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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, 2020
Jun. 30, 2019
Environmental Remediation Obligations [Abstract]    
Reserve - beginning of period $ 186 $ 187
Disbursements (28) (23)
Revised obligation estimates and accretion 40 28
Reserve - end of period $ 198 $ 192