Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Millions	Total	Common Stock, Net of Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total Cabot Corporation Stockholders' Equity [Member]	Noncontrolling Interests [Member]
Beginning Balance at Sep. 30, 2017	$ 1,625	$ 56		$ 1,707	$ (259)	$ 1,504	$ 121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(112)			(122)		(122)	10
Total other comprehensive income (loss)	(1)				(4)	(4)	3
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(10)						(10)
Issuance of stock under equity compensation plans, Shares		178
Amortization of share-based compensation	6		$ 6			6
Purchase and retirement of common stock	(16)		(6)	(10)		(16)
Purchase and retirement of common stock, Shares		(265)
Ending Balance at Dec. 31, 2017	1,472	$ 56		1,555	(263)	1,348	124
Ending Balance, Shares at Dec. 31, 2017		61,797
Beginning Balance at Sep. 30, 2017	1,625	$ 56		1,707	(259)	1,504	121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(275)
Total other comprehensive income (loss)	57
Ending Balance at Mar. 31, 2018	1,343	$ 55		1,370	(209)	1,216	127
Ending Balance, Shares at Mar. 31, 2018		61,819
Beginning Balance at Dec. 31, 2017	1,472	$ 56		1,555	(263)	1,348	124
Beginning Balance, Shares at Dec. 31, 2017		61,797
Net income (loss)	(163)			(173)		(173)	10
Total other comprehensive income (loss)	58				54	54	4
Cash dividends paid, Common stock, per share	(30)			(19)		(19)	(11)
Issuance of stock under equity compensation plans	1		1			1
Issuance of stock under equity compensation plans, Shares		30
Amortization of share-based compensation	6		6			6
Purchase and retirement of common stock	(1)	$ (1)	(7)	7		(1)
Purchase and retirement of common stock, Shares		(8)
Ending Balance at Mar. 31, 2018	1,343	$ 55		1,370	(209)	1,216	127
Ending Balance, Shares at Mar. 31, 2018		61,819
Beginning Balance at Sep. 30, 2018	1,279	$ 54		1,417	(317)	1,154	125
Beginning Balance, Shares at Sep. 30, 2018		60,367
Net income (loss)	77			69		69	8
Total other comprehensive income (loss)	(3)				(3)	(3)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans, Shares		344
Amortization of share-based compensation	5		5			5
Purchase and retirement of common stock	(62)	$ (1)	(5)	(56)		(62)
Purchase and retirement of common stock, Shares		(1,201)
Ending Balance at Dec. 31, 2018	1,276	$ 53		1,410	(320)	1,143	133
Ending Balance, Shares at Dec. 31, 2018		59,510
Beginning Balance at Sep. 30, 2018	1,279	$ 54		1,417	(317)	1,154	125
Beginning Balance, Shares at Sep. 30, 2018		60,367
Net income (loss)	106
Total other comprehensive income (loss)	4
Ending Balance at Mar. 31, 2019	1,232	$ 52		1,367	(315)	1,104	128
Ending Balance, Shares at Mar. 31, 2019		58,450
Beginning Balance at Dec. 31, 2018	1,276	$ 53		1,410	(320)	1,143	133
Beginning Balance, Shares at Dec. 31, 2018		59,510
Net income (loss)	29			23		23	6
Total other comprehensive income (loss)	7				5	5	2
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(13)						(13)
Issuance of stock under equity compensation plans	1	$ 1				1
Issuance of stock under equity compensation plans, Shares		41
Amortization of share-based compensation	2		2			2
Purchase and retirement of common stock	(50)	$ (2)	$ (2)	(46)		(50)
Purchase and retirement of common stock, Shares		(1,101)
Ending Balance at Mar. 31, 2019	$ 1,232	$ 52		$ 1,367	$ (315)	$ 1,104	$ 128
Ending Balance, Shares at Mar. 31, 2019		58,450