Leases - Lease Asset and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	$ 5,013	$ 5,013	$ 0			$ 0
Operating lease liabilities current liabilities	4,271	4,271	0			0
Operating lease liabilities noncurrent liabilities	721	721	0			0
Total operating lease liabilities	4,992	4,992
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	2,391	2,391
Lessee, Operating Lease, Liability, Payments, Due Year Two	2,082	2,082
Lessee, Operating Lease, Liability, Payments, Due Year Three	129	129
Lessee, Operating Lease, Liability, Payments, Due Year Four	93	93
Lessee, Operating Lease, Liability, Payments, Due Year Five	71	71
Lessee, Operating Lease, Liability, Payments, Due after Year Five	497	497
Lessee, Operating Lease, Liability, Payments, Due	5,263	5,263
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	271	271
Total operating lease liabilities	4,992	4,992
Operating lease liabilities current liabilities	4,271	4,271	0			0
Operating lease liabilities noncurrent liabilities	721	721	0			0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	5,263	5,263
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,159	2,293
Operating leases		7,291
Finance Leases [Abstract]
Finance Lease, Right-of-Use Asset	300	300
Prior Period Disclosures [Abstract]
Operating Leases, Rent Expense			5,900	$ 7,300	$ 5,900
Other Northwest Holdings [Member] | Corporate, Non-Segment [Member]
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	216	216
Operating lease liabilities current liabilities	130	130
Operating lease liabilities noncurrent liabilities	87	87
Total operating lease liabilities	217	217
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	93	93
Lessee, Operating Lease, Liability, Payments, Due Year Two	102	102
Lessee, Operating Lease, Liability, Payments, Due Year Three	28	28
Lessee, Operating Lease, Liability, Payments, Due Year Four	0	0
Lessee, Operating Lease, Liability, Payments, Due Year Five	0	0
Lessee, Operating Lease, Liability, Payments, Due after Year Five	0	0
Lessee, Operating Lease, Liability, Payments, Due	223	223
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	6	6
Total operating lease liabilities	217	217
Operating lease liabilities current liabilities	130	130
Operating lease liabilities noncurrent liabilities	87	87
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	223	223
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	44	87
Operating leases		304
Northwest Natural Gas Company [Member]
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	4,797	4,797	0			0
Operating lease liabilities current liabilities	4,141	4,141	0			0
Operating lease liabilities noncurrent liabilities	634	634	0			0
Total operating lease liabilities	$ 4,775	$ 4,775
Operating Lease, Weighted Average Remaining Lease Term	1 year 21 days	1 year 21 days
Operating Lease, Weighted Average Discount Rate, Percent	3.82%	3.82%
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 2,298	$ 2,298
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months			5,368
Lessee, Operating Lease, Liability, Payments, Due Year Two	1,980	1,980	4,812
Lessee, Operating Lease, Liability, Payments, Due Year Three	101	101	7,077
Lessee, Operating Lease, Liability, Payments, Due Year Four	93	93	7,223
Lessee, Operating Lease, Liability, Payments, Due Year Five	71	71	7,304
Lessee, Operating Lease, Liability, Payments, Due after Year Five	497	497	149,881
Lessee, Operating Lease, Liability, Payments, Due	5,040	5,040	181,665
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	265	265
Total operating lease liabilities	4,775	4,775
Operating lease liabilities current liabilities	4,141	4,141	0			0
Operating lease liabilities noncurrent liabilities	634	634	0			$ 0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	5,040	5,040	$ 181,665
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,115	2,206
Operating leases		6,987
New Headquarters Lease [Member]
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	$ 160,000	$ 160,000
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	20 years	20 years
Lessee, Operating Lease, Liability, Payments, Due	$ 160,000	$ 160,000
New Headquarters Lease Renewal Term 2 of 2 [Member] | New Headquarters Lease [Member]
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Lease Not yet Commenced, Renewal Term	7 years	7 years
New Headquarters Lease Renewal Term 1 of 2 [Member] | New Headquarters Lease [Member]
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Lease Not yet Commenced, Renewal Term	7 years	7 years