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Litigation, Claims and Contingencies - Summary of Reconciliation of Changes in Environmental Remediation Reserves (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Environmental Remediation Obligations [Abstract]    
Reserve - beginning of period $ 186 $ 187
Disbursements (9) (6)
Revised obligation estimates and accretion 2 3
Reserve - end of period $ 179 $ 184