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Litigation, Claims and Contingencies - Summary of Reconciliation of Changes in Environmental Remediation Reserves (Details) - USD ($)
$ in Millions
9 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Environmental Remediation Obligations [Abstract]    
Reserve - beginning of period $ 187 $ 163
Disbursements (23) (26)
Revised obligation estimates and accretion 28 55
Reserve - end of period $ 192 $ 192