Cabot (CBT) 10-Q 6 Aug 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Jun. 30, 2014	Jul. 31, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Jun-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'CBT	'
Entity Registrant Name	'CABOT CORP	'
Entity Central Index Key	'0000016040	'
Current Fiscal Year End Date	'--09-30	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	64,580,369

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	3 Months Ended				9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Income Statement [Abstract]	'		'		'		'
Net sales and other operating revenues	$940	[1]	$901	[1]	$2,736	[1]	$2,560	[1]
Cost of sales	756		725		2,197		2,094
Gross profit	184		176		539		466
Selling and administrative expenses	76		72		245		222
Research and technical expenses	15		17		46		50
Income from operations	93		87		248		194
Interest and dividend income	1		2		3		4
Interest expense	-14		-15		-41		-47
Other income	'		'		27		3
Income from continuing operations before income taxes and equity in (loss) earnings of affiliated companies	80	[2]	74	[2]	237	[2]	154	[2]
Provision for income taxes	-20		-16		-51		-52
Equity in (loss) earnings of affiliated companies, net of tax	-2		3		-2		9
Income from continuing operations	58		61		184		111
(Loss) income from discontinued operations, net of tax	-1		1		-2		-2
Net income	57		62		182		109
Net income attributable to noncontrolling interests, net of tax	5		3		14		3
Net income attributable to Cabot Corporation	$52		$59		$168		$106
Weighted-average common shares outstanding, in millions:	'		'		'		'
Basic	64.5		63.8		64.3		63.6
Diluted	65.2		64.5		65		64.3
Basic:	'		'		'		'
Income from continuing operations attributable to Cabot Corporation	$0.80		$0.88		$2.61		$1.66
(Loss) income from discontinued operations	($0.01)		$0.04		($0.03)		($0.01)
Net income attributable to Cabot Corporation	$0.79		$0.92		$2.58		$1.65
Diluted:	'		'		'		'
Income from continuing operations attributable to Cabot Corporation	$0.79		$0.87		$2.58		$1.64
(Loss) income from discontinued operations	($0.01)		$0.03		($0.03)		($0.01)
Net income attributable to Cabot Corporation	$0.78		$0.90		$2.55		$1.63
Dividends per common share	$0.22		$0.20		$0.62		$0.60
[1]	Revenue from external customers that are categorized as Unallocated and Other reflects royalties, other operating revenues, external shipping and handling costs, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable. Details are provided in the table below:
[2]	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]	'	'	'	'
Net income	$57	$62	$182	$109
Other comprehensive income (loss), net of tax	'	'	'	'
Foreign currency translation adjustment (net of tax provision (benefit) of $-, $2, $1, and $(12))	11	-16	-18	-38
Unrealized holding gains arising during the period (net of tax provision of $1, $-, $1, and $-)	2	1	2	1
Pension and other postretirement benefit liability adjustments	'	'	'	'
Pension and other postretirement benefit liability adjustments arising during the period (net of tax provision of $-, $6, $-, and $6)	'	13	'	15
Amortization of net loss and prior service credit included in net periodic pension cost (net of tax provision of $-, $-, $-, and $2)	'	'	1	1
Other comprehensive income (loss)	13	-2	-15	-21
Comprehensive income	70	60	167	88
Net income attributable to noncontrolling interests	5	3	14	3
Noncontrolling interests foreign currency translation adjustment, net of tax	'	2	-2	1
Comprehensive income attributable to noncontrolling interests, net of tax	5	5	12	4
Comprehensive income attributable to Cabot Corporation	$65	$55	$155	$84

Consolidated_Statements_of_Com1

Consolidated Statements of Comprehensive Income (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]	'	'	'	'
Foreign currency translation adjustment, tax provision (benefit)	'	$2	$1	($12)
Unrealized holding gains arising during the period, tax provision	1	'	1	'
Pension and other postretirement benefit liability adjustments arising during the period, tax provision	'	6	'	6
Amortization of net loss and prior service credit included in net periodic pension cost, tax provision	'	'	'	$2

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Jun. 30, 2014	Sep. 30, 2013
In Millions, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$101	$95
Accounts and notes receivable, net of reserve for doubtful accounts of $8 and $8	724	633
Inventories:	'	'
Raw materials	119	100
Work in process	2	2
Finished goods	366	309
Other	45	44
Total inventories	532	455
Prepaid expenses and other current assets	103	58
Notes receivable from sale of business	0	214
Deferred income taxes	37	36
Current assets held for sale	4	4
Total current assets	1,501	1,495
Property, plant and equipment, net	1,616	1,600
Goodwill	555	502
Equity affiliates	71	119
Intangible assets, net	354	308
Assets held for rent	55	49
Deferred income taxes	80	68
Other assets	57	83
Noncurrent assets held for sale	9	9
Total assets	4,298	4,233
Current liabilities:	'	'
Notes payable	119	264
Accounts payable and accrued liabilities	537	534
Income taxes payable	39	30
Deferred income taxes	2	2
Current portion of long-term debt	18	14
Total current liabilities	715	844
Long-term debt	1,026	1,020
Deferred income taxes	49	21
Other liabilities	273	265
Redeemable preferred stock	29	'
Commitments and contingencies (Note I)	'	'
Preferred stock:	'	'
Authorized: 2,000,000 shares of $1 par value Issued and Outstanding: None and none	0	0
Common stock:	'	'
Authorized: 200,000,000 shares of $1 par value Issued: 64,832,912 and 64,223,985 shares Outstanding: 64,580,371 and 63,970,502 shares	65	64
Less cost of 252,541 and 253,483 shares of common treasury stock	-8	-8
Additional paid-in capital	57	39
Retained earnings	1,883	1,755
Deferred employee benefits	'	-2
Accumulated other comprehensive income	90	103
Total Cabot Corporation stockholders' equity	2,087	1,951
Noncontrolling interests	119	132
Total stockholders' equity	2,206	2,083
Total liabilities and stockholders' equity	$4,298	$4,233

