217 NGD work in progress 204,978 159,924 Less: Accumulated depreciation 974,252 942,879 NGD plant, net 2,364,848 2,192,262 Other plant in service 66,009 64,997 Other construction work in progress 5,330 4,122 Less: Accumulated depreciation 18,603 17,406 Other plant, net (1) 52,736 51,713 Total property, plant, and equipment $ 2,417,584 $ 2,243,975 Other (NW Holdings): Other plant in service $ 4,051 $ 375 Less: Accumulated depreciation 263 192 Other plant, net (1) 3,788 183 NW Holdings: Total property, plant, and equipment $ 2,421,372 $ 2,244,158 NW Natural and NW Holdings: Capital expenditures in accrued liabilities $ 23,676 $ 34,761 (1) NW Natural's previously reported other balances were restated due to certain assets and liabilities now being classified as discontinued operations assets and liabilities in its balance sheets. See Note 18 for further discussion.

Gas Reserves (Tables)

Gas Reserves (Tables)	12 Months Ended
	Dec. 31, 2018
Gas Reserves [Abstract]
Gas Reserves Table [Text Block]	The following table outlines NW Natural's net gas reserves investment at December 31: In thousands 2018 2017 Gas reserves, current $ 16,647 $ 15,704 Gas reserves, non-current 170,660 171,832 Less: Accumulated amortization 104,463 87,779 Total gas reserves (1) 82,844 99,757 Less: Deferred taxes on gas reserves 20,071 22,712 Net investment in gas reserves $ 62,773 $ 77,045 (1) The net investment in additional wells included in total gas reserves was $4.8 million and $5.8 million at December 31, 2018 and 2017 , respectively.

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments [Abstract]
Schedule Of Long Term Investments	The following table summarizes other investments at December 31: NW Holdings NW Natural In thousands 2018 2017 2018 2017 Investments in life insurance policies $ 49,922 $ 50,792 $ 49,922 $ 50,792 Investments in gas pipeline 13,571 13,669 — — Other 65 1,902 — 1,862 Total other investments $ 63,558 $ 66,363 $ 49,922 $ 52,654

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions	The following table presents the absolute notional amounts related to open positions on NW Natural derivative instruments: At December 31, In thousands 2018 2017 Natural gas (in therms): Financial 408,850 429,100 Physical 472,275 520,268 Foreign exchange $ 6,936 $ 7,669
Income Statement Presentation of Derivative Instruments	The following table reflects the income statement presentation for the unrealized gains and losses from NW Natural's derivative instruments, which also represents all derivative instruments at NW Holdings: December 31, 2018 December 31, 2017 In thousands Natural gas commodity Foreign exchange Natural gas commodity Foreign exchange Benefit (expense) to cost of gas $ (1,239 ) $ (284 ) $ (26,000 ) $ 107 Operating revenues 1,660 — (1,021 ) — Amounts deferred to regulatory accounts on balance sheet (211 ) 284 26,665 (107 ) Total gain (loss) in pre-tax earnings $ 210 $ — $ (356 ) $ —
Table Of Estimated Collateral Calls Table	Credit Rating Downgrade Scenarios In thousands (Current Ratings) A+/A3 BBB+/Baa1 BBB/Baa2 BBB-/Baa3 Specu-lative With Adequate Assurance Calls $ — $ — $ — $ (3,940 ) $ (6,059 ) Without Adequate Assurance Calls — — — (3,940 ) (4,452 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule Of Future Minimum Lease Payments For Operating And Capital Leases	In thousands Minimum lease payments 2019 $ 5,368 2020 4,812 2021 7,077 2022 7,223 2023 7,304 Thereafter 149,881 Total $ 181,665
Long-term Purchase Commitment	The aggregate amounts of these agreements were as follows at December 31, 2018 : In thousands Gas Pipeline Pipeline 2019 $ 144,500 $ 78,449 $ 4,272 2020 2,776 76,613 3,560 2021 2,313 66,656 — 2022 — 61,075 — 2023 — 60,619 — Thereafter — 580,022 — Total 149,589 923,434 7,832 Less: Amount representing interest 1,314 201,224 183 Total at present value $ 148,275 $ 722,210 $ 7,649

Environmental Matters (Tables)

Environmental Matters (Tables)	12 Months Ended
	Dec. 31, 2018
Environmental Remediation Obligations [Abstract]
Schedule of Environmental Loss Contingencies by Site	The following table summarizes information regarding liabilities related to environmental sites, which are recorded in other current liabilities and other noncurrent liabilities in NW Natural's balance sheet at December 31: Current Liabilities Non-Current Liabilities In thousands 2018 2017 2018 2017 Portland Harbor site: Gasco/Siltronic Sediments $ 5,117 $ 2,683 $ 44,351 $ 45,346 Other Portland Harbor 2,600 1,949 6,273 4,163 Gasco/Siltronic Upland site 13,983 13,422 44,830 47,835 Central Service Center site 10 25 — — Front Street site 11,402 1,009 3 10,757 Oregon Steel Mills — — 179 179 Total $ 33,112 $ 19,088 $ 95,636 $ 108,280
Environmental Regulatory Table	The following table presents information regarding the total regulatory asset deferred as of December 31: In thousands 2018 2017 Deferred costs and interest (1) $ 41,883 $ 45,546 Accrued site liabilities (2) 128,369 126,950 Insurance proceeds and interest (88,502 ) (94,170 ) Total regulatory asset deferral (1) $ 81,750 $ 78,326 Current regulatory assets (3) 5,601 6,198 Long-term regulatory assets (3) 76,149 72,128 (1) Includes pre-review and post-review deferred costs, amounts currently in amortization, and interest, net of amounts collected from customers. (2) Excludes 3.32% of the Front Street site liability, or $0.4 million in 2018 and $0.4 million in 2017 , as the OPUC only allows recovery of 96.68% of costs for those sites allocable to Oregon, including those that historically served only Oregon customers. (3) Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, NW Natural earns a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. NW Natural also accrues a carrying charge on insurance proceeds for amounts owed to customers. In Washington, a carrying charge related to deferred amounts will be determined in a future proceeding. Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5.0 million tariff rider. The amounts allocable to Oregon are recoverable through NGD rates, subject to an earnings test.

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Groups, Including Discontinued Operations	The following table presents the operating results prior to the holding company reorganization effective October 1, 2018 of NWN Energy, NWN Gas Storage, Gill Ranch, NNG Financial, NWN Water, and NW Holdings, which were historically reported within the gas storage segment and other, and is presented net of tax on NW Natural's consolidated statements of comprehensive income: NW Natural Discontinued Operations In thousands, except per share data 2018 2017 2016 Revenues $ 3,016 $ 7,360 $ 8,018 Expenses Operations and maintenance 4,151 7,423 7,387 General taxes 448 1,410 1,317 Depreciation and amortization 420 4,555 4,714 Other expenses and interest 342 650 1,097 Impairment expense — 192,478 — Total expenses 5,361 206,516 14,515 Loss from discontinued operations before income tax (2,345 ) (199,156 ) (6,497 ) Income tax benefit (1) (622 ) (71,813 ) (2,557 ) Loss from discontinued operations, net of tax $ (1,723 ) $ (127,343 ) $ (3,940 ) (1) 2017 income tax benefit includes approximately $18 million of tax benefit from the enactment of the TCJA. The TCJA was enacted December 22, 2017 and resulted in the federal tax rate changing from 35% to 21% .
Northwest Holdings [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Groups, Including Discontinued Operations	The following table presents the operating results of Gill Ranch, which was historically reported within the gas storage segment, and is presented net of tax on NW Holdings' consolidated statements of comprehensive income: NW Holdings Discontinued Operations In thousands, except per share data 2018 2017 2016 Revenues $ 3,579 $ 7,135 $ 7,794 Expenses Operations and maintenance 5,771 7,245 6,643 General taxes 479 1,373 1,295 Depreciation and amortization 430 4,525 4,685 Other expenses and interest 609 975 992 Impairment expense — 192,478 — Total expenses 7,289 206,596 13,615 Loss from discontinued operations before income tax (3,710 ) (199,461 ) (5,821 ) Income tax benefit (1) (968 ) (71,765 ) (2,297 ) Loss from discontinued operations, net of tax $ (2,742 ) $ (127,696 ) $ (3,524 ) Loss from discontinued operations per share of common stock: Basic $ (0.10 ) $ (4.45 ) $ (0.13 ) Diluted $ (0.09 ) $ (4.44 ) $ (0.13 ) (1) 2017 income tax benefit includes approximately $18 million of tax benefit from the enactment of the TCJA. The TCJA was enacted December 22, 2017 and resulted in the federal tax rate changing from 35% to 21% .
Northwest Natural Gas Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Groups, Including Discontinued Operations	The following table presents the carrying amounts of the major components of NWN Energy, NWN Gas Storage, Gill Ranch, NNG Financial, NWN Water, and NW Holdings that are classified as discontinued operations assets and liabilities on NW Natural's consolidated balance sheets: NW Natural Discontinued Operations In thousands 2017 Assets: Cash $ 362 Accounts receivable 2,126 Intercompany receivables 3,664 Inventories 396 Other current assets 622 Property, plant, and equipment 11,191 Less: Accumulated depreciation 192 Other investments 13,710 Other non-current assets — Discontinued operations - current assets 7,170 Discontinued operations - non-current assets 24,709 Total discontinued operations assets $ 31,879 Liabilities: Accounts payable $ 1,954 Intercompany payables 266 Other current liabilities 345 Deferred tax liabilities (16,862 ) Other non-current liabilities 12,130 Discontinued operations - current liabilities 2,565 Discontinued operations - non-current liabilities (4,732 ) Total discontinued operations liabilities $ (2,167 )
Northwest Holdings [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Groups, Including Discontinued Operations	The following table presents the carrying amounts of the major components of Gill Ranch that are classified as discontinued operations assets and liabilities on NW Holdings' consolidated balance sheets: NW Holdings Discontinued Operations In thousands 2018 2017 Assets: Accounts receivable $ 390 $ 2,126 Inventories 685 396 Other current assets 333 535 Property, plant, and equipment 11,621 10,816 Less: Accumulated depreciation 7 — Other non-current assets 247 1 Discontinued operations - current assets 13,269 3,057 Discontinued operations - non-current assets — 10,817 Total discontinued operations assets $ 13,269 $ 13,874 Liabilities: Accounts payable $ 873 $ 1,287 Other current liabilities 307 306 Other non-current liabilities 11,779 12,043 Discontinued operations - current liabilities 12,959 1,593 Discontinued operations - non-current liabilities — 12,043 Total discontinued operations liabilities $ 12,959 $ 13,636 (1) The total assets and liabilities of Gill Ranch are classified as current as of December 31, 2018 because it is probable that the sale will be completed within one year.

