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Supplementary Financial Information - Future Estimated Amortization Expense (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Mar. 31, 2018
Future estimated amortization expense    
2020 $ 895  
2021 542  
2022 327  
2023 266  
2024 266  
Thereafter 990  
Net Book Value $ 3,286 $ 3,751