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2014	Sep. 30, 2013
In Millions, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]	'	'
Accounts and notes receivable, reserve for doubtful accounts	$8	$8
Preferred stock, Authorized shares	2,000,000	2,000,000
Preferred stock, par value	$1	$1
Preferred stock, Issued shares	0	0
Preferred stock, Outstanding shares	0	0
Common stock, Authorized shares	200,000,000	200,000,000
Common stock, par value	$1	$1
Common stock, Issued shares	64,832,912	64,223,985
Common stock, Outstanding shares	64,580,371	63,970,502
Common treasury stock, shares	252,541	253,483

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Cash Flows from Operating Activities:	'	'
Net income	$182	$109
Adjustments to reconcile net income to cash provided by operating activities:	'	'
Depreciation and amortization	150	144
Impairment of assets	4	16
Deferred tax provision	-16	7
Gain on existing investment in NHUMO	-29	'
Equity in earnings of affiliated companies	2	-9
Non-cash compensation	11	13
Other	1	2
Changes in assets and liabilities:	'	'
Accounts and notes receivable	-62	-38
Inventories	-66	-9
Prepaid expenses and other assets	-20	-4
Accounts payable and accrued liabilities	'	-55
Income taxes payable	2	-36
Other liabilities	4	'
Cash dividends received from equity affiliates	22	7
Other	'	-5
Cash provided by operating activities	185	142
Cash Flows from Investing Activities:	'	'
Additions to property, plant and equipment	-115	-195
Receipts from notes receivable from sale of business	215	39
Change in assets held for rent	-5	-4
Cash paid for acquisition of business, net of cash acquired of $7	-73	'
Cash provided by (used in) investing activities	22	-160
Cash Flows from Financing Activities:	'	'
Borrowings under financing arrangements	13	6
Repayments under financing arrangements	-10	-29
Proceeds from long-term debt, net of issuance costs	17	99
Repayments of long-term debt	-7	-265
(Decrease) increase in notes payable, net	-11	18
(Repayments) proceeds from issuance of commercial paper, net	-138	202
Proceeds from cash contributions received from noncontrolling stockholders	'	13
Purchases of common stock	-6	-6
Proceeds from sales of common stock	13	6
Cash dividends paid to noncontrolling interests	-19	-17
Cash dividends paid to common stockholders	-40	-39
Cash used in financing activities	-188	-12
Effect of exchange rate changes on cash	-13	-14
Increase (decrease) in cash and cash equivalents	6	-44
Cash and cash equivalents at beginning of period	95	120
Cash and cash equivalents at end of period	$101	$76

Consolidated_Statements_of_Cas1

Consolidated Statements of Cash Flows (Parenthetical) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Statement Of Cash Flows [Abstract]	'	'
Cash acquired in acquisition of business	$7	$7

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Basis of Presentation	'
	A. Basis of Presentation
	The consolidated financial statements include the accounts of Cabot Corporation (“Cabot” or the “Company”) and its wholly owned subsidiaries and majority-owned and controlled U.S. and non-U.S. subsidiaries. Additionally, Cabot considers consolidation of entities over which control is achieved through means other than voting rights, of which there were none in the periods presented. Intercompany transactions have been eliminated in consolidation.
	The unaudited consolidated financial statements have been prepared in accordance with the requirements of Form 10-Q and consequently do not include all disclosures required by Form 10-K. Additional information may be obtained by referring to Cabot’s Annual Report on Form 10-K for the fiscal year ended September 30, 2013 (“2013 10-K”).
	The financial information submitted herewith is unaudited and reflects all adjustments which are, in the opinion of management, necessary to provide a fair statement of the results for the interim periods ended June 30, 2014 and 2013. All such adjustments are of a normal recurring nature. The results for interim periods are not necessarily indicative of the results to be expected for the fiscal year.
	In November 2013, the Company purchased all of Grupo Kuo S.A.B. de C.V.’s (“KUO”) common stock in NHUMO, S.A. de C.V. (“NHUMO”), a carbon black joint venture between the Company and KUO in Mexico, which represented approximately 60% of the outstanding common stock of NHUMO (the “NHUMO transaction”). Prior to this transaction, the Company owned approximately 40% of the outstanding common stock of NHUMO, and the NHUMO entity was accounted for as an equity affiliate of the Company. The financial position, results of operations and cash flows of NHUMO are included in the Company’s consolidated financial statements from the date of acquisition.
	In March 2014, the Company entered into an agreement to sell its Security Materials business to SICPA SA (“SICPA”). The business is being accounted for as discontinued operations and the applicable assets of the business have been classified as held for sale in the Consolidated Balance Sheets as of June 30, 2014 and September 30, 2013. The Consolidated Statements of Operations for all periods presented have been recast to reflect the Security Materials business in discontinued operations. Unless otherwise indicated, all disclosures and amounts in the Notes to Consolidated Financial Statements relate to the Company’s continuing operations. The sale closed on July 31, 2014 and the Company expects to record a gain in discontinued operations as a result of this transaction. The Company’s Consolidated Statements of Cash Flows include the cash flows from both continuing and discontinued operations.
	Certain amounts in prior years’ Consolidated Statement of Cash Flows have been combined to conform to the current presentation.

