Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Less: current maturities	$ (28,453)	$ (50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Carrying Amount [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(2,212)	(2,489)
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,520,790	1,400,080
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Carrying Amount [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Less: current maturities	(28,453)	(25,101)
Long-term debt, less current maturities	211,191	174,903
Unamortized debt issuance costs	(383)	(516)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	239,644	200,004
Less: current maturities	(28,453)	(25,101)
Long-term debt, less current maturities	211,191	174,903
Centuri term loan/secured revolving credit facility	106,867	106,184
Carrying Amount [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	81,250	41,185
Carrying Amount [Member] \| IDRBs [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	197,788	197,511
Carrying Amount [Member] \| Centuri Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	107,250	106,700
Carrying Amount [Member] \| Centuri Other Debt Obligations [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	51,527	52,635
Carrying Amount [Member] \| Debentures [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(9,498)	(9,931)
Notes payable	1,173,002	1,197,569
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	75,000	75,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable		25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	7,500	7,500
Carrying Amount [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility	150,000	5,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	107,403	106,819
Centuri other debt obligations	51,978	52,840
Market Value [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	81,402	41,292
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	130,325	129,703
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	154,434	149,734
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	258,943	254,900
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	275,168	266,793
Market Value [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	292,578	283,029
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	97,218	94,691
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		25,040
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,174	29,290
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,964	31,905
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,920	8,769
Market Value [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000	5,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000	$ 50,000

Debt instrument unamortized discount and debt issuance costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Fair value portion of notes payable.

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Amount of long-term debt classified as other.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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