Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - 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
Other income (deductions)
Total other income (deductions)	$ 889	$ (1,980)	$ (6,151)	$ (5,780)	$ (6,401)	$ (8,007)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	836	(1,775)	(5,861)	(5,824)	(6,425)	(9,200)
Operating Segments \| Natural Gas Operations \| Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	4,700	2,100	6,000	6,800	9,500	8,800
Interest income	1,506	670	4,301	1,848	5,237	2,417
Equity AFUDC	448	968	1,034	2,077	1,253	2,473
Other components of net periodic benefit cost	(5,265)	(4,856)	(15,794)	(14,568)	(20,650)	(19,508)
Miscellaneous income and (expense)	(553)	(657)	(1,402)	(1,981)	(1,765)	(3,382)
Total other income (deductions)	836	(1,775)	(5,861)	(5,824)	(6,425)	(9,200)
Operating Segments \| Infrastructure Services \| Centuri Construction Group Inc.
Other income (deductions)
Interest income	4	1	6	2	7	2
Foreign transaction gain (loss)	(91)	(442)	258	(640)	144	(640)
Miscellaneous income and (expense)	125	231	(595)	676	(175)	1,825
Total other income (deductions)	38	(210)	(331)	38	(24)	1,187
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 15	$ 5	$ 41	$ 6	$ 48	$ 6

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Amount of gain (loss), recognized immediately as component of net periodic benefit (cost) credit, for change in value of benefit obligation or plan assets from experience different from that assumed, change in actuarial assumption, or consequence of temporarily deviating from substantive plan. Includes, but is not limited to, amount in excess of ten percent of greater of fair value of plan assets or benefit obligation.

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Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

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The change in cash surrender or contract value during the period which adjusted the amount of premiums paid in determining the expense or income recognized under the contract for the period.

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The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

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