Accumulated Other Comprehensive Loss (Tables)

6 Months Ended

Mar. 31, 2016

Equity [Abstract]

Changes in Each Component of AOCI, Net of Tax

Changes in each component of AOCI, net of tax, are as follows:

	Currency Translation Adjustment			Unrealized Gains on Investments		Pension and Other Postretirement Benefit Liability Adjustments			Total
	(Dollars in millions)
Balance at September 30, 2015, attributable to Cabot    Corporation	$	(239	)	$	2	$	(62	)	$	(299	)
Other comprehensive income (loss) before reclassifications		(47	)		—		—			(47	)
Amounts reclassified from accumulated other    comprehensive (loss) income		—			—		(1	)		(1	)
Net other comprehensive items		(286	)		2		(63	)		(347	)
Less: Noncontrolling interest		(3	)		—		—			(3	)
Balance at December 31, 2015, attributable to Cabot    Corporation	$	(283	)	$	2	$	(63	)	$	(344	)
Other comprehensive income (loss) before reclassifications		65			—		—			65
Amounts reclassified from accumulated other    comprehensive income (loss)		—			—		1			1
Net other comprehensive items		(218	)		2		(62	)		(278	)
Less: Noncontrolling interest		1			—		—			1
Balance at March 31, 2016, attributable to Cabot    Corporation	$	(219	)	$	2	$	(62	)	$	(279	)

Amounts Reclassified Out of AOCI

The amounts reclassified out of AOCI and into the Consolidated Statements of Operations in the three months and six months ended March 31, 2016 and 2015 are as follows:

		Three Months Ended					Six Months Ended
	Affected Line Item in the Consolidated	March 31,					March 31,
	Statements of Operations	2016			2015		2016			2015
		(Dollars in millions)
Pension and other postretirement    benefit liability adjustment
Amortization of actuarial losses	Net Periodic Benefit Cost - see    Note C for details	$	1		$	1	$	2		$	2
Amortization of prior service cost	Net Periodic Benefit Cost - see    Note C for details		(1	)		—		(2	)		(1	)
Settlement and curtailment (credit)    cost	Net Periodic Benefit Cost - see    Note C for details		—			—		(1	)		27
Total before tax			—			1		(1	)		28
Tax impact	Benefit (provision) for income    taxes		1			—		1			(6	)
Total after tax		$	1		$	1	$	—		$	22