Significant_Accounting_Policie

Significant Accounting Policies	9 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Significant Accounting Policies	'
	B. Significant Accounting Policies
	Revenue Recognition and Accounts Receivable
	Cabot recognizes revenue when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price is fixed or determinable and collectability is reasonably assured. Cabot generally is able to ensure that products meet customer specifications prior to shipment. If the Company is unable to determine that the product has met the specified objective criteria prior to shipment or if title has not transferred because of sales terms, the revenue is considered “unearned” and is deferred until the revenue recognition criteria are met.
	Shipping and handling charges related to sales transactions are recorded as sales revenue when billed to customers or included in the sales price.
	The following table shows the relative size of the revenue recognized in each of the Company’s reportable segments for the periods presented. The revenue of the Advanced Technologies reportable segment excludes the Security Materials business from all periods as the Company began reporting the business under discontinued operations.
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
	Reinforcement Materials		59	%		56	%		58	%		57	%
	Performance Materials		27	%		27	%		27	%		27	%
	Purification Solutions		9	%		9	%		9	%		10	%
	Advanced Technologies		5	%		8	%		6	%		6	%
	Cabot derives the substantial majority of its revenues from the sale of products in Reinforcement Materials and Performance Materials. Revenue from these products is typically recognized when the product is shipped and title and risk of loss have passed to the customer. The Company offers certain of its customers cash discounts and volume rebates as sales incentives. The discounts and volume rebates are recorded as a reduction in sales at the time revenue is recognized and are estimated based on historical experience and contractual obligations. Cabot periodically reviews the assumptions underlying its estimates of discounts and volume rebates and adjusts its revenues accordingly.
	Revenue in Purification Solutions is typically recognized when the product is shipped and title and risk of loss have passed to the customer. For major activated carbon injection systems projects, revenue is recognized using the percentage-of-completion method.
	Revenue in Advanced Technologies, excluding the Specialty Fluids business, is typically recognized when the product is shipped and title and risk of loss have passed to the customer. Depending on the nature of the contract with the customer, a portion of the segment’s revenue may be recognized using proportional performance. Cabot has technology and licensing agreements with one customer that are accounted for as multiple element arrangements. Revenue is recognized ratably over the term of the agreements, limited by the cumulative amounts that become due, some of which are through 2022.
	A significant portion of the revenue in the Specialty Fluids business, included in Advanced Technologies, arises from the rental of cesium formate. This revenue is recognized throughout the rental period based on the contracted rental terms. Customers are also billed and revenue is recognized, typically at the end of the job, for cesium formate product that is not returned. The Company also generates revenue from cesium formate sold outside of a rental process and revenue is recognized upon delivery of the fluid.
	Cabot maintains allowances for doubtful accounts based on an assessment of the collectability of specific customer accounts, the aging of accounts receivable and other economic information on both a historical and prospective basis. Customer account balances are charged against the allowance when it is probable the receivable will not be recovered. There is no material off-balance sheet credit exposure related to customer receivable balances.
	Property, Plant and Equipment
	Property, plant and equipment are recorded at cost. Depreciation of property, plant and equipment is calculated using the straight-line method over the estimated useful lives. The depreciable lives for buildings, machinery and equipment and other fixed assets are twenty to twenty-five years, ten to twenty-five years, and three to twenty-five years, respectively. The cost and accumulated depreciation for property, plant and equipment sold, retired, or otherwise disposed of are removed from the Consolidated Balance Sheets and resulting gains or losses are included in earnings in the Consolidated Statements of Operations. Expenditures for repairs and maintenance are charged to expenses as incurred. Expenditures for major renewals and betterments, which significantly extend the useful lives of existing plant and equipment, are capitalized and depreciated.
	Intangible Assets and Goodwill
	The Company records tangible and intangible assets acquired and liabilities assumed in business combinations under the acquisition method of accounting. Amounts paid for an acquisition are allocated to the assets acquired and liabilities assumed based on their fair values at the date of acquisition. Goodwill is comprised of the purchase price of business acquisitions in excess of the fair value assigned to the net tangible and identifiable intangible assets acquired. Goodwill is not amortized but is reviewed for impairment annually, as of May 31, or when events or changes in the business environment indicate that the carrying value of the reporting unit may exceed its fair value. A reporting unit, for the purpose of the impairment test, is at or below the operating segment level, and constitutes a business for which discrete financial information is available and regularly reviewed by segment management. The separate businesses included within Performance Materials are considered separate reporting units. The goodwill balance relative to this segment is recorded in the Fumed Metal Oxides reporting unit within Performance Materials.
