Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Total lease cost	$ 8,437	$ 21,310
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease cost	350	1,165
Centuri
Lessee, Lease, Description [Line Items]
Operating lease cost	3,026	9,069
Amortization of ROU assets	34	103
Interest on lease liabilities	10	24
Total finance lease cost	44	127
Short-term lease cost	$ 5,017	$ 10,949