Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Summary of Income Statement Components of Discontinued Operations - Construction Services in Condensed Consolidated Statements of Income (Detail) - Southwest Gas Corporation [Member] $ in Thousands	12 Months Ended
	Jun. 30, 2017 USD ($)
Operating expenses:
Income before income taxes	$ 45,669
Income tax expense	16,550
Net income	29,119
Net income attributable to noncontrolling interests	949
Discontinued operations - construction services - net income	28,170
Construction Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction revenues	640,830
Operating expenses:
Construction expenses	567,115
Depreciation and amortization	25,727
Operating income	47,988
Other income (deductions)	1,193
Net interest deductions	3,512
Income before income taxes	45,669
Income tax expense	16,550
Net income	29,119
Net income attributable to noncontrolling interests	949
Discontinued operations - construction services - net income	$ 28,170