Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 80	$ 52	$ 138	$ 49
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment (net of tax provision of $1, $—, $3 and $—)	56	65	(69)	18
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period, net of tax				(1)
Amortization of net loss and prior service credit included in net periodic benefit cost, net of tax	1	1	2	1
Other comprehensive income (loss)	57	66	(67)	18
Comprehensive income (loss)	137	118	71	67
Net income (loss) attributable to noncontrolling interests, net of tax	6	4	10	8
Noncontrolling interests foreign currency translation adjustment, net of tax	1	1	(3)	(2)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	7	5	7	6
Comprehensive income (loss) attributable to Cabot Corporation	$ 130	$ 113	$ 64	$ 61