Quarterly Financial Informati_2

Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2018
Entity Information [Line Items]
Schedule of Quarterly Financial Information [Table Text Block]	NORTHWEST NATURAL HOLDING COMPANY QUARTERLY FINANCIAL INFORMATION (UNAUDITED) NW Holdings Quarter ended (1) In thousands, except per share data March 31 June 30 September 30 December 31 2018 Operating revenues $ 263,635 $ 124,567 $ 91,239 $ 226,702 Net income (loss) from continuing operations 42,011 (339 ) (11,144 ) 36,783 Loss from discontinued operations, net of tax (474 ) (659 ) (650 ) (959 ) Net income (loss) 41,537 (998 ) (11,794 ) 35,824 Average common shares outstanding: Basic 28,753 28,791 28,815 28,851 Diluted 28,803 28,791 28,815 28,940 Earnings (loss) from continuing operations per share of common stock: Basic 1.46 (0.01 ) (0.39 ) 1.27 Diluted 1.46 (0.01 ) (0.39 ) 1.27 Loss from discontinued operations per share of common stock: Basic (0.02 ) (0.02 ) (0.02 ) (0.03 ) Diluted (0.02 ) (0.02 ) (0.02 ) (0.03 ) Earnings (loss) per share of common stock: Basic 1.44 (0.03 ) (0.41 ) 1.24 Diluted 1.44 (0.03 ) (0.41 ) 1.24 2017 Operating revenues $ 295,724 $ 134,476 $ 86,212 $ 238,626 Net income (loss) from continuing operations 41,397 4,075 (7,887 ) 34,488 Loss from discontinued operations, net of tax (1,087 ) (1,346 ) (608 ) (124,655 ) Net income (loss) 40,310 2,729 (8,495 ) (90,167 ) Average common shares outstanding: Basic 28,633 28,648 28,678 28,716 Diluted 28,723 28,717 28,678 28,797 Earnings (loss) from continuing operations per share of common stock: Basic 1.45 0.14 (0.28 ) 1.20 Diluted 1.44 0.14 (0.28 ) 1.20 Loss from discontinued operations per share of common stock: Basic (0.04 ) (0.04 ) (0.02 ) (4.34 ) Diluted (0.04 ) (0.04 ) (0.02 ) (4.33 ) Earnings (loss) per share of common stock: Basic 1.41 0.10 (0.30 ) (3.14 ) Diluted 1.40 0.10 (0.30 ) (3.13 ) (1) Quarterly earnings (loss) per share are based upon the average number of common shares outstanding during each quarter. Variations in earnings between quarterly periods are due primarily to the seasonal nature of our business.
Northwest Natural Gas Company [Member]
Entity Information [Line Items]
Schedule of Quarterly Financial Information [Table Text Block]	NORTHWEST NATURAL GAS COMPANY QUARTERLY FINANCIAL INFORMATION (UNAUDITED) NW Natural Quarter ended In thousands March 31 June 30 September 30 December 31 2018 Operating revenues $ 263,635 $ 124,563 $ 91,227 $ 226,146 Net income (loss) from continuing operations 42,014 (271 ) (11,275 ) 37,581 Loss from discontinued operations, net of tax (477 ) (727 ) (519 ) — Net income (loss) 41,537 (998 ) (11,794 ) 37,581 2017 Operating revenues $ 295,668 $ 134,420 $ 86,157 $ 238,793 Net income (loss) from continuing operations 41,438 4,072 (7,876 ) 34,086 Loss from discontinued operations, net of tax (1,128 ) (1,343 ) (619 ) (124,253 ) Net income (loss) 40,310 2,729 (8,495 ) (90,167 )

Condensed Financials Standalo_2

Condensed Financials Standalone Holdco NW Holdings Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2018
Condensed Income Statements, Captions [Line Items]
Condensed Income Statement [Table Text Block]	NORTHWEST NATURAL HOLDING COMPANY CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (PARENT COMPANY ONLY) In thousands Inception through December 31, 2018 Operating expenses: Operations and maintenance $ 838 Total operating expenses 838 Loss from operations (838 ) Earnings from investment in subsidiaries, net of tax 36,469 Other income (expense), net 36 Interest expense, net 53 Income before income taxes 35,614 Income tax expense (benefit) (225 ) Net income $ 35,839 See Notes to Condensed Financial Statements
Condensed Balance Sheet [Table Text Block]	NORTHWEST NATURAL HOLDING COMPANY CONDENSED BALANCE SHEETS (PARENT COMPANY ONLY) As of December 31, In thousands 2018 Assets: Current assets: Cash and cash equivalents $ 4,011 Receivables from affiliates 2,796 Income taxes receivable 6,000 Other current assets 3,078 Total current assets 15,885 Non-current assets: Investments in subsidiaries 754,971 Other investments 65 Other non-current assets 310 Total non-current assets 755,346 Total assets $ 771,231 Liabilities and equity: Current liabilities: Accounts payable 168 Payables to affiliates 9,166 Interest accrued 32 Total current liabilities 9,366 Long-term debt (1 ) Deferred credits and other non-current liabilities: Deferred tax liabilities 7 Total deferred credits and other non-current liabilities 7 Equity: Common stock 739,722 Retained earnings 22,137 Total equity 761,859 Total liabilities and equity $ 771,231 See Notes to Condensed Financial Statements
Condensed Cash Flow Statement [Table Text Block]	NORTHWEST NATURAL HOLDING COMPANY CONDENSED STATEMENTS OF CASH FLOWS (PARENT COMPANY ONLY) In thousands Inception through December 31, 2018 Operating activities: Net income $ 35,839 Adjustments to reconcile net income to cash used in operations: Equity in earnings of subsidiaries, net of tax (36,469 ) Deferred income taxes 7 Other 15 Changes in assets and liabilities: Receivables, net (585 ) Income and other taxes (9,034 ) Accounts payable 9,304 Interest accrued 32 Other, net (44 ) Cash used in operating activities (935 ) Investing activities: Contributions to subsidiaries (1,804 ) Cash used in investing activities (1,804 ) Financing activities: Cash dividend payments on common stock (12,923 ) Capital contributions 20,000 Other (327 ) Cash provided by financing activities 6,750 Increase in cash and cash equivalents 4,011 Cash and cash equivalents, beginning of period — Cash and cash equivalents, end of period $ 4,011 See Notes to Condensed Financial Statements

Valuation Allowances and Rese_2

Valuation Allowances and Reserves (Tables)	12 Months Ended
	Dec. 31, 2018
Entity Information [Line Items]
Summary of Valuation Allowance	NORTHWEST NATURAL HOLDING COMPANY SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES COLUMN A COLUMN B COLUMN C COLUMN D COLUMN E Additions Deductions In thousands (year ended December 31) Balance at beginning of period Charged to costs and expenses Charged to other accounts Net write-offs Balance at end of period 2018 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 956 $ 680 $ — $ 659 $ 977 2017 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 1,290 $ 865 $ — $ 1,199 $ 956 2016 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 870 $ 1,246 $ — $ 826 $ 1,290
Northwest Natural Gas Company [Member]
Entity Information [Line Items]
Summary of Valuation Allowance	NORTHWEST NATURAL GAS COMPANY SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES COLUMN A COLUMN B COLUMN C COLUMN D COLUMN E Additions Deductions In thousands (year ended December 31) Balance at beginning of period Charged to costs and expenses Charged to other accounts Net write-offs Balance at end of period 2018 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 956 $ 678 $ — $ 659 $ 975 2017 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 1,290 $ 865 $ — $ 1,199 $ 956 2016 Reserves deducted in balance sheet from assets to which they apply: Allowance for uncollectible accounts $ 870 $ 1,246 $ — $ 826 $ 1,290

Significant Accounting Polici_4

Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 41,930	$ 45,781
Regulatory Assets, Noncurrent	371,786	356,608
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	41,930	45,781
Regulatory Assets, Noncurrent	371,786	356,608
Northwest Natural Gas Company [Member] | Unrealized loss on derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	12,381	18,712
Regulatory Assets, Noncurrent	3,025	4,649
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,873	154
Regulatory Assets, Noncurrent	9,978	84
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,601	6,198
Regulatory Assets, Noncurrent	76,149	72,128
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,140	11,227
Regulatory Assets, Noncurrent	2,545	3,970
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,218	2,218
Regulatory Assets, Noncurrent	19,185	19,991
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	74,173	60,383
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	174,993	179,824
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,717	7,272
Regulatory Assets, Noncurrent	$ 11,738	$ 15,579

Significant Accounting Polici_5

Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 47,436	$ 34,013
Regulatory Liability, Noncurrent	611,560	586,093
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	47,436	34,013
Regulatory Liability, Noncurrent	611,560	586,093
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	17,182	14,886
Regulatory Liability, Noncurrent	552	4,630
Northwest Natural Gas Company [Member] | Unrealized gain on derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	8,740	1,674
Regulatory Liability, Noncurrent	725	1,306
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,264	322
Regulatory Liability, Noncurrent	0	957
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	19,250	17,131
Regulatory Liability, Noncurrent	4,411	4,965
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	225,408	213,306
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 380,500	$ 360,929

- Narrative (Details)

- Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Accrued Asset Removal Costs [Abstract]
Weighted Average Depreciation Rates Transmission Distribution		2.70%
Weighted Average Depreciation Rates Gas Storage		2.10%
Weighted Average Depreciation Rates General Plant		4.50%
Weighted Average Depreciation Rates Intangible Other		3.10%
Public Utilities, AFUDC, Rate		5.20%	5.50%	0.70%
Revenue Recognition [Abstract]
Accrued unbilled revenue	$ 57,827	$ 57,827	$ 62,381
Inventory Disclosure [Abstract]
Inventory, Raw Materials and Supplies, Net of Reserves	14,200	14,200	10,900
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	8,954	8,954	0
Goodwill and Intangible Asset Impairment	0
Operating Expenses		573,981	604,136	$ 517,456
Accounts Payable [Member]
Cash and Cash Equivalents [Abstract]
Bank Overdrafts	2,700	2,700	4,800
Gill Ranch [Member] | Discontinued Operations, Held-for-sale [Member]
Property, Plant and Accrued Asset Removal Costs [Abstract]
Impairment of Long-Lived Assets		$ 0	$ 192,478	$ 0
Natural Gas Distribution [Member]
Property, Plant and Accrued Asset Removal Costs [Abstract]
Weighted-average depreciation rate, assets in service		2.80%	2.80%	2.80%
Northwest Natural Gas Company [Member]
Industry Regulation [Abstract]
Regulatory Asset, Amortization Period		12 months
Regulatory Asset, Amortization Period, Low End of Range		less than one year
Regulatory Liability, Amortization Period		12 months
Regulatory Liability, Amortization Period, Low End Of Range		less than one year
Revenue Recognition [Abstract]
Accrued unbilled revenue	57,773	$ 57,773	$ 62,381
Revenue Taxes [Abstract]
Revenue Taxes		30,100	19,100	$ 17,100
Inventory Disclosure [Abstract]
Inventory - Gas	$ 29,900	29,900	36,700
Goodwill and Intangible Assets Disclosure [Abstract]
Operating Expenses		$ 572,472	$ 604,116	$ 516,662
Northwest Natural Gas Company [Member] | Oregon [Member]
Derivative Instruments and Hedging Activities Disclosure [Abstract]
PGA deferral rate		90.00%	90.00%	90.00%
Northwest Natural Gas Company [Member] | Oregon [Member] | Minimum [Member]
Derivative Instruments and Hedging Activities Disclosure [Abstract]
PGA deferral rate		80.00%
Impact on earnings from gas cost sharing		10.00%
Northwest Natural Gas Company [Member] | Oregon [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosure [Abstract]
PGA deferral rate		90.00%
Impact on earnings from gas cost sharing		20.00%
Northwest Natural Gas Company [Member] | Washington [Member]
Derivative Instruments and Hedging Activities Disclosure [Abstract]
PGA deferral rate		100.00%	100.00%	100.00%
Asset Recoverable Gas Costs [Member] | Northwest Natural Gas Company [Member]
Industry Regulation [Abstract]
Regulatory Liability, Amortization Period		12 months

Significant Accounting Polici_6

Significant Accounting Policies - Retirement Benefits ASU 2017-07 (Details) - Northwest Natural Gas Company [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 155,225	$ 152,180	$ 135,979
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		$ 5,600	$ 6,600

Significant Accounting Polici_7

Significant Accounting Policies - Revenue from Contract with Customer (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 706,143	$ 755,038	$ 668,173
Operating Expenses									573,981	604,136	517,456
Revenue taxes									30,082	0	0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues									676,000
Operating Expenses									543,900
Northwest Holdings and Northwest Natural [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes									30,100	32,200	28,300
Northwest Holdings and Northwest Natural [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues									30,100
Operating Expenses									$ 30,100
Northwest Holdings and Northwest Natural [Member] | Revenue Taxes in Operating Revenues [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes										19,100	17,100
Northwest Holdings and Northwest Natural [Member] | Revenue Taxes on Balance Sheet [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes										$ 13,100	$ 11,200

