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Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Operating Leases (Detail)
$ in Thousands
Dec. 31, 2018
USD ($)
Operating Leased Assets [Line Items]  
2019 $ 10,951
2020 8,019
2021 6,059
2022 5,326
2023 3,760
Thereafter 10,688
Total minimum lease payments 44,803
Natural Gas Operations  
Operating Leased Assets [Line Items]  
2019 898
2020 363
2021 299
2022 163
2023 79
Thereafter 177
Total minimum lease payments 1,979
Centuri  
Operating Leased Assets [Line Items]  
2019 10,053
2020 7,656
2021 5,760
2022 5,163
2023 3,681
Thereafter 10,511
Total minimum lease payments $ 42,824