Background, Organization, and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Other property and investments	$ 31,153	$ 23,961
Total	1,305,334	834,245
Southwest Gas Corporation
Property, Plant and Equipment [Line Items]
Net cash surrender value of COLI policies	147,187	140,874
Other property and investments	2,739	2,737
Total	149,926	143,611
Centuri
Property, Plant and Equipment [Line Items]
Centuri property, equipment, and intangibles	1,592,461	1,089,414
Centuri accumulated provision for depreciation and amortization	$ (468,206)	$ (422,741)