Significant Accounting Polici_8

Significant Accounting Policies - Leases ASU 2016-02 (Details) - Accounting Standards Update 2016-02 [Member] $ in Thousands	3 Months Ended
	Dec. 31, 2018USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	$ 7,300
Northwest Natural Gas Company [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	$ 7,000

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Line Items]
Net income from continuing operations	$ 36,783	$ (11,144)	$ (339)	$ 42,011	$ 34,488	$ (7,887)	$ 4,075	$ 41,397	$ 67,311	$ 72,073	$ 62,419
Loss from discontinued operations, net of tax	(959)	(650)	(659)	(474)	(124,655)	(608)	(1,346)	(1,087)	(2,742)	(127,696)	(3,524)
Net income (loss)	$ 35,824	$ (11,794)	$ (998)	$ 41,537	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	$ 64,569	$ (55,623)	$ 58,895
Average common shares outstanding:
Average common shares outstanding - basic (in shares)	28,851	28,815	28,791	28,753	28,716	28,678	28,648	28,633	28,803	28,669	27,647
Additional shares for stock-based compensation plans									70	84	132
Average common shares outstanding - diluted (in shares)	28,940	28,815	28,791	28,803	28,797	28,678	28,717	28,723	28,873	28,753	27,779
Earnings from continuing operations per share of common stock:
Basic (in dollars per share)	$ 1.27	$ (0.39)	$ (0.01)	$ 1.46	$ 1.20	$ (0.28)	$ 0.14	$ 1.45	$ 2.34	$ 2.51	$ 2.26
Diluted (in dollars per share)	1.27	(0.39)	(0.01)	1.46	1.20	(0.28)	0.14	1.44	2.33	2.51	2.25
Loss from discontinued operations per share of common stock:
Basic (in dollars per share)	(0.03)	(0.02)	(0.02)	(0.02)	(4.34)	(0.02)	(0.04)	(0.04)	(0.10)	(4.45)	(0.13)
Diluted (in dollars per share)	(0.03)	(0.02)	(0.02)	(0.02)	(4.33)	(0.02)	(0.04)	(0.04)	(0.09)	(4.44)	(0.13)
Earnings (loss) per share of common stock
Basic (in dollars per share)	1.24	(0.41)	(0.03)	1.44	(3.14)	(0.30)	0.10	1.41	2.24	(1.94)	2.13
Diluted (in dollars per share)	$ 1.24	$ (0.41)	$ (0.03)	$ 1.44	$ (3.13)	$ (0.30)	$ 0.10	$ 1.40	$ 2.24	$ (1.93)	$ 2.12
Antidilutive shares									2	13	5
Discontinued Operations, Held-for-sale [Member] | Gill Ranch [Member]
Income Statement [Line Items]
Loss from discontinued operations, net of tax									$ (2,742)	$ (127,696)	$ (3,524)
Loss from discontinued operations per share of common stock:
Basic (in dollars per share)									$ (0.10)	$ (4.45)	$ (0.13)
Diluted (in dollars per share)									$ (0.09)	$ (4.44)	$ (0.13)

Segment Information - Narrative

Segment Information - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 3,242,662	$ 3,039,746
Operating Segments [Member]
Segment Reporting Information [Line Items]
Number of Reportable Segments	1
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Assets	$ 3,141,969	$ 2,961,326	$ 2,806,627
Northwest Holdings and Northwest Natural [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are Not In Utility Rates	80.00%
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are In Utility Rates	33.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account	20.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account When In Rates	67.00%
Northwest Holdings and Northwest Natural [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment Reporting, Disclosure of Major Customers	0
Customer Total [Member] | Geographic Concentration Risk [Member] | Oregon [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	89.00%
Customer Total [Member] | Geographic Concentration Risk [Member] | Washington [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	11.00%
Volumes Delivered [Member] | Residential and Commerical Customers [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	60.00%
Gross Profit Margin [Member] | Residential and Commerical Customers [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage	90.00%

Segment Information - Schedule

Segment Information - Schedule of Summarized Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 706,143	$ 755,038	$ 668,173
Depreciation and amortization									85,156	81,053	77,604
Income from operations									132,162	150,902	150,717
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	36,783	(11,144)	(339)	42,011	34,488	(7,887)	4,075	41,397	67,311	72,073	62,419
Capital expenditures									214,636	213,325	138,357
Total assets	3,242,662				3,039,746				3,242,662	3,039,746
Discontinued operations - current assets	13,269				3,057				13,269	3,057
Discontinued operations - total assets					13,900					13,900
Discontinued operations - non-current assets	0				10,817				0	10,817	210,400
Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues									24,923	22,096	17,472
Depreciation and amortization									1,254	1,290	1,286
Income from operations									15,004	12,472	14,109
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									10,558	11,211	8,268
Capital expenditures									2,005	1,653	283
Total assets	50,767				50,471				50,767	50,471	48,719
Income Tax Expense (Benefit)										(4,000)
Northwest Natural Gas Company [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									705,571	755,038	667,949
Depreciation and amortization									84,986	81,024	77,575
Income from operations									133,099	150,922	151,287
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									68,049	71,720	62,835
Capital expenditures									214,328	213,325	138,357
Total assets	3,192,736				3,011,797				3,192,736	3,011,797	2,855,346
Northwest Holdings Other [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues									572	0	224
Depreciation and amortization									170	29	29
Income from operations									(937)	(20)	(570)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									(738)	353	(416)
Capital expenditures									308	0	0
Total assets	36,657				14,075				36,657	14,075	14,040
Income Tax Expense (Benefit)										(400)
Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									680,648	732,942	650,477
Depreciation and amortization									83,732	79,734	76,289
Income from operations									118,095	138,450	137,178
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									57,491	60,509	54,567
Capital expenditures									212,323	211,672	138,074
Total assets	3,141,969				2,961,326				3,141,969	2,961,326	2,806,627
Income Tax Expense (Benefit)										1,000
Regulatory disallowance of environmental expense, net of tax											2,000
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	226,146	91,227	124,563	263,635	238,793	86,157	134,420	295,668	705,571	755,038	667,949
Depreciation and amortization									84,986	81,024	77,575
Income from operations									133,099	150,922	151,287
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	37,581	$ (11,275)	$ (271)	$ 42,014	34,086	$ (7,876)	$ 4,072	$ 41,438	68,049	71,720	62,835
Capital expenditures									214,328	213,325	138,357
Total assets	3,192,736				3,043,676				3,192,736	3,043,676
Discontinued operations - current assets	0				7,170				0	7,170
Discontinued operations - total assets	0				4,732				0	4,732
Discontinued operations - non-current assets	0				24,709				0	24,709
Northwest Natural Gas Company [Member] | Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues									24,923
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues									705,571
Northwest Natural Gas Company [Member] | Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									680,648
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Discontinued operations - total assets					31,900					31,900
Discontinued operations - non-current assets											226,100
Parent Company [Member]
Segment Reporting Information [Line Items]
Operating revenues									706,143	755,038	668,173
Depreciation and amortization									85,156	81,053	77,604
Income from operations									132,162	150,902	150,717
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									67,311	72,073	62,419
Capital expenditures									214,636	213,325	138,357
Total assets	$ 3,229,393				$ 3,025,872				$ 3,229,393	$ 3,025,872	$ 2,869,386

Segment Information - Additiona

Segment Information - Additional Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 706,143	$ 755,038	$ 668,173
Cost of gas									255,519	324,795	260,588
Environmental remediation									11,127	15,291	13,298
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating revenues									680,648	732,942	650,477
Cost of gas									255,743	325,019	260,588
Environmental remediation									11,127	15,291	13,298
Revenue taxes									30,082	0	0
NGD margin									$ 383,696	$ 392,632	$ 376,591

Common Stock - Narrative (Detai

Common Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 16, 2016	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Authorized (in shares)			100,000,000		100,000,000
Issuance of common stock, net of issuance costs				$ 52,761
Restated Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number			55,938	180,163	91,688	352,688
Stock Repurchase Plan [Member]
Stock Repurchase Program
Stock Repurchase Program, Number of Shares Authorized to be Repurchased			2,800,000
Stock Repurchase Program, Authorized Amount			$ 100,000
Treasury Stock, Shares, Acquired			0
Repurchases Since Plan Inception in Year 2000 [Member] | Stock Repurchase Plan [Member]
Stock Repurchase Program
Treasury Stock, Shares, Acquired			2,100,000
Treasury Stock, Value, Acquired, Cost Method			$ 83,300
Northwest Holdings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserved Shares Dividend Reinvestment			394,204
Northwest Holdings [Member] | Employee Stock Purchase Plan Member [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance			24,339
Northwest Natural Gas Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Authorized (in shares)			100,000,000		100,000,000
Share Price	$ 54.63
Issuance of common stock, net of issuance costs	$ 52,800			$ 52,761
Northwest Natural Gas Company [Member] | Base shares offered [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity Issuance		880,000
Northwest Natural Gas Company [Member] | Over-Allotment Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity Issuance		132,000
Sale of Stock, Offering Period		30 days
Northwest Natural Gas Company [Member] | Total shares issued [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity Issuance	1,012,000

Common Stock - Schedule of Summ

Common Stock - Schedule of Summary of Changes in Common Stock (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Common Stock [Roll Forward]
Beginning Balance	28,736
Ending Balance	28,880	28,736
Common Stock [Member]
Increase (Decrease) in Common Stock [Roll Forward]
Beginning Balance	28,736	28,630	27,427
Sales to employees under ESPP	19	18	18
Stock-based compensation	64	88	173
Equity Issuance			1,012
Stock Issued During Period, Shares, Dividend Reinvestment Plan	61
Ending Balance	28,880	28,736	28,630

Revenue - Disaggregation of Rev

Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									$ 696,157
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	706,143	$ 755,038	$ 668,173
Natural Gas Distribution [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									670,662
Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									10,780
Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									8,548
Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									5,595
Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									572
Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									8,989
Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating Lease, Lease Income									997
Operating Segments [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									705,571	755,038	667,949
Operating Segments [Member] | Natural Gas Distribution [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									680,648	732,942	650,477
Corporate, Non-Segment [Member] | Other Northwest Natural [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									24,923	22,096	17,472
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									572
Operating revenues									572	0	224
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Natural Gas Distribution [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									572
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									0
Corporate, Non-Segment [Member] | Northwest Holdings Other [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating Lease, Lease Income									0
Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	$ 226,146	$ 91,227	$ 124,563	$ 263,635	$ 238,793	$ 86,157	$ 134,420	$ 295,668	705,571	$ 755,038	$ 667,949
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									695,585
Operating revenues									705,571
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Natural Gas Distribution [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									670,662
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									10,780
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									8,548
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									5,595
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									0
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									8,989
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating Lease, Lease Income									997
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									670,662
Operating revenues									680,648
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Natural Gas Distribution [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									670,662
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									0
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									0
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									0
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									0
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									8,989
Northwest Natural Gas Company [Member] | Operating Segments [Member] | Natural Gas Distribution [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating Lease, Lease Income									997
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									24,923
Operating revenues									24,923
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Natural Gas Distribution [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									10,780
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									8,548
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									5,595
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									0
Northwest Natural Gas Company [Member] | Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating Lease, Lease Income									$ 0

Revenue - Revenue, Remaining Pe

Revenue - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction (Details) $ in Millions	Dec. 31, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 14.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Year	2,019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 11.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Year	2,020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 10.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Year	2,021
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Year	2,022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 6.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Year	2,023
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 56