	For the purpose of the goodwill impairment test, the Company first assesses qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying value amount and as a basis for determining whether it is necessary to perform the two-step goodwill impairment test. Alternatively, the Company may elect to proceed directly to the two-step goodwill impairment test. If an initial qualitative assessment identifies that it is more likely than not that the carrying value of a reporting unit exceeds its estimated fair value, an additional quantitative evaluation is performed under the two-step impairment test. If based on the quantitative evaluation the fair value of the reporting unit is less than its carrying amount, the Company performs an analysis of the fair value of all assets and liabilities of the reporting unit. If the implied fair value of the reporting unit’s goodwill is determined to be less than its carrying amount, an impairment is recognized for the difference. The fair value of a reporting unit is based on discounted estimated future cash flows. The fair value is also benchmarked against a market approach using the guideline public companies method. The assumptions used to estimate fair value include management’s best estimates of future growth rates, operating cash flows, capital expenditures and discount rates over an estimate of the remaining operating period at the reporting unit level. Should the fair value of any of the Company’s reporting units decline because of reduced operating performance, market declines, as a result of changes in the discount rate, or other indicators of impairment, charges for impairment may be necessary. Based on the Company’s most recent annual goodwill impairment test performed as of May 31,2014, the fair values of the Reinforcement Materials and Fumed Metal Oxides reporting units were substantially in excess of their carrying values. The fair value of the Purification Solutions reporting unit exceeded its carrying value by approximately 9%. At June 30, 2014, the Purification Solutions reporting unit had the most significant goodwill balance, in the amount of $468 million. The future growth in the Purification Solutions business is highly dependent on achieving the expected volumes and margins in the activated carbon based mercury removal business. These volumes and margins are highly dependent on the overall size of the mercury removal market and the Company’s successful realization of its anticipated market share over the next 3 years. The size of the mercury removal market significantly depends on, among other factors, the adoption and enforcement of environmental laws and regulations, particularly those that would require U.S. based coal fired electrical utilities to reduce the quantity of air pollutants they release, including mercury, to comply with the Mercury and Air Toxics Standards that become effective in April 2015.
	The Company uses assumptions and estimates in determining the fair value of assets acquired and liabilities assumed in a business combination. The determination of the fair value of intangible assets requires the use of significant judgment with regard to (i) assumptions used in the valuation model; and (ii) determination of the intangible assets’ useful lives. The Company estimates the fair value of identifiable acquisition-related intangible assets principally based on projections of cash flows that will arise from these assets. The projected cash flows are discounted to determine the fair value of the assets at the dates of acquisition. The Company’s intangible assets are primarily comprised of trademarks that are considered indefinite-lived, customer relationships, patented and unpatented technology and other intellectual property, all of which are considered definite-lived intangible assets. The Company reviews definite-lived intangible assets for impairment when indication of potential impairment exists, such as a significant reduction in cash flows associated with the assets. Actual cash flows arising from a particular intangible asset could vary from projected cash flows which could imply different carrying values from those established at the dates of acquisition and which could result in impairment of such asset.
	The Company evaluates indefinite-lived intangible assets, which are comprised of the trademarks of Purification Solutions, for impairment annually or when events occur or circumstances change that may reduce the fair value of the asset below its carrying amount. The annual review is performed as of May 31. The Company may first perform a qualitative assessment to determine whether it is necessary to perform the quantitative impairment test or bypass the qualitative assessment and proceed directly to performing the quantitative impairment test. The quantitative impairment test is based on discounted estimated future cash flows. The assumptions used to estimate fair value include management’s best estimates of future growth rates and discount rates over an estimate of the remaining operating period at the unit of accounting level. These future growth rates depend on achieving the expected volumes and pricing levels of the products of Purification Solutions. Finite lived intangible assets are amortized over their estimated useful lives.
	Long-lived Assets
	The Company’s long-lived assets primarily include property, plant and equipment, long-term investments and assets held for rent. The carrying values of long-lived assets are reviewed for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset may not be recoverable. An asset impairment is recognized when the carrying value of the asset is not recoverable based on the probability-weighted undiscounted estimated future cash flows to be generated by the asset. The Company’s estimates reflect management’s assumptions about selling prices, production and sales volumes, costs and market conditions over an estimate of the remaining operating period. If an impairment is indicated, the asset is written down to fair value. If the asset does not have a readily determinable market value, a discounted cash flow model may be used to determine the fair value of the asset. The key inputs to the discounted cash flow would be the same as the undiscounted cash flow noted above, with the addition of the discount rate used. In circumstances when an asset does not have separate identifiable cash flows, an impairment charge is recorded when the Company no longer intends to use the asset.
	To test for impairment of assets the Company generally uses a probability-weighted estimate of the future undiscounted net cash flows of the assets over their remaining lives to determine if the value of the asset is recoverable. Long-lived assets are grouped with other assets and liabilities at the lowest level for which independent identifiable cash flows are determinable.
	Income Tax in Interim Periods
