Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Less: current maturities	$ (169,574)	$ (163,512)
Long-term debt, less current maturities	2,310,084	2,300,482
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(169,574)	(163,512)
Long-term debt, less current maturities	2,310,084	2,300,482
Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	(125,000)	(125,000)
Long-term debt, less current maturities	1,991,624	1,991,333
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(125,000)	(125,000)
Long-term debt, less current maturities	$ 1,991,624	1,991,333
Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.45%
Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.10%
Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.875%
Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.875%
Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.80%
Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.70%
Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.15%
Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	8.00%
Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.78%
Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.92%
Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.76%
Carrying Amount
Debt Instrument [Line Items]
Less: current maturities	$ (169,574)	(163,512)
Long-term debt, less current maturities	2,310,084	2,300,482
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(169,574)	(163,512)
Long-term debt, less current maturities	2,310,084	2,300,482
Carrying Amount \| Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	(125,000)	(125,000)
Long-term debt, less current maturities	1,991,624	1,991,333
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	2,116,624	2,116,333
Less: current maturities	(125,000)	(125,000)
Long-term debt, less current maturities	1,991,624	1,991,333
Carrying Amount \| Centuri
Debt Instrument [Line Items]
Less: current maturities	(44,574)	(38,512)
Long-term debt, less current maturities	318,460	309,149
Unamortized debt issuance costs	(1,031)	(1,101)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	363,034	347,661
Less: current maturities	(44,574)	(38,512)
Long-term debt, less current maturities	318,460	309,149
Centuri secured revolving credit facility	231,611	243,711
Carrying Amount \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	42,666	60,021
Carrying Amount \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	232,642	244,812
Carrying Amount \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	88,757	43,929
Carrying Amount \| Debentures \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	1,768,280	1,768,050
Unamortized discount and debt issuance costs	(14,220)	(14,450)
Carrying Amount \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount \| Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	75,000	75,000
Carrying Amount \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	7,500	7,500
Carrying Amount \| Debentures \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	150,000	150,000
Carrying Amount \| Industrial development revenue bonds \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,656)	(1,717)
Unsecured debt	198,344	198,283
Carrying Amount \| Industrial development revenue bonds \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount \| Industrial development revenue bonds \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount \| Industrial development revenue bonds \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount \| Industrial development revenue bonds \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	42,658	60,057
Market Value \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	228,304	252,182
Market Value \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	87,758	44,787
Market Value \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	125,975	126,673
Market Value \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	178,950	162,666
Market Value \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	252,688	258,550
Market Value \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	290,645	291,928
Market Value \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	278,106	308,307
Market Value \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	311,565	320,685
Market Value \| Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	297,987	330,138
Market Value \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	96,435	96,905
Market Value \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,293	27,500
Market Value \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	32,621	32,543
Market Value \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,185	9,156
Market Value \| Debentures \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000	150,000
Market Value \| Industrial development revenue bonds \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value \| Industrial development revenue bonds \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value \| Industrial development revenue bonds \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value \| Industrial development revenue bonds \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000	$ 50,000

Debt instrument unamortized discount and debt issuance costs.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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