Leases - Supplemental Information Related to Leases Included in Balance Sheet (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, weighted average remaining lease term (in years)	9 years 3 months 29 days
Operating lease, weighted average discount rate	4.08%
Finance lease ROU assets	$ 639
Finance lease liabilities, current	256
Finance lease liabilities, noncurrent	239
Finance lease liabilities	$ 495
Finance lease, weighted average remaining lease term (in years)	1 year 7 months 17 days
Finance lease, weighted average discount rate	5.84%
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 1,977
Operating lease liabilities, current	745
Operating lease liabilities, noncurrent	1,253
Operating lease liabilities	$ 1,998
Operating lease, weighted average remaining lease term (in years)	4 years 2 months 19 days
Operating lease, weighted average discount rate	3.39%
Centuri
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 57,267
Operating lease liabilities, current	7,178
Operating lease liabilities, noncurrent	52,647
Operating lease liabilities	59,825
Finance lease liabilities	$ 495