	The Company records its tax provision or benefit on an interim basis using an estimated annual effective tax rate. This rate is applied to the current period ordinary income or loss to determine the income tax provision or benefit allocated to the interim period. Losses from jurisdictions for which no benefit can be recognized and the income tax effects of unusual or infrequent items are excluded from the estimated annual effective tax rate and are recognized in the impacted interim period.
	Valuation allowances are provided against the future tax benefits that arise from the deferred tax assets in jurisdictions for which no benefit can be recognized. The estimated annual effective tax rate may be significantly impacted by nondeductible expenses and the Company’s projected earnings mix by tax jurisdiction. Adjustments to the estimated annual effective income tax rate are recognized in the period when such estimates are revised.
	Inventory Valuation
	Inventories are stated at the lower of cost or market. The cost of all carbon black inventories in the U.S. is determined using the last-in, first-out (“LIFO”) method. Had the Company used the first-in, first-out (“FIFO”) method instead of the LIFO method for such inventories, the value of those inventories would have been $53 million and $55 million higher as of June 30, 2014 and September 30, 2013, respectively. The cost of Specialty Fluids inventories is determined using the average cost method. The cost of other U.S. and non-U.S. inventories is determined using the FIFO method.
	Cabot reviews inventory for both potential obsolescence and potential declines in anticipated selling prices. In this review, the Company makes assumptions about the future demand for and market value of the inventory, and based on these assumptions estimates the amount of any obsolete, unmarketable, slow moving or overvalued inventory. Cabot writes down the value of these inventories by an amount equal to the difference between the cost of the inventory and its estimated market value.
	Pensions and Other Postretirement Benefits
	The Company recognizes the funded status of defined benefit pension and other postretirement benefit plans as an asset or liability. This amount is defined as the difference between the fair value of plan assets and the benefit obligation. The Company is required to recognize as a component of other comprehensive income, net of tax, the actuarial gains/losses and prior service costs/credits that arise but were not previously required to be recognized as components of net periodic benefit cost. Other comprehensive income is adjusted as these amounts are later recognized in income as components of net periodic benefit cost.
	Accumulated Other Comprehensive Income
	Accumulated other comprehensive income (“AOCI”), which is included as a component of stockholders’ equity, includes unrealized gains or losses on available-for-sale marketable securities, currency translation adjustments in foreign subsidiaries, translation adjustments on foreign equity securities and pension liability adjustments.
	At the beginning of fiscal 2014, the Company adopted new accounting guidance for the presentation of amounts reclassified from AOCI. The guidance specifically requires, either on the face of the financial statements or in the notes, presentation of significant amounts reclassified from AOCI to net income for each component of AOCI and the respective line items within the Consolidated Statements of Operations. These disclosures are included in Note H to the consolidated financial statements.
	Accumulated other comprehensive items in the accompanying Consolidated Balance Sheets consist of the following items net of tax:
		June 30,			September 30,
		2014			2013
	Foreign currency translation adjustments at beginning of period	$	154		$	167
	Net foreign currency translation adjustments during the period		(16	)		(13	)
	Balance at end of period		138			154
	Unrealized gain on investments at beginning of period		2			—
	Net unrealized gains during the period		2			2
	Balance at end of period		4			2
	Pension and other postretirement benefit plans at beginning of period		(53	)		(75	)
	Net change in pension and other postretirement benefit plans during the period		1			22
	Balance at end of period		(52	)		(53	)
	Total accumulated other comprehensive income	$	90		$	103
	Recent Accounting Pronouncements
	In July 2013, the Financial Accounting Standards Board (FASB) issued a new standard related to the “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists”. The standard requires, unless certain conditions exist, an unrecognized tax benefit or a portion of an unrecognized tax benefit be presented in the financial statements as a reduction to a deferred tax asset for a net operating loss carryforward, similar to a tax loss or a tax credit carryforward. This standard is applicable for fiscal years beginning after December 15, 2013, and for interim periods within those years. The Company will adopt this standard on October 1, 2014. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements and it does not expect the impact to be material.
	In April 2014, the FASB issued a new standard related to the “Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity”. The standard requires discontinued operations treatment for disposals of a component or group of components that represents a strategic shift that has or will have a major impact on an entity’s operations or financial results and requires additional disclosures for discontinued operations and new disclosures for individually material disposal transactions that do not meet the definition of a discontinued operation. This standard is applicable for fiscal years beginning after December 15, 2014 and for interim periods within those years with early adoption permitted but only for disposals that have not been reported in financial statements previously issued. The Company expects to adopt this standard beginning on October 1, 2015. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements.
	In May 2014, the FASB issued a new standard related to the “Revenue from Contracts with Customers” which amends the existing accounting standards for revenue recognition. The standard requires entities to recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration the entity expects to be entitled to in exchange for those goods or services. This standard is applicable for fiscal years beginning after December 15, 2016 and for interim periods within those years and early adoption is not permitted. The Company expects to adopt this standard on October 1, 2017. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements.

