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Revenue noncash revenue (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Other Significant Noncash Transactions [Line Items]      
Other Significant Noncash Transaction, Value of Consideration Given $ 10 $ 50 $ 100