Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Summary of Income Statement Components of Discontinued Operations - Construction Services in Condensed Consolidated Income Statements (Detail) - Southwest Gas Corporation [Member] $ in Thousands	12 Months Ended
	Mar. 31, 2017 USD ($)
Operating expenses:
Income before income taxes	$ 56,890
Income tax expense	21,030
Net income	35,860
Net income attributable to noncontrolling interests	1,105
Discontinued operations - construction services - net income	34,755
Construction Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction revenues	932,930
Operating expenses:
Construction expenses	831,041
Depreciation and amortization	41,054
Operating income	60,835
Other income (deductions)	1,227
Net interest deductions	5,172
Income before income taxes	56,890
Income tax expense	21,030
Net income	35,860
Net income attributable to noncontrolling interests	1,105
Discontinued operations - construction services - net income	$ 34,755