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Debt - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2019
May 31, 2019
Dec. 31, 2018
Nov. 30, 2018
Oct. 31, 2018
Mar. 31, 2017
Debt Instrument [Line Items]                  
Debt instrument percentage       19.00%          
Issuance of additional debt       $ 2,400,000,000          
Equity cushion relating to minimum net worth requirement       $ 1,500,000,000          
Percentage of maximum dividend payout restriction       60.00%          
Short term borrowings outstanding       $ 211,000,000   $ 152,000,000      
Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Debt instrument percentage       13.00%          
Short term borrowings outstanding       $ 194,000,000   152,000,000      
Centuri                  
Debt Instrument [Line Items]                  
Issuance of additional debt       184,000,000          
Equity cushion relating to minimum fixed charge ratio coverage requirement       53,000,000          
LIBOR | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Borrowings outstanding under facility       294,000,000          
LIBOR | Centuri                  
Debt Instrument [Line Items]                  
Borrowings outstanding under facility       188,000,000          
Southwest Gas Credit Facility | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity       400,000,000          
Line of credit designated as long term debt       150,000,000          
Line of credit designated for working capital purposes       $ 250,000,000          
Commitment fee percentage       0.10%          
Long-term debt       $ 150,000,000          
Effective interest rates       2.57%          
Minimum current borrowing usage $ 44,000,000                
Maximum current borrowing usage   $ 366,000,000              
Southwest Gas Credit Facility | LIBOR | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Applicable margin       1.00%          
Southwest Gas Credit Facility | Alternative Base Rate | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Applicable margin       0.00%          
Commercial Paper Program | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity       $ 50,000,000          
Borrowings outstanding under facility       50,000,000          
Notes, 4.15%, due 2049 | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Notes payable         $ 300,000,000        
Debt instrument interest rate         4.15%        
Debt instrument discount rate         0.051%        
Secured Revolving Credit Facility and Term Loan | Centuri                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity       590,000,000     $ 590,000,000 $ 450,000,000  
Borrowings outstanding under facility       $ 305,000,000          
Effective interest rates       3.20%          
Minimum current borrowing usage     $ 6,000,000            
Maximum current borrowing usage $ 99,000,000                
Debt secured by assets       $ 1,300,000,000          
Secured Revolving Credit Facility and Term Loan | Centuri | Minimum                  
Debt Instrument [Line Items]                  
Commitment fee percentage       0.125%          
Secured Revolving Credit Facility and Term Loan | Centuri | Maximum                  
Debt Instrument [Line Items]                  
Commitment fee percentage       0.35%          
Secured Revolving Credit Facility and Term Loan | LIBOR | Centuri                  
Debt Instrument [Line Items]                  
Applicable margin       0.875%          
Secured Revolving Credit Facility and Term Loan | LIBOR | Centuri | Maximum                  
Debt Instrument [Line Items]                  
Applicable margin       2.25%          
Secured Revolving Credit Facility and Term Loan | Alternative Base Rate | Centuri | Minimum                  
Debt Instrument [Line Items]                  
Applicable margin       0.00%          
Secured Revolving Credit Facility and Term Loan | Alternative Base Rate | Centuri | Maximum                  
Debt Instrument [Line Items]                  
Applicable margin       1.25%          
Centuri term loan facility | Centuri                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity             265,000,000    
Line of Credit | Centuri                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity             $ 325,000,000    
Notes, 6.1%, due 2041                  
Debt Instrument [Line Items]                  
Debt instrument interest rate       6.10%          
$100 Million Credit Facility                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity                 $ 100,000,000
Commitment fee percentage       0.15%          
$100 Million Credit Facility | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Weighted average interest rate       2.749%          
Short term borrowings outstanding       $ 17,000,000   $ 0      
$100 Million Credit Facility | LIBOR                  
Debt Instrument [Line Items]                  
Applicable margin       1.125%          
$100 Million Credit Facility | Alternative Base Rate                  
Debt Instrument [Line Items]                  
Applicable margin       0.125%          
$400 Million Credit Facility | Southwest Gas Corporation                  
Debt Instrument [Line Items]                  
Credit facility maximum borrowing capacity       $ 400,000,000          
Line of credit designated for working capital purposes       $ 250,000,000          
Weighted average interest rate       2.61%   3.47%      
Short term borrowings outstanding       $ 194,000,000   $ 152,000,000