Leases - Lease Asset and Liabilities (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 USD ($)
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	$ 6,163	$ 0			$ 0
Operating lease liabilities current liabilities	4,656	0			0
Operating lease liabilities noncurrent liabilities	1,495	0			0
Total operating lease liabilities	6,151
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	3,601
Lessee, Operating Lease, Liability, Payments, Due Year Two	2,082
Lessee, Operating Lease, Liability, Payments, Due Year Three	129
Lessee, Operating Lease, Liability, Payments, Due Year Four	93
Lessee, Operating Lease, Liability, Payments, Due Year Five	71
Lessee, Operating Lease, Liability, Payments, Due after Year Five	497
Lessee, Operating Lease, Liability, Payments, Due	6,473
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	322
Total operating lease liabilities	6,151
Operating lease liabilities current liabilities	4,656	0			0
Operating lease liabilities noncurrent liabilities	1,495	0			0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	6,473
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,134
Operating leases	7,291
Finance Leases [Abstract]
Finance Lease, Right-of-Use Asset	200
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	260
Operating lease liabilities current liabilities	179
Operating lease liabilities noncurrent liabilities	82
Total operating lease liabilities	261
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	139
Lessee, Operating Lease, Liability, Payments, Due Year Two	102
Lessee, Operating Lease, Liability, Payments, Due Year Three	28
Lessee, Operating Lease, Liability, Payments, Due Year Four	0
Lessee, Operating Lease, Liability, Payments, Due Year Five	0
Lessee, Operating Lease, Liability, Payments, Due after Year Five	0
Lessee, Operating Lease, Liability, Payments, Due	269
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	8
Total operating lease liabilities	261
Operating lease liabilities current liabilities	179
Operating lease liabilities noncurrent liabilities	82
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	269
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	43
Operating leases	304
Northwest Natural Gas Company [Member]
Operating Lease Asset and Liabilities [Abstract]
Operating lease right of use asset	5,903	0			0
Operating lease liabilities current liabilities	4,477	0			0
Operating lease liabilities noncurrent liabilities	1,413	0			0
Total operating lease liabilities	$ 5,890
Operating Lease, Weighted Average Remaining Lease Term	1 year 4 months
Operating Lease, Weighted Average Discount Rate, Percent	3.79%
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 3,462
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months		5,368
Lessee, Operating Lease, Liability, Payments, Due Year Two	1,980	4,812
Lessee, Operating Lease, Liability, Payments, Due Year Three	101	7,077
Lessee, Operating Lease, Liability, Payments, Due Year Four	93	7,223
Lessee, Operating Lease, Liability, Payments, Due Year Five	71	7,304
Lessee, Operating Lease, Liability, Payments, Due after Year Five	497	149,881
Lessee, Operating Lease, Liability, Payments, Due	6,204	181,665
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	314
Total operating lease liabilities	5,890
Operating lease liabilities current liabilities	4,477	0			0
Operating lease liabilities noncurrent liabilities	1,413	0			$ 0
Lease Not Yet Commenced [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	6,204	$ 181,665
Leases Cash Flow [Abstract]
Operating cash flows from operating leases	1,091
Operating leases	6,987
New Headquarters Lease [Member]
Maturity of Operating Lease Liabilities [Abstract]
Lessee, Operating Lease, Liability, Payments, Due	$ 160,000
Lease Not Yet Commenced [Abstract]
Operating Lease, Lease Not yet Commenced, Number of Renewal Terms	2
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	20 years
Lessee, Operating Lease, Lease Not yet Commenced, Renewal Term	7 years
Lessee, Operating Lease, Liability, Payments, Due	$ 160,000