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - USD ($)	12 Months Ended			24 Months Ended	36 Months Ended			72 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares)	28,880,000	28,736,000		28,880,000	28,880,000	28,736,000		28,880,000
Proceeds from stock options exercised	$ 1,546,000	$ 4,819,000	$ 8,404,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	659,000	(930,000)	(924,000)
Allocated Share-based Compensation Expense, Net of Tax	1,830,000	1,424,000	1,446,000
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount	$ 531,000	$ 528,000	$ 554,000
Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,100,000			1,100,000	1,100,000			1,100,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant, Any Award Type	574,787			574,787	574,787			574,787
Number of RSUs [Roll Forward]
Nonvested, beginning of period	0
Nonvested, end of period	0	0		0	0	0		0
Long Term Incentive Plan [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement by Share Based Payment Award Performance Period	3 years
Share Based Compensation Award Target Share	55,793			55,793	55,793			55,793
Unrecognized compensation expense	$ 1,100,000			$ 1,100,000	$ 1,100,000			$ 1,100,000
Unrecognized Compensation Cost, Period for Recognition	through 2,019
Allocated Share-based Compensation Expense	$ 1,056,000
Number of RSUs [Roll Forward]
Nonvested, end of period	0			0	0			0
Weighted Average Price Per LTIP
Nonvested, beginning of period	$ 56.40
Vested	56.23
Nonvested, end of period	$ 57.05	$ 56.40		$ 57.05	$ 57.05	$ 56.40		$ 57.05
Long Term Incentive Plan [Member] | Performance Shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	111,586			111,586	111,586			111,586
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2016 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	24,421			24,421	24,421			24,421
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted	28,218
Allocated Share-based Compensation Expense	$ 598,000				$ 1,413,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2016 Award [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	48,842			48,842	48,842			48,842
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2014 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted			31,388
Allocated Share-based Compensation Expense			$ 168,000				$ 1,685,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2015 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted		18,304
Allocated Share-based Compensation Expense		$ (346,000)				$ 1,169,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2017 Award [Member
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	31,372			31,372	31,372			31,372
Allocated Share-based Compensation Expense	$ 458,000			$ 1,400,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2017 Award [Member | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	62,744			62,744	62,744			62,744
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2018 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	0			0	0			0
Allocated Share-based Compensation Expense	$ 0
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2018 Award [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	0			0	0			0
Long Term Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 3,100,000			$ 3,100,000	$ 3,100,000			$ 3,100,000
Unrecognized Compensation Cost, Period for Recognition	through 2,023
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Allocated Share-based Compensation Expense	$ 1,800,000	$ 1,600,000	$ 1,500,000
Number of RSUs [Roll Forward]
Nonvested, beginning of period	84,522	89,973	88,587	89,973	88,587
Granted	32,450	32,168	40,271
Vested	(32,689)	(35,341)	(29,488)
Forfeit	(1,603)	(2,278)	(9,397)
Nonvested, end of period	82,680	84,522	89,973	82,680	82,680	84,522	89,973	82,680
Weighted Average Price Per LTIP
Nonvested, beginning of period	$ 53.90	$ 48.85	$ 44.78	$ 48.85	$ 44.78
Granted	57.59	60.51	54.36
Vested	50.75	47.07	45.56
Forfeited	59.95	53.78	44.59
Nonvested, end of period	$ 56.47	53.90	$ 48.85	$ 56.47	$ 56.47	$ 53.90	$ 48.85	$ 56.47
Restated Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross								0
Proceeds from stock options exercised	$ 1,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years 7 days
Share Based Compensation Arrangement By Share Based Payment Award Exchange Period	6 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	1 year 8 months 10 days
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ (200,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 43.29	$ 41.15
Option Shares
Exercised	(35,450)	(88,275)	(172,525)
Forfeited	(300)	(200)	0
Balance outstanding, end of period	55,938	91,688	180,163	55,938	55,938	91,688	180,163	55,938
Weighted-Average Price Per Share
Balance outstanding, beginning of period	$ 44.43	$ 44.38	$ 44	$ 44.38	$ 44
Exercised	43.61	44.33	43.61
Balance outstanding, end of period	$ 44.96	$ 44.43	$ 44.38	$ 44.96	$ 44.96	$ 44.43	$ 44.38	$ 44.96
Intrinsic Value balance, beginning of period	$ 1,400,000	$ 2,800,000	$ 2,300,000	$ 2,800,000	$ 2,300,000
Intrinsic value, Exercised	800,000	1,800,000	2,000,000
Intrinsic Value balance, end of period	$ 900,000	1,400,000	2,800,000	$ 900,000	$ 900,000	$ 1,400,000	$ 2,800,000	$ 900,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent Of Offering Date Price Employees Are Allowed To Purchase Stock	85.00%
Maximum Value Of Stock Employees Are Allowed To Purchase	$ 21,205
Operations and Maintenance Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense	$ 2,489,000	$ 2,354,000	$ 2,370,000
Stock Repurchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Treasury Stock, Shares, Acquired	0
Stock Repurchase Plan [Member] | Repurchases Since Plan Inception in Year 2000 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Treasury Stock, Shares, Acquired	2,100,000
Treasury Stock, Value, Acquired, Cost Method	$ 83,300,000
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares)	28,880,000	28,736,000	28,630,000	28,880,000	28,880,000	28,736,000	28,630,000	28,880,000	27,427,000
Target Achieved [Member] | Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2018 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share	34,702			34,702	34,702			34,702

Debt (Details)

Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instruments [Abstract]
Long-term Debt, Gross	$ 741,813	$ 786,700
Long-Term Debt, Current Maturities, Gross	30,000	97,000
Long-term Debt, Excluding Current Maturities, Gross	711,813	689,700
Debt Outstanding
Proceeds from Issuance of Debt		100,000
Short-term Debt, Other Disclosures [Abstract]
Short-term debt	217,620	54,200
Bank Loans	$ 0	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	3.00%	1.90%
Commercial Paper, Maximum Maturity	46 days
Commercial Paper, Average Maturity	22 days
Letters of Credit Outstanding, Amount	$ 2,800
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt, Gross	741,813	$ 786,700
Note 4110 Series Due 2048 [Member]
Debt Outstanding
Proceeds from Issuance of Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.11%
Note 1545 Series Due 2018 [Member]
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	1.55%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 75,000
Note 6600 Series B Due 2018 [Member]
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.60%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 22,000
Note 2822 Series Due 2027 [Member] [Domain]
Debt Outstanding
Proceeds from Issuance of Debt		$ 25,000
Debt Instrument, Interest Rate, Stated Percentage		2.822%
Note 3685 Series Due 2047 [Member] [Domain]
Debt Outstanding
Proceeds from Issuance of Debt		$ 75,000
Debt Instrument, Interest Rate, Stated Percentage		3.685%
Note 7000 Series Due 2017 [Member]
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage		7.00%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount		$ 40,000
Line of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	100,000
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity	150,000
Line of Credit Facility, Amount Outstanding	$ 0	0
Debt Instrument Covenant Consolidated Indebtedness To Capitalization Ratio	70.00%
Letter of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 40,000
Letters of Credit Outstanding, Amount	0	0
Northwest Holdings Other [Member]
Debt Instruments [Abstract]
Long-term Debt, Gross	2,113	0
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt, Gross	2,113	0
Northwest Natural Gas Company [Member]
Debt Instruments [Abstract]
Long-term Debt, Gross	739,700	786,700
Long-Term Debt, Current Maturities, Gross	30,000	97,000
Long-term Debt, Excluding Current Maturities, Gross	709,700	689,700
Liabilities, Fair Value Disclosure [Abstract]
Long-term Debt, Gross	739,700	786,700
Unamortized Debt Issuance Expense	5,577	6,813
Long-term Debt	734,123	779,887
Debt Instrument, Fair Value Disclosure	760,222	853,339
Northwest Natural Gas Company [Member] | Note 4110 Series Due 2048 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	0
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.11%
Northwest Natural Gas Company [Member] | Note 1545 Series Due 2018 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 0	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage		1.55%
Northwest Natural Gas Company [Member] | Note 6600 Series B Due 2018 [Member]
Debt Instruments [Abstract]
Secured Debt	0	$ 22,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage		6.60%
Northwest Natural Gas Company [Member] | Note 8310 Series B Due 2019 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	8.31%	8.31%
Northwest Natural Gas Company [Member] | Note 7630 Series B Due 2019 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.63%	7.63%
Northwest Natural Gas Company [Member] | Note 5370 Series B Due 2020 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 75,000	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.37%	5.37%
Northwest Natural Gas Company [Member] | Note 9050 Series A Due 2021 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	9.05%	9.05%
Northwest Natural Gas Company [Member] | Note 3176 Series B Due 2021 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.18%	3.18%
Northwest Natural Gas Company [Member] | Note 3542 Series B Due 2023 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.54%	3.54%
Northwest Natural Gas Company [Member] | Note 5620 Series B Due 2023 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.62%	5.62%
Northwest Natural Gas Company [Member] | Note 7720 Series B Due 2025 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.72%	7.72%
Northwest Natural Gas Company [Member] | Note 6520 Series B Due 2025 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.52%	6.52%
Northwest Natural Gas Company [Member] | Note 7050 Series B Due 2026 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.05%	7.05%
Northwest Natural Gas Company [Member] | Note 3211 Series B Due 2026 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 35,000	$ 35,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.21%	3.21%
Northwest Natural Gas Company [Member] | Note 2822 Series Due 2027 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 25,000	$ 25,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	2.82%	2.82%
Northwest Natural Gas Company [Member] | Note 7000 Series Due 2027 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Northwest Natural Gas Company [Member] | Note 6650 Series Due 2027 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 19,700	$ 19,700
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Northwest Natural Gas Company [Member] | Note 6650 Series Due 2028 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Northwest Natural Gas Company [Member] | Note 7740 Series Due 2030 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.74%	7.74%
Northwest Natural Gas Company [Member] | Note 7850 Series Due 2030 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.85%	7.85%
Northwest Natural Gas Company [Member] | Note 5820 Series Due 2032 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 30,000	$ 30,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.82%	5.82%
Northwest Natural Gas Company [Member] | Note 5660 Series Due 2032 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.66%	5.66%
Northwest Natural Gas Company [Member] | Note 5250 Series Due 2035 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.25%	5.25%
Northwest Natural Gas Company [Member] | Note 4000 Series Due 2042 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.00%	4.00%
Northwest Natural Gas Company [Member] | Note 3685 Series Due 2047 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 75,000	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.69%	3.69%
Northwest Natural Gas Company [Member] | Note 4136 Series Due 2046 [Member]
Debt Instruments [Abstract]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.14%	4.14%
Northwest Natural Gas Company [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	$ 30,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	75,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	60,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	90,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	484,700
Short-term Debt, Other Disclosures [Abstract]
Short-term debt	217,500	$ 54,200
Northwest Natural Gas Company [Member] | Line of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	300,000
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity	450,000
Line of Credit Facility, Amount Outstanding	$ 0
Debt Instrument Covenant Consolidated Indebtedness To Capitalization Ratio	70.00%
Northwest Natural Gas Company [Member] | Letter of Credit [Member]
Short-term Debt, Other Disclosures [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 60,000
Letters of Credit Outstanding, Amount	$ 0