Acquisition_of_NHUMO

Acquisition of NHUMO	9 Months Ended
	Jun. 30, 2014
Business Combinations [Abstract]	'
Acquisition of NHUMO	'
	C. Acquisition of NHUMO
	In November 2013, the Company purchased all of KUO’s common stock in the NHUMO joint venture, which represented approximately 60% of the outstanding common stock of the joint venture. Prior to this transaction, the Company owned approximately 40% of the outstanding common stock of NHUMO, and the NHUMO entity was accounted for as an equity affiliate of the Company.
	At the close of the transaction, the Company paid KUO $80 million in cash and NHUMO issued redeemable preferred stock to KUO with a redemption value of $25 million. The preferred stock accumulates dividends at a fixed rate of 6% annually and is redeemable at the option of KUO or the Company for $25 million starting in November 2018 or upon the occurrence of certain other conditions. Annual payment by NHUMO of the dividends will be contingent on NHUMO achieving a minimum EBITDA (earnings before interest, taxes, depreciation and amortization) level and if such minimum EBITDA is not achieved in any year, the dividend will be accumulated and paid at the time the preferred shares are redeemed. The preferred stock issued in connection with the transaction is not mandatorily redeemable and has embedded put and call rights at the fixed redemption price. Accordingly, the instrument is accounted for as a financing obligation and has been separately presented in the Consolidated Balance Sheets as a long term liability. Upon acquisition, the Company began consolidating NHUMO into its consolidated financial statements. Prior to closing, the Company received a $14 million dividend from NHUMO.
	The Company incurred acquisition costs of approximately $1 million through June 30, 2014 associated with the transaction, which are included in Selling and administrative expenses in the Consolidated Statements of Operations.
	The allocation of the purchase price set forth below was based on preliminary estimates of the fair value of assets acquired, liabilities assumed, and Cabot’s previously held equity interest in NHUMO as of the acquisition date. The Company is continuing to obtain information to complete its valuation of these accounts and the associated tax accounting.
		(in millions)
	Assets
	Cash	$	7
	Accounts receivable		33
	Inventories		14
	Property, plant and equipment		48
	Other non-current assets		1
	Intangible assets		57
	Goodwill		51
	Total assets acquired		211
	Liabilities
	Current liabilities		(18	)
	Deferred tax liabilities		(31	)
	Total liabilities assumed		(49	)
	Net assets acquired	$	162
	Cash consideration paid		80
	Fair value of redeemable preferred stock		28
	Previously held equity interest in NHUMO		54
	Total	$	162
	As a result of the acquisition, the Company recorded a gain of $29 million for the difference between the carrying value and the fair value of the previously held equity interest in NHUMO, which was included in Other income. The fair value of $54 million for the previously held equity interest was determined based on the fair value of Cabot’s pre-existing interest in NHUMO as adjusted for a control premium derived from synergies gained as a result of the Company obtaining control of NHUMO.
	As part of the preliminary purchase price allocation, the Company determined that a separately identifiable intangible asset was customer relationships in the amount of $57 million, which is being amortized over a period of 17 years. The Company estimated the fair value of the identifiable acquisition-related intangible asset based on projections of cash flows that will arise from the asset. The projected cash flows are discounted to determine the fair value of the asset at the date of acquisition. The determination of the fair value of the intangible asset acquired required the use of significant judgment with regard to (i) assumptions in the discounted cash flow model used and (ii) determination of the useful life of customer relationships.
	The fair value of the redeemable preferred stock was determined based on a discounted cash flow model, using the expected timing of the cash flows and an appropriate discount rate.
	The excess of the purchase price over the fair value of the tangible net assets and intangible asset acquired, the issuance of redeemable preferred stock and the previously held equity interest in NHUMO was recorded as goodwill. The goodwill recognized is attributable to the expected growth and operating synergies that the Company expects to realize from this acquisition. Goodwill generated from the acquisition will not be deductible for tax purposes.

Discontinued_Operations_and_No

Discontinued Operations and Notes Receivable from Sale of Business	9 Months Ended
	Jun. 30, 2014
Discontinued Operations And Disposal Groups [Abstract]	'
Discontinued Operations and Notes Receivable from Sale of Business	'
	D. Discontinued Operations and Notes Receivable from Sale of Business
	In January 2012, the Company sold its Supermetals business to Global Advanced Metals Pty Ltd., an Australian company (“GAM”), for $452 million, including cash consideration of $175 million received on the closing date and notes receivable (“GAM Notes”) totaling $277 million payable at various dates through March 2014. During the third quarter of fiscal 2014, Cabot received the final payment on the GAM Notes in the amount of $215 million.
	In March 2014, the Company entered into an agreement to sell its Security Materials business to SICPA, for approximately $20 million in cash. The sale closed on July 31, 2014 and the Company expects to record a gain in discontinued operations as a result of this transaction.
	The following table summarizes the results from discontinued operations during the three and nine months ended June 30, 2014 and 2013:
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
		(Dollars in millions)
	Net sales and other operating revenues	$	2		$	1		$	5		$	4
	Loss from operations before income taxes		(1	)		(1	)		(3	)		(6	)
	Benefit from income taxes		—			2			1			4
	(Loss) Income from discontinued operations, net of tax	$	(1	)	$	1		$	(2	)	$	(2	)
	The following table summarizes the assets held for sale in the Company’s Consolidated Balance Sheets:
		June 30, 2014			September 30, 2013
		(Dollars in millions)
	Assets
	Inventories	$	3		$	3
	Other current assets		1			1
	Total current assets held for sale	$	4		$	4
	Property, plant and equipment, net	$	5		$	5
	Goodwill		2			2
	Intangible assets, net		2			2
	Total noncurrent assets held for sale	$	9		$	9

Employee_Benefit_Plans

Employee Benefit Plans	9 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Employee Benefit Plans	'
	E. Employee Benefit Plans
	Net periodic defined benefit pension and other postretirement benefit costs include the following:
		Three Months Ended June 30
		2014						2013						2014					2013
		Pension Benefits												Postretirement Benefits
		U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
		(Dollars in millions)
	Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
	Interest cost		2			4			1			4			1			1		—			1
	Expected return on plan assets		(3	)		(5	)		(2	)		(4	)		—			—		—			—
	Amortization of prior service credit		—			—			—			—			(1	)		—		—			—
	Amortization of actuarial loss		—			1			1			1			—			—		—			—
	Net periodic benefit cost	$	—		$	2		$	1		$	3		$	—		$	1	$	—		$	1
		Nine Months Ended June 30
		2014						2013						2014					2013
		Pension Benefits												Postretirement Benefits
		U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
		(Dollars in millions)
	Service cost	$	2		$	7		$	4		$	7		$	—		$	—	$	—		$	—
	Interest cost		6			12			4			11			2			1		1			1
	Expected return on plan assets		(8	)		(15	)		(6	)		(13	)		—			—		—			—
	Amortization of prior service credit		—			—			—			—			(2	)		—		(2	)		—
	Amortization of actuarial loss		—			3			2			3			—			—		—			—
	Net periodic benefit cost	$	—		$	7		$	4		$	8		$	—		$	1	$	(1	)	$	1

