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Revenue noncash revenue (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2018
Other Significant Noncash Transactions [Line Items]      
Other Significant Noncash Transaction, Value of Consideration Given $ 10 $ 25 $ 100