Earnings Per Share - Calculation of Undistributed Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss) attributable to Cabot Corporation	$ 94	$ 88	$ (173)	$ (122)	$ 72	$ 47	$ 74	$ 55	$ (113)	$ 248	$ 147
Less: Dividends declared on common stock									79	77	65
Less: Dividends and dividend equivalents to participating securities									1
Undistributed earnings (loss)									(193)	171	82
Undistributed earnings (loss) allocated to common shareholders									$ (193)	169	81
Undistributed earnings allocated to participating securities										$ 2	$ 1