Pension and Other Postretirem_3

Pension and Other Postretirement Benefits Costs - Changes in Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	$ 287,925
Fair value of plan assets at December 31	257,797	$ 287,925
Northwest Natural Gas Company [Member] | Other Postretirement Benefits Plan [Member]
Reconciliation of change in benefit obligation:
Obligation at January 1	28,927	29,395
Service cost	282	341
Interest cost	964	1,141
Net actuarial (gain) loss	327	213
Benefits paid	(1,674)	(1,737)	$ (1,732)
Obligation at December 31	28,172	28,927	29,395
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Employer contributions	1,674	1,737
Benefits paid	(1,674)	(1,737)	(1,732)
Fair value of plan assets at December 31	0	0	0
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Obligation at January 1	486,289	457,839
Service cost	7,185	7,090
Interest cost	16,991	18,111
Net actuarial (gain) loss	32,979	(34,829)
Benefits paid	(21,918)	(31,580)	(20,959)
Obligation at December 31	455,568	486,289	457,839
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	287,925	257,714
Actual return on plan assets	(25,925)	40,308
Employer contributions	17,715	21,483
Benefits paid	(21,918)	(31,580)	(20,959)
Fair value of plan assets at December 31	257,797	287,925	$ 257,714
Northwest Holdings and Northwest Natural [Member] | Other Postretirement Benefits Plan [Member]
Reconciliation of change in plan assets:
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(28,172)	(28,927)
Northwest Holdings and Northwest Natural [Member] | Pension Plan [Member]
Reconciliation of change in plan assets:
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (197,771)	(198,364)
Partial Buy-out, Assets Transferred to Insurer [Member] | Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Benefits paid		(9,300)
Reconciliation of change in plan assets:
Benefits paid		(9,300)
Partial Buy-out, Liabilities Transferred to Insurer [Member] | Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Benefits paid		(8,700)
Reconciliation of change in plan assets:
Benefits paid		$ (8,700)

Pension and Other Postretirem_4

Pension and Other Postretirement Benefits Costs - Narrative (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 22, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year	6.75%	6.75%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	4.75%	4.75%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate		2,025
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage		25.00%
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage		35.00%
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan Assets, Amount	$ 257,797	$ 257,797	$ 287,925
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	385,900	385,900	410,300
Non-Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan Assets, Amount	0	0
Defined Benefit Plan, Benefit Obligation	35,400	35,400	36,600
Northwest Natural Gas Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Amounts That Will Be Amortized From Accumulated Other Comprehensive Income Loss In Next Fiscal Year Regulatory		13,700
Defined Benefit Plan Amortization Of Net Gains Losses Regulatory		14,200
Defined Benefit Plan Amortization Of Net Prior Service Cost Credit Regulatory		(500)
Settlement expense	(600)	(600)
Payment for Pension and Other Postretirement Benefits		15,540
Defined Contribution Plan, Cost		6,500	5,400	$ 4,600
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan Assets, Amount	257,797	257,797	287,925	257,714
Total Pension Asset Investments Net Of Receivables And Payables	257,796	257,796	287,895
Defined Benefit Plan, Benefit Obligation	455,568	455,568	486,289	457,839
Payment for Pension and Other Postretirement Benefits		17,715	21,483
Expected future employer contributions	13,318	13,318
Northwest Natural Gas Company [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan Assets, Amount	0	0	0	0
Defined Benefit Plan, Benefit Obligation	28,172	28,172	28,927	29,395
Payment for Pension and Other Postretirement Benefits		1,674	1,737
Expected future employer contributions	1,787	1,787
Northwest Natural Gas Company [Member] | Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(162,400)	(162,400)
Plan Assets, Amount	257,800	257,800	287,900
Defined Benefit Plan, Benefit Obligation	420,200	420,200	449,700
Expected future employer contributions	11,000	11,000
Northwest Natural Gas Company [Member] | Western States Office and Professional Employees Union Pension Fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plans, Withdrawal Obligation	6,813	6,813			$ 8,300
Multiemployer Plans, Plan Contributions		600
Multiemployer Plan, Contributions by Employer		$ 600	600	600
Northwest Natural Gas Company [Member] | Western States Office and Professional Employees Union Pension Fund [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plan, Collective-Bargaining Arrangement, Percentage of Contributions Required for Multiple Collective-Bargaining Arrangements		4.00%
Northwest Natural Gas Company [Member] | Western States Office and Professional Employees Union Pension Fund [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plan, Collective-Bargaining Arrangement, Percentage of Contributions Required for Multiple Collective-Bargaining Arrangements		6.00%
Northwest Natural Gas Company Consolidated [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement expense	0	$ 0	0	(193)
Additional Amount Authorized for Inclusion in Rates	8,100
Northwest Natural Gas Company Consolidated [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement expense	$ 0	$ 0	$ 0	$ 0

Pension and Other Postretirem_5

Pension and Other Postretirement Benefits Costs - Schedule of Amounts Realized in Other Comprehensive Loss (Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Asset, Net actuarial loss (gain)	$ (14,261)	$ (12,177)	$ (14,005)
Settlement Loss	0	0	0
Regulatory Asset, Prior service cost	(42)	(127)	(230)
Regulatory Asset, Actuarial loss	(18,761)	(14,802)	(13,238)
Regulatory Asset, Total	(4,542)	(2,752)	537
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Asset, Net actuarial loss (gain)	327	214	1,488
Settlement Loss	0	0	0
Regulatory Asset, Prior service cost	468	468	468
Regulatory Asset, Actuarial loss	(448)	(696)	(705)
Regulatory Asset, Total	(307)	(442)	(1,725)
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Amounts reclassified to AOCL	(677)	2,777	(1,196)
Settlement Loss	0	0	193
Other Comprehensive Loss (Income), Prior service cost	0	0	0
Loss from plan settlement	(1,052)	(946)	1,386
Total reclassifications before tax	$ (1,729)	$ 1,831	$ 383

Pension and Other Postretirem_6

Pension and Other Postretirement Benefits Costs - Schedule of Amount Recognized in AOCL (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
AOCL, Prior Service Costs (Credit)	$ 0	$ 0
AOCL, Net Actuarial Loss	11,537	13,266
AOCL, Total	11,537	13,266
Pension Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets, Prior Service Cost (Credit)	7	49
Regulatory Assets, Net Actuarial Loss	170,535	175,035
Regulatory Assets, Total	170,542	175,084
Postretirement Benefit Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets, Prior Service Cost (Credit)	(1,738)	(2,206)
Regulatory Assets, Net Actuarial Loss	6,189	6,964
Regulatory Assets, Total	$ 4,451	$ 4,758

Pension and Other Postretirem_7

Pension and Other Postretirement Benefits Costs - Schedule of AOCL and Changes in AOCL, Non-Qualified Employee Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (8,438)
Ending balance	(7,188)	$ (8,438)
Northwest Holdings and Northwest Natural [Member] | Non-Qualified Pension Plans, Defined Benefit [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8,438)	(6,951)
Amounts reclassified to AOCL	642	(2,794)
Amortization of actuarial losses	1,052	946
Total reclassifications before tax	1,694	(1,848)
Tax expense (benefit)	(444)	361
Total reclassifications for the period	1,250	(1,487)
Ending balance	$ (7,188)	$ (8,438)

Pension and Other Postretirem_8

Pension and Other Postretirement Benefits Costs - Schedule of Pension Plan Target Allocations (Details)	Dec. 31, 2018
Long government/credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	20.00%
U.S. small/mid cap equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	10.00%
Emerging markets equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	5.00%
Northwest Natural Gas Company [Member] | U.S. large cap equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	18.00%
Northwest Natural Gas Company [Member] | Non-U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	18.00%
Northwest Natural Gas Company [Member] | Absolute Return Strategy Investment Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	12.00%
Northwest Natural Gas Company [Member] | Real estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	7.00%
Northwest Natural Gas Company [Member] | High yield bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	5.00%
Northwest Natural Gas Company [Member] | Emerging market debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation, percentage	5.00%

Pension and Other Postretirem_9

Pension and Other Postretirement Benefits Costs - Schedule of Components of Net Periodic Benefit Cost for NW Natural (Details) - Northwest Natural Gas Company Consolidated [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 282	$ 341	$ 391
Interest cost	964	1,141	1,175
Expected return on plan assets	0	0	0
Amortization of prior service costs	468	468	468
Amortization of net actuarial loss	448	696	705
Settlement expense	0	0	0
Net periodic benefit cost	1,226	1,710	1,803
Amount allocated to construction	(98)	(587)	(600)
Amount deferred to regulatory balancing account	0	0	0
Net amount charged to expense	1,128	1,123	1,203
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7,185	7,090	7,083
Interest cost	16,991	18,111	18,399
Expected return on plan assets	20,639	20,433	20,054
Amortization of prior service costs	(43)	(127)	(231)
Amortization of net actuarial loss	19,813	15,748	14,624
Settlement expense	0	0	(193)
Net periodic benefit cost	23,393	20,643	20,476
Amount allocated to construction	(2,764)	(6,597)	(5,746)
Amount deferred to regulatory balancing account	(10,314)	(6,542)	(6,252)
Net amount charged to expense	$ 10,315	$ 7,504	$ 8,478

Pension and Other Postretire_10

Pension and Other Postretirement Benefits Costs - Schedule of Assumptions Used (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefits Plan [Member]
Assumptions for net periodic benefit cost:
Weighted-average discount rate	3.44%	3.85%	4.00%
Assumptions for year-end funded status:
Weighted-average discount rate	4.13%	3.44%	3.85%
Pension Plan [Member]
Assumptions for net periodic benefit cost:
Weighted-average discount rate	3.51%	3.99%	4.17%
Expected long-term rate of return	7.50%	7.50%	7.50%
Assumptions for year-end funded status:
Weighted-average discount rate	4.20%	3.52%	4.00%
Expected long-term rate of return	7.50%	7.50%	7.50%
Minimum [Member] | Pension Plan [Member]
Assumptions for net periodic benefit cost:
Rate of Compensation Increase	3.25%	3.25%	3.25%
Assumptions for year-end funded status:
Rate of Compensation Increase	3.25%	3.25%	3.25%
Maximum [Member] | Pension Plan [Member]
Assumptions for net periodic benefit cost:
Rate of Compensation Increase	4.50%	4.50%	4.50%
Assumptions for year-end funded status:
Rate of Compensation Increase	3.50%	4.50%	4.50%

Pension and Other Postretire_11

Pension and Other Postretirement Benefits Costs - Schedule of Effect of One Percentage Point Change in Assumed Health Care Cost (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
1% Increase
Effect on net periodic postretirement health care benefit cost	$ 43
Effect on the accumulated postretirement benefit obligation	622
1% Decrease
Effect on net periodic postretirement health care benefit cost	39
Effect on the accumulated postretirement benefit obligation	$ 560

Pension and Other Postretire_12

Pension and Other Postretirement Benefits Costs - Schedule of Future Employer Contributions and Benefit Payments (Details) - Northwest Natural Gas Company [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension and Other Postretirement Benefits	$ 15,540
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension and Other Postretirement Benefits	17,715	$ 21,483
Expected future employer contributions	13,318
Benefits paid	21,918	31,580	$ 20,959
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2,019	22,699
2,020	23,622
2,021	24,516
2,022	25,316
2,023	26,074
2024-2028	145,917
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension and Other Postretirement Benefits	1,674	1,737
Expected future employer contributions	1,787
Benefits paid	1,674	$ 1,737	$ 1,732
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2,019	1,787
2,020	1,846
2,021	1,930
2,022	1,941
2,023	1,993
2024-2028	$ 9,628

