Background, Organization, and Summary of Significant Accounting Policies - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Other income (deductions)
Total other income (deductions)	$ (20,770)	$ 6,839	$ (17,524)	$ (6,253)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	(20,536)	5,946	(16,965)	(6,691)
Operating Segments | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	(15,500)	7,600	(5,700)	5,100
Interest income	1,388	1,597	6,147	6,199
Equity AFUDC	1,061	960	4,262	4,358
Other components of net periodic benefit cost	(5,005)	(3,765)	(16,299)	(19,560)
Miscellaneous income and (expense)	(2,480)	(446)	(5,375)	(2,788)
Total other income (deductions)	(20,536)	5,946	(16,965)	(6,691)
Operating Segments | Utility Infrastructure Services | Centuri
Other income (deductions)
Interest income	0	0	0	87
Foreign transaction gain (loss)	(10)	531	5	162
Miscellaneous income and (expense)	(232)	344	(656)	125
Total other income (deductions)	(242)	875	(651)	374
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 8	$ 18	$ 92	$ 64