Background, Organization, and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Other property	$ 20,698	$ 18,814
Total	785,522	784,173
Southwest Gas Corporation
Property, Plant and Equipment [Line Items]
Net cash surrender value of COLI policies	116,615	132,072
Other property	1,932	1,715
Total	118,547	133,787
Centuri
Property, Plant and Equipment [Line Items]
Centuri property, equipment, and intangibles	1,009,351	983,905
Centuri accumulated provision for depreciation and amortization	$ (363,074)	$ (352,333)