Pension and Other Postretire_13

Pension and Other Postretirement Benefits Costs - Schedule of Fair Value of Plan Assets (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	$ 257,797	$ 287,925
Northwest Natural Gas Company [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	257,797	287,925	$ 257,714
Total Pension Asset Investments Net Of Receivables And Payables	257,796	287,895
Accrued interest and dividend income	1	30
Due from broker for securities sold	0	0
Total receivables	1	30
Due to broker for securities purchased	0	0
Northwest Natural Gas Company [Member] | United States Equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	85,233	98,375
Northwest Natural Gas Company [Member] | Non-U.S. equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	95,011	106,029
Northwest Natural Gas Company [Member] | Liability hedging [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	45,659	53,981
Northwest Natural Gas Company [Member] | Opportunistic [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	23,186	23,895
Northwest Natural Gas Company [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	8,707	5,615
Northwest Natural Gas Company [Member] | Level 1 [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	24,994
Total Pension Asset Investments Net Of Receivables And Payables		21,293
Northwest Natural Gas Company [Member] | Level 1 [Member] | United States Equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 1 [Member] | Non-U.S. equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	24,994	21,211
Northwest Natural Gas Company [Member] | Level 1 [Member] | Liability hedging [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 1 [Member] | Opportunistic [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	82
Northwest Natural Gas Company [Member] | Level 2 [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0
Total Pension Asset Investments Net Of Receivables And Payables		0
Northwest Natural Gas Company [Member] | Level 2 [Member] | United States Equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 2 [Member] | Non-U.S. equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 2 [Member] | Liability hedging [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 2 [Member] | Opportunistic [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 3 [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0
Total Pension Asset Investments Net Of Receivables And Payables		0
Northwest Natural Gas Company [Member] | Level 3 [Member] | United States Equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 3 [Member] | Non-U.S. equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 3 [Member] | Liability hedging [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 3 [Member] | Opportunistic [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	0	0
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	232,802
Total Pension Asset Investments Net Of Receivables And Payables		266,602
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member] | United States Equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	85,233	98,375
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member] | Non-U.S. equity [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	70,017	84,818
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member] | Liability hedging [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	45,659	53,981
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member] | Opportunistic [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	23,186	23,895
Northwest Natural Gas Company [Member] | Fair Value Measured at NAV [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Plan Assets, Amount	$ 8,707	$ 5,533

Income Tax (Details)

Income Tax (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				$ (76,000)	$ 0		$ 0
Deferred Tax Liabilities, Property, Plant and Equipment	$ 278,735,000	$ 288,385,000		288,385,000	278,735,000
Deferred Tax Liabilities Non Regulated Liabilities	2,766,000	1,061,000		1,061,000	2,766,000
Deferred Tax Liabilities, Gross	327,062,000	337,984,000		337,984,000	327,062,000
Deferred Tax Liabilities, Net	270,526,000	280,463,000		280,463,000	270,526,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	52,000		52,000	66,000
Deferred Tax Assets, Net	$ 56,536,000	57,521,000		57,521,000	56,536,000
Federal Statutory Income Tax Rate, Percent	21.00%					35.00%
Increase (Decrease) in Regulatory Liabilities				7,929,000	0		0
Effect of TCJA				0	(3,376,000)		0
Regulatory Liability, Noncurrent	$ 586,093,000	611,560,000		611,560,000	586,093,000
Unrecognized Tax Benefits	0	0		0	0		0
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				19,222,000	39,578,000		36,901,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,927,000	5,066,000		4,844,000
Income Tax Reconciliation, Differences Regulatory Commission				1,302,000	2,357,000		2,357,000
Effect of TCJA				0	(3,376,000)		0
Income Tax Reconciliation, Other Adjustments				(1,184,000)	(2,617,000)		(1,091,000)
Income Tax Expense (Benefit)				$ 24,191,000	$ 41,008,000		$ 43,011,000
Effective Income Tax Rate				26.40%	36.30%		40.80%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 8,953,000	$ 19,345,000		$ 10,042,000
Current State and Local Tax Expense (Benefit)				3,785,000	5,963,000		3,116,000
Current Income Tax Expense (Benefit)				12,738,000	25,308,000		13,158,000
Deferred Federal Income Tax Expense (Benefit)				9,001,000	13,869,000		25,473,000
Deferred State and Local Income Tax Expense (Benefit)				2,452,000	1,831,000		4,380,000
Deferred Income Tax Expense (Benefit)				11,453,000	15,700,000		29,853,000
Income Tax Expense (Benefit)				24,191,000	41,008,000		43,011,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				24,191,000	41,008,000		43,011,000
Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	23,352,000	27,135,000		27,135,000	23,352,000
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	22,209,000	21,403,000		21,403,000	22,209,000
Deferred Tax Assets, Regulatory Assets and Liabilities	56,470,000	57,469,000		57,469,000	56,470,000
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				(75,000)	0		0
Deferred Tax Liabilities, Property, Plant and Equipment	296,113,000	303,186,000		303,186,000	296,113,000
Deferred Tax Liabilities Non Regulated Liabilities	2,250,000	537,000		537,000	2,250,000
Deferred Tax Liabilities, Gross	343,924,000	352,260,000		352,260,000	343,924,000
Deferred Tax Liabilities, Net	287,388,000	294,739,000		294,739,000	287,388,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	52,000		52,000	66,000
Deferred Tax Assets, Net	56,536,000	57,521,000		57,521,000	56,536,000
Increase (Decrease) in Prepaid Taxes		500,000	$ 7,400,000
Increase (Decrease) in Deferred Income Taxes		300,000	4,100,000
Increase (Decrease) in Regulatory Liabilities		200,000	$ 3,300,000	7,929,000	0		0
Effect of TCJA				0	(2,956,000)		0
Regulatory Liability, Noncurrent	586,093,000	611,560,000		611,560,000	586,093,000
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				19,434,000	39,624,000		37,137,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,982,000	5,072,000		4,858,000
Income Tax Reconciliation, Differences Regulatory Commission				1,302,000	2,357,000		2,357,000
Effect of TCJA				0	(2,956,000)		0
Income Tax Reconciliation, Other Adjustments				(1,184,000)	(2,619,000)		(1,077,000)
Income Tax Expense (Benefit)				$ 24,459,000	$ 41,478,000		$ 43,275,000
Effective Income Tax Rate				26.40%	36.60%		40.80%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 9,127,000	$ 19,304,000		$ 10,158,000
Current State and Local Tax Expense (Benefit)				3,846,000	5,956,000		3,131,000
Current Income Tax Expense (Benefit)				12,973,000	25,260,000		13,289,000
Deferred Federal Income Tax Expense (Benefit)				9,025,000	14,371,000		25,581,000
Deferred State and Local Income Tax Expense (Benefit)				2,461,000	1,847,000		4,405,000
Deferred Income Tax Expense (Benefit)				11,486,000	16,218,000		29,986,000
Income Tax Expense (Benefit)				24,459,000	41,478,000		43,275,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				24,459,000	41,478,000		$ 43,275,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets	900,000	2,300,000		2,300,000	900,000
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	23,352,000	27,135,000		27,135,000	23,352,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	22,209,000	21,402,000		21,402,000	22,209,000
Deferred Tax Assets, Regulatory Assets and Liabilities	56,470,000	57,469,000		57,469,000	56,470,000
Regulatory Liabilities	213,300,000	217,100,000		217,100,000	213,300,000
Regulatory Liability, Noncurrent	213,306,000	225,408,000		225,408,000	213,306,000
Northwest Natural Gas Company [Member] | Tax reform deferral [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Noncurrent		8,200,000		8,200,000
Excluding AFUDC [Member] | Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 21,300,000	$ 19,100,000		$ 19,100,000	$ 21,300,000

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	$ 3,414,490	$ 3,204,635
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	993,118	960,477
Total property, plant, and equipment, net	2,421,372	2,244,158
Regulatory Liability, Noncurrent	611,560	586,093
Acquired Equipment Capital Leases	0	0
Natural Gas Distribution [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	3,134,122	2,975,217
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	204,978	159,924
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	974,252	942,879
Total property, plant, and equipment, net	2,364,848	2,192,262
Northwest Holdings and Northwest Natural [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, Equipment non-cash	23,676	34,761
Other Northwest Natural [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	66,009	64,997
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	5,330	4,122
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	18,603	17,406
Total property, plant, and equipment, net	52,736	51,713
Northwest Natural Gas Company [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total property, plant, and equipment, net	2,417,584	2,243,975
Northwest Holdings Other [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	4,051	375
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	263	192
Total property, plant, and equipment, net	$ 3,788	$ 183
Natural Gas Distribution [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%	2.80%
Other Northwest Natural [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Weighted Average Depreciation Rates	2.20%	1.90%	2.00%
Northwest Natural Gas Company [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	$ 3,410,439	$ 3,204,260
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	992,855	960,285
Total property, plant, and equipment, net	2,417,584	2,243,975
Regulatory Liability, Noncurrent	611,560	586,093
Northwest Natural Gas Company [Member] | Asset Removal Costs [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Regulatory Liability, Noncurrent	$ 380,500	$ 360,929

Gas Reserves (Details)

Gas Reserves (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Gas Reserves, Current	$ 16,647,000	$ 15,704,000
Northwest Natural Gas Company [Member]
Schedule of Investments [Line Items]
Total Cumulative Gas Reserves Investment	188,000,000
Gas Reserves Investment	178,000,000
Cumulative Gross Investment	10,000,000
Additional Rate Per Therm	$ 0.4725
Gas Reserves Percent Gas Supplies YTD	6.00%	6.00%	8.00%
Gas Reserves, Current	$ 16,647,000	$ 15,704,000
Gas Reserves Noncurrent Gross	170,660,000	171,832,000
Gas Reserves Amortization	104,463,000	87,779,000
Total Gas Reserves	82,844,000	99,757,000
Deferred Taxes Related To Gas Reserves	20,071,000	22,712,000
Net Gas Reserves Investment	62,773,000	77,045,000
Investment in Additional Wells, Gas Reserves	$ 4,800,000	$ 5,800,000

Investments (Details)

Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Investments [Abstract]
Life Insurance, Corporate or Bank Owned, Amount	$ 49,922	$ 50,792
Equity Method Investments	13,571	13,669
Other Investments	65	1,902
Total Other investments	$ 63,558	66,363
Trail West Holdings [Member]
Investment [Line Items]
Ownership percentage of TWH	50.00%
Equity Method Investment, Underlying Equity in Net Assets	$ 13,400	13,400
Equity Method Investment Exposure	13,400
Northwest Natural Gas Company [Member]
Other Investments [Abstract]
Life Insurance, Corporate or Bank Owned, Amount	49,922	50,792
Equity Method Investments	0	0
Other Investments	0	1,862
Total Other investments	$ 49,922	$ 52,654

Business Combinations (Details)

Business Combinations (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
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	$ 8,954	$ 0
Falls Water Co [Member]
Business Acquisition [Line Items]
Consideration transferred	$ 8,500
Consideration transferred, shares | shares	125,000
Connections of acquired business	5,300 connections
Goodwill	$ 6,400
Other insignificant acquisitions [Member]
Business Acquisition [Line Items]
Consideration transferred	$ 2,800
Number of Businesses Acquired	3

Derivative Instruments - Narrat

Derivative Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative, Loss on Derivative	$ 7,400,000	$ 7,800,000
Unrealized Gain (Loss) on Derivatives	(7,800,000)
Derivative, Fair Value, Net	(5,700,000)	20,300,000
Northwest Natural Gas Company [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	3,600,000	2,900,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 9,300,000	23,300,000
Northwest Natural Gas Company [Member] | Gas Year 2018 - 2019 [Member] | Financial [Member]
Derivative [Line Items]
TargetHedgeAchieved	48.00%
Northwest Natural Gas Company [Member] | Gas Year 2018 - 2019 [Member] | Physical [Member]
Derivative [Line Items]
TargetHedgeAchieved	24.00%
Northwest Natural Gas Company [Member] | Gas Year 2017 - 2018 [Member] | Financial [Member]
Derivative [Line Items]
TargetHedgeAchieved	49.00%
Northwest Natural Gas Company [Member] | Gas Year 2017 - 2018 [Member] | Physical [Member]
Derivative [Line Items]
TargetHedgeAchieved	26.00%
Collateral Posted with NW Natural Counterparties [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 0	0
Collateral Posted by NW Natural Counterparties [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 0	$ 0

