Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Name:	us-gaap_DisaggregationOfRevenueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Revenue - Schedule of Gas Operating Revenue (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Other revenues	$ 3,587	$ 1,785	$ (7,867)	$ 2,979	$ (6,766)	$ 3,945
Tax Reform Savings Revenue Adjustments [Member] \| Southwest Gas Holdings, Inc. [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 1,600		$ 12,500		$ 12,500