Leases - Lease Asset and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	$ 3,917	$ 3,917	$ 0			$ 0
Operating lease liabilities current liabilities	3,171	3,171	0			0
Operating lease liabilities noncurrent liabilities	732	732	0			0
Total operating lease liabilities	3,903	3,903
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	1,205	1,205
Lessee, Operating Lease, Liability, Payments, Due Year Two	2,115	2,115
Lessee, Operating Lease, Liability, Payments, Due Year Three	154	154
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	4,135	4,135
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	232	232
Total operating lease liabilities	3,903	3,903
Operating lease liabilities current liabilities	3,171	3,171	0			0
Operating lease liabilities noncurrent liabilities	732	732	0			0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	4,135	4,135
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,158	3,451
Operating leases		7,358
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	172	172
Operating lease liabilities current liabilities	96	96
Operating lease liabilities noncurrent liabilities	78	78
Total operating lease liabilities	174	174
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	48	48
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	178	178
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	4	4
Total operating lease liabilities	174	174
Operating lease liabilities current liabilities	96	96
Operating lease liabilities noncurrent liabilities	78	78
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	178	178
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	45	132
Operating leases		304
Northwest Natural Gas Company [Member]
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	3,745	3,745	0			0
Operating lease liabilities current liabilities	3,075	3,075	0			0
Operating lease liabilities noncurrent liabilities	654	654	0			0
Total operating lease liabilities	$ 3,729	$ 3,729
Operating Lease, Weighted Average Discount Rate, Percent	3.86%	3.86%
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 1,157	$ 1,157
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months			5,368
Lessee, Operating Lease, Liability, Payments, Due Year Two	2,013	2,013	4,812
Lessee, Operating Lease, Liability, Payments, Due Year Three	126	126	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	3,957	3,957	181,665
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	228	228
Total operating lease liabilities	3,729	3,729
Operating lease liabilities current liabilities	3,075	3,075	0			0
Operating lease liabilities noncurrent liabilities	654	654	0			$ 0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	3,957	3,957	$ 181,665
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,113	3,319
Operating leases		7,054
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
Maximum [Member] | Northwest Natural Gas Company [Member]
Operating Lease Asset and Liabilities [Abstract]
Operating Lease, Weighted Average Remaining Lease Term	1 year	1 year