Goodwill_and_Other_Intangible_

Goodwill and Other Intangible Assets	9 Months Ended
	Jun. 30, 2014
Goodwill And Intangible Assets Disclosure [Abstract]	'
Goodwill and Other Intangible Assets	'
	F. Goodwill and Other Intangible Assets
	Cabot had goodwill balances of $555 million and $502 million at June 30, 2014 and September 30, 2013, respectively. Goodwill of $2 million which was presented under the Advanced Technologies reportable segment as of September 30, 2013 has been reclassified to Assets held for sale in the Company’s Consolidated Balance Sheets for all periods presented as a result of the then pending divestiture of the Security Materials business discussed in Note D. The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the period ended June 30, 2014 are as follows:
			Reinforcement		Performance			Purification		Total
			Materials		Materials			Solutions
			(Dollars in millions)
	Balance at September 30, 2013		$	25	$	11		$	466	$	502
	Goodwill acquired(1)			51		—			—		51
	Foreign currency impact			—		—			2		2
	Balance at June 30, 2014		$	76	$	11		$	468	$	555
	(1)	Goodwill acquired relates to the NHUMO transaction as described in Note C.
	Goodwill impairment tests are performed at least annually. The Company performed its last annual impairment assessment as of May 31, 2014 and determined there was no impairment.
	Net intangible assets of $2 million which were presented as other intangible assets as of September 30, 2013 have been reclassified to Assets held for sale in the Company’s Consolidated Balance Sheets for all periods presented as a result of the then pending divestiture of the Security Materials business discussed in Note D. The following table provides information regarding the Company’s intangible assets:
			June 30, 2014							September 30, 2013
			Gross		Accumulated			Net		Gross		Accumulated			Net
			Carrying		Amortization			Intangible		Carrying		Amortization			Intangible
			Value					Assets		Value					Assets
			(Dollars in millions)
	Intangible assets with finite lives
	Developed technology		$	155	$	(15	)	$	140	$	154	$	(9	)	$	145
	Customer relationships(1)			171		(14	)		157		113		(7	)		106
	Total intangible assets, finite lives		$	326	$	(29	)	$	297	$	267	$	(16	)	$	251
	Trademarks, indefinite lives			57		—			57		57		—			57
	Total intangible assets		$	383	$	(29	)	$	354	$	324	$	(16	)	$	308
	(1)	The change in the gross carrying value of the Customer relationships intangible asset is primarily due to the NHUMO transaction as described in Note C.
	Intangible assets with finite lives are amortized over their estimated useful lives, which range from sixteen to twenty years, with a weighted average amortization period of approximately nineteen years. Amortization expense for the three months ended June 30, 2014 and 2013 was $4 million and is included in Cost of sales and Selling and administrative expenses in the Consolidated Statements of Operations. Amortization expense for the nine months ended June 30, 2014 and 2013 was $13 million and $10 million, respectively, and is included in Cost of sales and Selling and administrative expenses in the Consolidated Statements of Operations. Total amortization expense is estimated to be approximately $17 million each year for the next five fiscal years. Intangible assets with indefinite lives are evaluated for impairment at least annually. The Company performed its last annual impairment assessment as of May 31, 2014, and determined there was no impairment.

Stockholders_Equity

Stockholders' Equity	9 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Stockholders' Equity	'
	G. Stockholders’ Equity
	In fiscal 2007, the Board of Directors authorized Cabot to repurchase up to ten million shares of Cabot’s common stock in the open market or in privately negotiated transactions. This authorization does not have a set expiration date. During the first nine months of both fiscal 2014 and 2013, Cabot repurchased 379 shares of its common stock under this authorization. As of June 30, 2014, approximately 1.6 million shares remain available for repurchase under the current authorization.
	During the nine months ended June 30 of both fiscal 2014 and 2013, Cabot paid cash dividends to common stockholders of $40 million and $39 million, respectively.
	Noncontrolling interests
	The following table illustrates the noncontrolling interests activity for the periods presented:
		2014			2013
		(Dollars in millions)
	Balance at September 30	$	132		$	126
	Net income attributable to noncontrolling interests		14			3
	Noncontrolling interest foreign currency translation adjustment		(2	)		1
	Contribution from noncontrolling interests		—			13
	Noncontrolling interest dividends		(25	)		(17	)
	Balance at June 30	$	119		$	126

