CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 52	$ 27	$ 49	$ 76
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment (net of tax benefit of $-, $-, $- and $1)	65	(131)	18	(234)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period (net of tax provision of $-, $-, $- and $6)			(1)	21
Amortization of net loss and prior service credit included in net periodic pension cost (net of tax benefit of $1, $-, $1, and $-)	1	1	1	1
Other comprehensive income (loss)	66	(130)	18	(212)
Comprehensive income (loss)	118	(103)	67	(136)
Net income attributable to noncontrolling interests	4	1	8	5
Noncontrolling interests foreign currency translation adjustment, net of tax	1	(1)	(2)	(3)
Comprehensive income attributable to noncontrolling interests, net of tax	5		6	2
Comprehensive income (loss) attributable to Cabot Corporation	$ 113	$ (103)	$ 61	$ (138)