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Long-Term Debt - Estimated Maturities of Long-Term Debt (Detail)
$ in Thousands
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]  
2019 $ 33,060
2020 163,296
2021 33,571
2022 460,515
2023 178,614
Southwest Gas Corporation  
Debt Instrument [Line Items]  
2019 0
2020 125,000
2021 0
2022 425,000
2023 0
Centuri  
Debt Instrument [Line Items]  
2019 33,060
2020 38,296
2021 33,571
2022 35,515
2023 $ 178,614