Revenue - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Type Customer Contract Segment
Segment Reporting Information [Line Items]
Number of customers | Customer	2,000,000
Number of customers party to contracts with rate components subject to negotiation | Customer	24
Southwest Gas Corporation
Segment Reporting Information [Line Items]
Number of operating segment | Segment	1
Number of types of services provided to customers | Type	2
Centuri Construction Group Inc.
Segment Reporting Information [Line Items]
Number of types of services provided to customers | Type	3
Number of types of agreements with customers | Type	2
Revenue recognized that included in amounts billed in excess of revenue earned on contracts | $	$ 51.7
Increases due to amounts billed to customers in excess of revenue earned during the period | $	33.8
Change in contract liability, revenue recognized | $	$ 9.6
Number of contracts with original duration more than one year | Contract	16