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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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The carrying amount of capitalized costs of materials, supplies, and other assets, which are not included in inventory but used in production.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions.

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Nature of Operations and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
	Jun. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Nov. 30, 2017	Aug. 31, 2017
Significant Accounting Policies [Line Items]
Previous tax effects from TCJA reclassified to retained earnings from AOCI		$ 9.3
Corporate federal income tax rate		21.00%	35.00%
Other current liabilities, excess cost-of-service rates amount to be refunded		$ 2.4
New England Utility Constructors Inc.
Significant Accounting Policies [Line Items]
Net consideration of acquisition less assumed debt				$ 99.0
Gas Pipe Materials and Supplies \| Prepaids and other current assets
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies		53.0	$ 33.0
Prepaid and other current assets		73.0	$ 40.0
Centuri Construction Group Inc.
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary	96.60%
Previous Owners \| Link-Line and WS Nicholls \| Centuri Construction Group Inc.
Significant Accounting Policies [Line Items]
Percentage of earnings attribution by Centuri to previous owners					3.40%
Southwest Gas Corporation
Significant Accounting Policies [Line Items]
Dividends declared but not yet paid		22.0
Pooled Funds and Mutual Funds \| Level 2 - Significant Other Observable Inputs
Significant Accounting Policies [Line Items]
Money market fund investments		39.6
Pooled Funds and Mutual Funds \| Southwest Gas Corporation \| Level 2 - Significant Other Observable Inputs
Significant Accounting Policies [Line Items]
Money market fund investments		$ 38.8