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Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Earnings Per Share [Abstract]        
Net income (loss) attributable to Cabot Corporation $ 88 $ 47 $ (207) $ 176
Less: Dividends declared on common stock 21 19 60 57
Undistributed earnings (loss) 67 28 (267) 119
Undistributed earnings (loss) allocated to common shareholders $ 67 $ 28 $ (267) 118
Undistributed earnings (loss) allocated to participating shareholders       $ 1