Derivative Instruments - Schedu

Derivative Instruments - Schedule of Notional Amounts (Details) - Northwest Natural Gas Company [Member] therm in Thousands, $ in Thousands	Dec. 31, 2018USD ($)therm	Dec. 31, 2017USD ($)therm
Natural Gas Therms [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	408,850	429,100
Natural Gas Therms [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	472,275	520,268
Foreign Exchange Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount | $	$ 6,936	$ 7,669

Derivative Instruments - Sche_2

Derivative Instruments - Schedule of Unrealized and Realized Gain/Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Derivative, Loss on Derivative	$ 7,400	$ 7,800
Northwest Holdings and Northwest Natural [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Northwest Holdings and Northwest Natural [Member] | Foreign Exchange Contract [Member] | Cost of Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(284)	107
Northwest Holdings and Northwest Natural [Member] | Foreign Exchange Contract [Member] | Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Northwest Holdings and Northwest Natural [Member] | Natural Gas Commodity [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	210	(356)
Northwest Holdings and Northwest Natural [Member] | Natural Gas Commodity [Member] | Cost of Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(1,239)	26,000
Northwest Holdings and Northwest Natural [Member] | Natural Gas Commodity [Member] | Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	1,660	(1,021)
Northwest Holdings and Northwest Natural [Member] | Deferred Derivative Gain (Loss) [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	284	(107)
Northwest Holdings and Northwest Natural [Member] | Deferred Derivative Gain (Loss) [Member] | Natural Gas Commodity [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ (211)	$ 26,665

- Schedule of Credit Rating Dow

- Schedule of Credit Rating Downgrade Scenarios (Details) - Northwest Natural Gas Company [Member] $ in Thousands	Dec. 31, 2018USD ($)
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)

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Land Buildings And Equipment Expiration Date	through 2,108
Operating Leases, Rent Expense, Minimum Rentals	$ 5,900	$ 7,300	$ 5,900
Property, Plant and Equipment, Gross	3,414,490	3,204,635
Other Liabilities, Noncurrent	147,763	135,292
Northwest Natural Gas Company [Member]
Leases [Abstract]
Operating Leases, Future Minimum Payments Due	181,665
Unrecorded Unconditional Purchase Obligation, Purchases	82,600	85,300	85,000
Unrecorded Unconditional Purchase Obligation Offsetting Sales	4,300	4,500	$ 4,500
Property, Plant and Equipment, Gross	3,410,439	3,204,260
Other Liabilities, Noncurrent	$ 147,668	135,205
Northwest Natural Gas Company [Member] | New Headquarters Lease 1st Renewal Period [Member]
Leases [Abstract]
Lessee, Operating Lease, Renewal Term	7 years
Northwest Natural Gas Company [Member] | New Headquarters Lease 2nd Renewal Period [Member]
Leases [Abstract]
Lessee, Operating Lease, Renewal Term	7 years
Northwest Natural Gas Company [Member] | New Headquarters Lease [Member]
Leases [Abstract]
Lessee, Operating Lease, Term of Contract	20 years
Operating Leases, Future Minimum Payments Due	$ 160,000
Lease Expected Commencement Year	1/1/2020
Northwest Natural Gas Company [Member] | Current Headquarters Lease [Member]
Leases [Abstract]
Lease Expiration Date	Jan. 1, 2020
Northwest Natural Gas Company [Member] | Build to Suit Lease [Member] | New Headquarters Lease [Member]
Leases [Abstract]
Property, Plant and Equipment, Gross	$ 26,000	500
Other Liabilities, Noncurrent	26,000	$ 500
Northwest Holdings and Northwest Natural [Member] | Build to Suit Lease [Member] | New Headquarters Lease [Member]
Leases [Abstract]
Property, Plant and Equipment, Gross	$ 26,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments Due, Fiscal Year Maturity (Details) - Northwest Natural Gas Company [Member] $ in Thousands	Dec. 31, 2018USD ($)
Minimum lease payments
Minimum lease payments, 2019	$ 5,368
Minimum lease payments, 2020	4,812
Minimum lease payments, 2021	7,077
Minimum lease payments, 2022	7,223
Minimum lease payments, 2023	7,304
Minimum lease payments, Thereafter	149,881
Minimum lease payments, Total	$ 181,665

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Gas Purchase and Pipeline Capacity Purchase and Release Commitments (Details) - Northwest Natural Gas Company [Member] $ in Thousands	Dec. 31, 2018USD ($)
Gas Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
2,018	$ 144,500
2,019	2,776
2,020	2,313
2,021	0
2,022	0
Thereafter	0
Total	149,589
Less: Amount representing interest	1,314
Total at present value	148,275
Pipeline Capacity Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
2,018	78,449
2,019	76,613
2,020	66,656
2,021	61,075
2,022	60,619
Thereafter	580,022
Total	923,434
Less: Amount representing interest	201,224
Total at present value	722,210
Pipeline Capacity Release Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
2,018	4,272
2,019	3,560
2,020	0
2,021	0
2,022	0
Thereafter	0
Total	7,832
Less: Amount representing interest	183
Total at present value	$ 7,649

Environmental Matters (Details)

Environmental Matters (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2016	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Environmental Regulatory Table [Abstract]
Regulatory Assets, Current			$ 41,930,000	$ 45,781,000
Regulatory Assets, Noncurrent			$ 371,786,000	356,608,000
Northwest Natural Gas Company [Member]
Environmental Activity [Line Items]
Remediation Recovery Percentage			96.68%
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			$ 33,112,000	19,088,000
Environmental Noncurrent Liabilities			95,636,000	108,280,000
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			$ 1,050,000,000
Number Of Potentially Responsible Parties			more than 100
Loss Contingency Range Of Possible Loss Gasco Siltronic			$49.5 million to $350 million
Environmental Project Liability			$ 49,500,000
NRD Liability Claim Yakama Nation			300,000
Undiscounted Environmental Remediation Expense		$ 10,500,000
Liability For Studies And Design Costs			900,000
Pre-tax Regulatory Disallowance			15,000,000
Deferred Environmental Charges			95,000,000
Disallowance From Order, Interest Portion	$ 2,800,000
Environmental Costs to be Amortized	13,800,000
Total Disallowance From Order	3,300,000
Disallowance From Order, Operations and Maintenance Portion	$ 500,000
Annual Tariff Rider Collection			5,000,000
Environmental Settlement Insurance Proceeds Received To Date			150,000,000
Annual insurance proceeds to appy against remediation costs			5,000,000
Insurance Proceeds Applied To Prudently Incurred Remediation Costs			73,200,000
Environmental Regulatory Table [Abstract]
Regulatory Assets, Current			41,930,000	45,781,000
Regulatory Assets, Noncurrent			$ 371,786,000	356,608,000
Remediation Non-recovery Percentage			3.32%
Annual Insurance Proceeds to Apply Against Remediation Costs			$ 5,000,000
Total Annual Remediation Expense and Interest			10,000,000
Northwest Natural Gas Company [Member] | Minimum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			(0.30)
Northwest Natural Gas Company [Member] | Maximum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			0.50
Northwest Natural Gas Company [Member] | Gasco Siltronic Sediments [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			5,117,000	2,683,000
Environmental Noncurrent Liabilities			44,351,000	45,346,000
Northwest Natural Gas Company [Member] | Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			2,600,000	1,949,000
Environmental Noncurrent Liabilities			6,273,000	4,163,000
Northwest Natural Gas Company [Member] | Gasco Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			13,983,000	13,422,000
Environmental Noncurrent Liabilities			44,830,000	47,835,000
Northwest Natural Gas Company [Member] | Central Service Center [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			10,000	25,000
Environmental Noncurrent Liabilities			0	0
Northwest Natural Gas Company [Member] | Front Street [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			11,402,000	1,009,000
Environmental Noncurrent Liabilities			3,000	10,757,000
Northwest Natural Gas Company [Member] | Oregon Steel Mills [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities			0	0
Environmental Noncurrent Liabilities			179,000	179,000
Northwest Natural Gas Company [Member] | Environmental Restoration Costs [Member]
Environmental Regulatory Table [Abstract]
Environmental Regulatory Deferred Cost and Interest			41,883,000	45,546,000
Environmental Regulatory Accrued Site Liabilities			128,369,000	126,950,000
Environmental Regulatory Insurance Proceeds and Interest			(88,502,000)	(94,170,000)
Environmental Regulatory Assets Noncurrent			81,750,000	78,326,000
Regulatory Assets, Current			5,601,000	6,198,000
Regulatory Assets, Noncurrent			76,149,000	72,128,000
Northwest Natural Gas Company [Member] | Environmental Restoration Costs [Member] | Front Street [Member]
Environmental Regulatory Table [Abstract]
Environmental Regulatory Accrued Site Liabilities			$ 400,000	$ 400,000
Northwest Natural Gas Company [Member] | Oregon [Member]
Environmental Activity [Line Items]
Remediation Recovery Percentage			96.68%

