Leases - Supplemental Information Related to Leases Included in Balance Sheet (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, weighted average remaining lease term (in years)	9 years 25 days
Operating lease, weighted average discount rate	4.08%
Finance lease ROU assets	$ 854
Finance lease liabilities, current	314
Finance lease liabilities, noncurrent	394
Finance lease liabilities	$ 708
Finance lease, weighted average remaining lease term (in years)	1 year 10 months 2 days
Finance lease, weighted average discount rate	5.90%
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 2,072
Operating lease liabilities, current	897
Operating lease liabilities, noncurrent	1,200
Operating lease liabilities	$ 2,097
Operating lease, weighted average remaining lease term (in years)	3 years 9 months 21 days
Operating lease, weighted average discount rate	3.36%
Centuri
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 60,301
Operating lease liabilities, current	7,965
Operating lease liabilities, noncurrent	55,185
Operating lease liabilities	63,150
Finance lease liabilities	$ 708