v3.26.1
Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Earnings Per Share [Abstract]        
Net income (loss) attributable to Cabot Corporation $ 6 $ 101 $ 147 $ 288
Less: Dividends declared on common stock 24 24 72 70
Less: Dividends declared on participating securities       1
Undistributed earnings (loss) (18) 77 75 217
Undistributed earnings (loss) allocated to common stockholders $ (18) 76 73 214
Undistributed earnings allocated to participating security holders   $ 1 $ 2 $ 3