Discontinued Operations (Detail

Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Federal Statutory Income Tax Rate, Percent	21.00%												35.00%
Disposal Group, Including Discontinued Operation, Classified Balance Sheet Disclosures [Abstract]
Discontinued operations - current assets	$ 3,057	$ 13,269				$ 3,057					$ 13,269	$ 3,057
Discontinued operations - non-current assets	10,817	0				10,817					0	10,817		$ 210,400
Discontinued operations - total assets	13,900					13,900						13,900
Discontinued operations - current liabilities	1,593	12,959				1,593					12,959	1,593
Discontinued operations - non-current liabilities	12,043	0				12,043					0	12,043
Discontinued Operation, Income Statement Disclosures [Abstract]
Loss from discontinued operations, net of tax		$ (959)	$ (650)	$ (659)	$ (474)	$ (124,655)	$ (608)	$ (1,346)	$ (1,087)		$ (2,742)	$ (127,696)		$ (3,524)
Loss from discontinued operations per share of common stock
Basic (in dollars per share)		$ (0.03)	$ (0.02)	$ (0.02)	$ (0.02)	$ (4.34)	$ (0.02)	$ (0.04)	$ (0.04)		$ (0.10)	$ (4.45)		$ (0.13)
Diluted (in dollars per share)		$ (0.03)	$ (0.02)	$ (0.02)	$ (0.02)	$ (4.33)	$ (0.02)	$ (0.04)	$ (0.04)		$ (0.09)	$ (4.44)		$ (0.13)
Gill Ranch [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Disposal Group, Including Discontinued Operation, Classified Balance Sheet Disclosures [Abstract]
Accounts receivable	2,126	$ 390				$ 2,126					$ 390	$ 2,126
Inventories	396	685				396					685	396
Other current assets	535	333				535					333	535
Property, plant, and equipment	10,816	11,621				10,816					11,621	10,816
Less: Accumulated depreciation	0	7				0					7	0
Other non-current assets	1	247				1					247	1
Discontinued operations - current assets	3,057	13,269				3,057					13,269	3,057
Discontinued operations - non-current assets	10,817	0				10,817					0	10,817
Discontinued operations - total assets	13,874	13,269				13,874					13,269	13,874
Accounts payable	1,287	873				1,287					873	1,287
Other current liabilities	306	307				306					307	306
Other non-current liabilities	12,043	11,779				12,043					11,779	12,043
Discontinued operations - current liabilities	1,593	12,959				1,593					12,959	1,593
Discontinued operations - non-current liabilities	12,043	0				12,043					0	12,043
Total discontinued operations liabilities	13,636	$ 12,959				13,636					12,959	13,636
Gill Ranch [Member] | Discontinued Operations, Held-for-sale [Member]
Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues											3,579	7,135		$ 7,794
Operations and maintenance											5,771	7,245		6,643
General taxes											479	1,373		1,295
Depreciation and amortization											430	4,525		4,685
Other expenses and interest											609	975		992
Impairment expense											0	192,478		0
Total expenses											7,289	206,596		13,615
Loss from discontinued operations before income tax											(3,710)	(199,461)		(5,821)
Income tax benefit											(968)	(71,765)		(2,297)
Loss from discontinued operations, net of tax											$ (2,742)	$ (127,696)		$ (3,524)
Loss from discontinued operations per share of common stock
Basic (in dollars per share)											$ (0.10)	$ (4.45)		$ (0.13)
Diluted (in dollars per share)											$ (0.09)	$ (4.44)		$ (0.13)
Transferred Subsidiaries - NW Natural Discontinued Operations [Member] | Discontinued Operations, Disposed of by Means Other than Sale [Member]
Disposal Group, Including Discontinued Operation, Classified Balance Sheet Disclosures [Abstract]
Cash	362					362						$ 362
Accounts receivable	2,126					2,126						2,126
Intercompany receivables	3,664					3,664						3,664
Inventories	396					396						396
Other current assets	622					622						622
Property, plant, and equipment	11,191					11,191						11,191
Less: Accumulated depreciation	192					192						192
Other investments	13,710					13,710						13,710
Other non-current assets	0					0						0
Discontinued operations - current assets	7,170					7,170						7,170
Discontinued operations - non-current assets	24,709					24,709						24,709
Discontinued operations - total assets	31,879					31,879						31,879
Accounts payable	1,954					1,954						1,954
Intercompany payables	266					266						266
Other current liabilities	345					345						345
Deferred tax liabilities	(16,862)					(16,862)						(16,862)
Other non-current liabilities	12,130					12,130						12,130
Discontinued operations - current liabilities	2,565					2,565						2,565
Discontinued Operation, Liabilities, Noncurrent	(4,732)					(4,732)						(4,732)
Total discontinued operations liabilities	$ (2,167)					$ (2,167)						(2,167)
Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues											$ 3,016	7,360		$ 8,018
Operations and maintenance											4,151	7,423		7,387
General taxes											448	1,410		1,317
Depreciation and amortization											420	4,555		4,714
Other expenses and interest											342	650		1,097
Impairment expense											0	192,478		0
Total expenses											5,361	206,516		14,515
Loss from discontinued operations before income tax											(2,345)	(199,156)		(6,497)
Income tax benefit											(622)	(71,813)		(2,557)
Loss from discontinued operations, net of tax											$ (1,723)	(127,343)		$ (3,940)
Gill Ranch [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percent ownership		75.00%									75.00%
Pacific Gas and Electric Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percent ownership		25.00%									25.00%
Scenario, Forecast [Member] | Minimum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of businesses										$ 25,000
Scenario, Forecast [Member] | Maximum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of businesses										$ 26,500
TCJA Revaluation [Member] | Gill Ranch [Member] | Discontinued Operations, Held-for-sale [Member]
Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit												18,000
TCJA Revaluation [Member] | Transferred Subsidiaries - NW Natural Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit												$ 18,000

Quarterly Financial Informati_3

Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity Information [Line Items]
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 706,143	$ 755,038	$ 668,173
Net income from continuing operations	36,783	(11,144)	(339)	42,011	34,488	(7,887)	4,075	41,397	67,311	72,073	62,419
Loss from discontinued operations, net of tax	(959)	(650)	(659)	(474)	(124,655)	(608)	(1,346)	(1,087)	(2,742)	(127,696)	(3,524)
Net income (loss)	$ 35,824	$ (11,794)	$ (998)	$ 41,537	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	$ 64,569	$ (55,623)	$ 58,895
Average common shares outstanding:
Average common shares outstanding - basic (in shares)	28,851	28,815	28,791	28,753	28,716	28,678	28,648	28,633	28,803	28,669	27,647
Diluted (in shares)	28,940	28,815	28,791	28,803	28,797	28,678	28,717	28,723	28,873	28,753	27,779
Earnings from continuing operations per share of common stock:
Basic (in dollars per share)	$ 1.27	$ (0.39)	$ (0.01)	$ 1.46	$ 1.20	$ (0.28)	$ 0.14	$ 1.45	$ 2.34	$ 2.51	$ 2.26
Diluted (in dollars per share)	1.27	(0.39)	(0.01)	1.46	1.20	(0.28)	0.14	1.44	2.33	2.51	2.25
Loss from discontinued operations per share of common stock:
Basic (in dollars per share)	(0.03)	(0.02)	(0.02)	(0.02)	(4.34)	(0.02)	(0.04)	(0.04)	(0.10)	(4.45)	(0.13)
Diluted (in dollars per share)	(0.03)	(0.02)	(0.02)	(0.02)	(4.33)	(0.02)	(0.04)	(0.04)	(0.09)	(4.44)	(0.13)
Earnings (loss) per share of common stock:
Basic (in dollars per share)	1.24	(0.41)	(0.03)	1.44	(3.14)	(0.30)	0.10	1.41	2.24	(1.94)	2.13
Diluted (in dollars per share)	$ 1.24	$ (0.41)	$ (0.03)	$ 1.44	$ (3.13)	$ (0.30)	$ 0.10	$ 1.40	$ 2.24	$ (1.93)	$ 2.12
Northwest Natural Gas Company [Member]
Entity Information [Line Items]
Operating revenues	$ 226,146	$ 91,227	$ 124,563	$ 263,635	$ 238,793	$ 86,157	$ 134,420	$ 295,668	$ 705,571	$ 755,038	$ 667,949
Net income from continuing operations	37,581	(11,275)	(271)	42,014	34,086	(7,876)	4,072	41,438	68,049	71,720	62,835
Loss from discontinued operations, net of tax	0	(519)	(727)	(477)	(124,253)	(619)	(1,343)	(1,128)	(1,723)	(127,343)	(3,940)
Net income (loss)	$ 37,581	$ (11,794)	$ (998)	$ 41,537	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	$ 66,326	$ (55,623)	$ 58,895

Condensed Financials Standalo_3

Condensed Financials Standalone Holdco Condensed Financials Standalone Holdco (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operations and maintenance									$ 156,698	$ 152,358	$ 136,723
Total operating expenses									573,981	604,136	517,456
Income from operations									132,162	150,902	150,717
Other income (expense), net									(3,601)	(295)	(7,151)
Interest expense, net									37,059	37,526	38,136
Income before income taxes									91,502	113,081	105,430
Income Tax Expense (Benefit)									24,191	41,008	43,011
Net income (loss)	$ 35,824	$ (11,794)	$ (998)	$ 41,537	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	64,569	(55,623)	58,895
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	12,633			3,472	3,472			3,521	3,472	3,521	4,211	$ 12,633	$ 3,472
Income taxes receivable												6,000	0
Other current assets												28,472	24,949
Total current assets												295,921	269,936
Other investments												63,558	66,363
Other non-current assets												14,149	6,505
Total non-current assets												2,946,741	2,769,810
Total assets												3,242,662	3,039,746
Accounts payable												115,878	111,021
Interest accrued												7,306	6,773
Total current liabilities												509,084	381,850
Long-term debt												(706,247)	(683,184)
Deferred tax liabilities												280,463	270,526
Total deferred credits and other non-current liabilities												1,264,697	1,231,936
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively												457,640	448,865
Retained earnings												312,182	302,349
Total equity												762,634	742,776
Total liabilities and equity												3,242,662	3,039,746
Statement of Cash Flows [Abstract]
Net income (loss)	35,824	$ (11,794)	$ (998)	41,537	(90,167)	$ (8,495)	$ 2,729	40,310	64,569	(55,623)	58,895
Deferred income taxes									11,453	15,700	29,853
Other									(1,596)	(2,102)	(2,846)
Receivables, net									(181)	(3,282)	6,395
Income and other taxes									(16,904)	6,734	9,467
Accounts payable									16,792	1,092	12,028
Interest accrued									526	807	93
Other, net									552	(4,093)	11,727
Cash provided by operating activities									168,771	206,704	222,147
Cash used in investing activities									(217,453)	(214,172)	(136,629)
Cash dividend payments on common stock									(51,311)	(53,957)	(51,508)
Other									(715)	(2,309)	(3,087)
Cash provided by (used in) financing activities									57,843	7,419	(86,208)
Increase (decrease) in cash and cash equivalents									9,161	(49)	(690)
Cash and cash equivalents, beginning of period				3,472				$ 3,521	3,472	3,521	4,211
Cash and cash equivalents, end of period	12,633				3,472				12,633	3,472	$ 3,521
Parent Company, Condensed Statement of Cash Flows [Member]
Income Statement [Abstract]
Equity in earnings of subsidiaries, net of tax									(36,469)
Net income (loss)									35,839
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	4,011			0	0				0	0		4,011	$ 0
Statement of Cash Flows [Abstract]
Net income (loss)									35,839
Equity in earnings of subsidiaries, net of tax									(36,469)
Deferred income taxes									7
Other									15
Receivables, net									(585)
Income and other taxes									(9,034)
Accounts payable									9,304
Interest accrued									32
Other, net									44
Cash provided by operating activities									(935)
Contributions to subsidiaries									(1,804)
Cash used in investing activities									(1,804)
Cash dividend payments on common stock									(12,923)
Capital contributions									20,000
Other									(327)
Cash provided by (used in) financing activities									6,750
Increase (decrease) in cash and cash equivalents									4,011
Cash and cash equivalents, beginning of period				$ 0					0
Cash and cash equivalents, end of period	4,011				$ 0				4,011	$ 0
Parent Company, Condensed Balance Sheet [Member]
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	4,011								4,011			4,011
Receivables from affiliates												2,796
Income taxes receivable												6,000
Other current assets												3,078
Total current assets												15,885
Investments in subsidiaries												754,971
Other investments												65
Other non-current assets												310
Total non-current assets												755,346
Total assets												771,231
Accounts payable												168
Payables to affiliates												9,166
Interest accrued												32
Total current liabilities												9,366
Long-term debt												(1)
Deferred tax liabilities												7
Total deferred credits and other non-current liabilities												7
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively												739,722
Retained earnings												22,137
Total equity												761,859
Total liabilities and equity												$ 771,231
Statement of Cash Flows [Abstract]
Cash and cash equivalents, end of period	$ 4,011								4,011
Parent Company, Condensed Income Statement [Member]
Income Statement [Abstract]
Operations and maintenance									838
Total operating expenses									838
Income from operations									(838)
Equity in earnings of subsidiaries, net of tax									(36,469)
Other income (expense), net									36
Interest expense, net									53
Income before income taxes									35,614
Income Tax Expense (Benefit)									(225)
Net income (loss)									35,839
Statement of Cash Flows [Abstract]
Net income (loss)									35,839
Equity in earnings of subsidiaries, net of tax									$ (36,469)

Valuation Allowances and Rese_3

Valuation Allowances and Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity Information [Line Items]
Allowance for Doubtful Accounts Receivable, beginning of period	$ 956	$ 1,290	$ 870
Valuation Allowances and Reserves, Charged to Cost and Expense	680	865	1,246
Valuation Allowances and Reserves, Charged to Other Accounts	0	0	0
SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction	659	1,199	826
Allowance for Doubtful Accounts Receivable, end of period	977	956	1,290
Northwest Natural Gas Company [Member]
Entity Information [Line Items]
Allowance for Doubtful Accounts Receivable, beginning of period	956	1,290	870
Valuation Allowances and Reserves, Charged to Cost and Expense	678	865	1,246
Valuation Allowances and Reserves, Charged to Other Accounts	0	0	0
SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction	659	1,199	826
Allowance for Doubtful Accounts Receivable, end of period	$ 975	$ 956	$ 1,290