Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation (Detail) - Southwest Gas Corporation - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Income before income taxes	$ 0	$ 0	$ 0	$ 0	$ 0	$ 21,649
Income tax expense	0	0	0	0	0	7,842
Net income from discontinued operations	0	0	0	0	0	13,807
Net income attributable to noncontrolling interests	0	0	0	0	0	514
Discontinued operations - construction services - income	$ 0	$ 0	$ 0	$ 0	$ 0	13,293
Infrastructure Services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction revenues						301,040
Operating expenses:
Construction expenses						266,504
Depreciation and amortization						12,318
Operating income						22,218
Other income (deductions)						1,149
Net interest deductions						1,718
Income before income taxes						21,649
Income tax expense						7,842
Net income from discontinued operations						13,807
Net income attributable to noncontrolling interests						514
Discontinued operations - construction services - income						$ 13,293