Long-Term Debt - Schedule of Long-Term Debt (Detail) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: current maturities	$ (33,429,000)		$ (25,346,000)
Long-term debt, less current maturities	2,123,641,000		1,798,576,000
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(33,429,000)		(25,346,000)
Long-term debt, less current maturities	2,123,641,000		1,798,576,000
Southwest Gas Corporation
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,818,621,000		1,521,031,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	$ 1,818,621,000		$ 1,521,031,000
Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure		$ 300,000,000
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate		3.70%
Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.45%		4.45%
Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.10%		6.10%
Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.875%		3.875%
Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.875%		4.875%
Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.80%		3.80%
Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.70%		3.70%
Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	8.00%		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%		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%		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%		6.76%
Carrying Amount
Debt Instrument [Line Items]
Long-term debt, less current maturities	$ 2,123,641,000		$ 1,798,576,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	2,123,641,000		1,798,576,000
Carrying Amount | Southwest Gas Corporation
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,818,621,000		1,521,031,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,818,621,000		1,521,031,000
Long-term debt, less current maturities	1,818,621,000		1,521,031,000
Carrying Amount | Centuri Construction Group Inc.
Debt Instrument [Line Items]
Less: current maturities	(33,429,000)		(25,346,000)
Long-term debt, less current maturities	305,020,000		277,545,000
Unamortized debt issuance costs	(941,000)		(1,111,000)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	338,449,000		302,891,000
Less: current maturities	(33,429,000)		(25,346,000)
Long-term debt, less current maturities	305,020,000		277,545,000
Centuri secured revolving credit facility	188,061,000		198,467,000
Carrying Amount | Centuri secured revolving credit facility | Centuri Construction Group Inc.
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	78,217,000		56,472,000
Carrying Amount | Centuri term loan facility | Centuri Construction Group Inc.
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	189,002,000		199,578,000
Carrying Amount | Centuri other debt obligations | Centuri Construction Group Inc.
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	72,171,000		47,952,000
Carrying Amount | Debentures | Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(12,037,000)		(9,350,000)
Notes payable	1,470,463,000		1,173,150,000
Carrying Amount | Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000,000		125,000,000
Carrying Amount | Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000,000		125,000,000
Carrying Amount | Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000,000		250,000,000
Carrying Amount | Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000,000		250,000,000
Carrying Amount | Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000,000		300,000,000
Carrying Amount | Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000,000		0
Carrying Amount | Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	75,000,000		75,000,000
Carrying Amount | Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000,000		25,000,000
Carrying Amount | Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000,000		25,000,000
Carrying Amount | Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	7,500,000		7,500,000
Carrying Amount | Debentures | Revolving credit facility and commercial paper | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	150,000,000		150,000,000
Carrying Amount | Industrial development revenue bonds | Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,842,000)		(2,119,000)
Unsecured debt	198,158,000		197,881,000
Carrying Amount | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Carrying Amount | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Carrying Amount | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Carrying Amount | Industrial development revenue bonds | 2009 Series A, due 2039 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Fair Value | Centuri Construction Group Inc.
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	195,247,000		207,588,000
Centuri other debt obligations	71,966,000		48,183,000
Fair Value | Centuri secured revolving credit facility | Centuri Construction Group Inc.
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	78,277,000		56,525,000
Fair Value | Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	126,808,000		129,273,000
Fair Value | Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	147,874,000		158,304,000
Fair Value | Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	251,003,000		256,163,000
Fair Value | Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	262,023,000		283,243,000
Fair Value | Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	273,843,000		302,970,000
Fair Value | Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	293,304,000		0
Fair Value | Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	93,836,000		96,063,000
Fair Value | Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,686,000		28,714,000
Fair Value | Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,693,000		31,542,000
Fair Value | Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,596,000		8,882,000
Fair Value | Debentures | Revolving credit facility and commercial paper | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000,000		150,000,000
Fair Value | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Fair Value | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000		50,000,000
Fair Value | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000,000		50,000,000
Fair Value | Industrial development revenue bonds | 2009 Series A, due 2039 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000,000		$ 50,000,000