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income	9 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Accumulated Other Comprehensive Income	'
	H. Accumulated Other Comprehensive Income
	Comprehensive income combines net income and other comprehensive income items, which are reported as components of stockholders’ equity in the accompanying Consolidated Balance Sheets.
	Changes in each component of Accumulated other comprehensive income, net of tax, are as follows:
		Currency				Unrealized				Pension and Other			Total
		Translation				Gains on				Postretirement
		Adjustment				Investments				Benefit Liability
										Adjustments
		(Dollars in millions)
	Balance at September 30, 2013, attributable to Cabot Corporation	$	154			$	2			$	(53	)	$	103
	Other comprehensive loss before reclassifications		(18	)			—				—			(18	)
	Amounts reclassified from accumulated other comprehensive income		—				2				1			3
	Net other comprehensive items		(18	)			2				1			(15	)
	Less: Noncontrolling interest		(2	)			—				—			(2	)
	Balance at June 30, 2014, attributable to Cabot Corporation	$	138			$	4			$	(52	)	$	90
	The amounts reclassified out of Accumulated other comprehensive income and into the Statement of Operations for the nine months ended June 30, 2014 and 2013 are as follows:
				Nine Months Ended June 30
		Affected Line Item in the Consolidated		2014				2013
		Statements of Operations
				(Dollars in millions)
	Pension and other postretirement benefit liability adjustment
	Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		$	3			$	5
	Amortization of prior service cost	Net Periodic Benefit Cost - see Note E for details			(2	)			(2	)
	Total before tax				1				3
	Tax impact	Provision for income taxes			—				(2	)
	Total after tax			$	1			$	1

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Jun. 30, 2014
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	I. Commitments and Contingencies
	Purchase Commitments
	Cabot has entered into long-term purchase agreements primarily for the purchase of raw materials. Under certain of these agreements the quantity of material being purchased is fixed, but the price paid changes as market prices change. For those commitments, the amounts included in the table below are based on market prices at June 30, 2014.
		Payments Due by Fiscal Year
		Remainder of		2015		2016		2017		2018		Thereafter		Total
		Fiscal 2014
		(Dollars in millions)
	Reinforcement Materials	$	102	$	318	$	280	$	189	$	181	$	2,628	$	3,698
	Performance Materials		12		39		33		29		30		236		379
	Advanced Technologies		1		2		1		1		1		—		6
	Purification Solutions		12		20		11		10		9		17		79
	Total	$	127	$	379	$	325	$	229	$	221	$	2,881	$	4,162
	Guarantee Agreements
	Cabot has provided certain indemnities pursuant to which it may be required to make payments to an indemnified party in connection with certain transactions and agreements. In connection with certain acquisitions and divestitures, Cabot has provided routine indemnities with respect to such matters as environmental, tax, insurance, product and employee liabilities. In connection with various other agreements, including service and supply agreements, Cabot may provide routine indemnities for certain contingencies and routine warranties. Cabot is unable to estimate the maximum potential liability for these types of indemnities as a maximum obligation is not explicitly stated in most cases and the amounts, if any, are dependent upon the outcome of future contingent events, the nature and likelihood of which cannot be reasonably estimated. The durations of the indemnities vary, and in many cases are indefinite. Cabot has not recorded any liability for these indemnities in the consolidated financial statements, except as otherwise disclosed.
	Contingencies
	Cabot is a defendant, or potentially responsible party, in various lawsuits and environmental proceedings wherein substantial amounts are claimed or at issue.
	Environmental Matters
	As of June 30, 2014 and September 30, 2013, Cabot had $17 million and $5 million, respectively, reserved for environmental matters, substantially all of which is accounted for on an undiscounted basis. Cabot recorded a $13 million charge in the second quarter of fiscal 2014. These environmental matters mainly relate to closed sites. These reserves represent Cabot’s best estimates of the probable costs to be incurred at those sites where costs are reasonably estimable based on its analysis of the extent of clean up required, alternative clean-up methods available, abilities of other responsible parties to contribute and its interpretation of laws and regulations applicable to each site. Cash payments related to these environmental matters were $2 million in the first nine months of both fiscal 2014 and 2013. Cabot reviews the adequacy of the reserves as circumstances change at individual sites and adjusts the reserves as appropriate. Almost all of Cabot’s reserves relate to environmental issues that are mature and have been investigated and studied and, in many cases, are subject to agreed upon remediation plans. However, depending on the results of future testing, changes in risk assessment practices, remediation techniques and regulatory requirements, newly discovered conditions, and other factors, it is reasonably possible that the Company could incur additional costs in excess of environmental reserves currently recorded. Management estimates, based on the latest available information, that any such future environmental remediation costs that are reasonably possible to be in excess of amounts already recorded would be immaterial to the Company’s consolidated financial statements.
	Other Matters
	Respirator Liabilities
	Cabot has exposure in connection with a safety respiratory products business that a subsidiary acquired from American Optical Corporation (“AO”) in an April 1990 asset purchase transaction. The subsidiary manufactured respirators under the AO brand and disposed of that business in July 1995. In connection with its acquisition of the business, the subsidiary agreed, in certain circumstances, to assume a portion of AO’s liabilities, including costs of legal fees together with amounts paid in settlements and judgments, allocable to AO respiratory products used prior to the 1990 purchase by the Cabot subsidiary. As more fully described in Cabot’s 2013 10-K, the respirator liabilities involve claims for personal injury, including asbestosis, silicosis and coal worker’s pneumoconiosis, allegedly resulting from the use of respirators that are alleged to have been negligently designed and/or labeled.
	As of June 30, 2014 and September 30, 2013, there were approximately 41,000 and 42,000 claimants, respectively, in pending cases asserting claims against AO in connection with respiratory products. Cabot has a reserve to cover its expected share of liability for existing and future respirator liability claims. At June 30, 2014 and September 30, 2013, the reserve was $9 million and $11 million, respectively, on a discounted basis ($13 million and $15 million on an undiscounted basis at June 30, 2014 and September 30, 2013, respectively). The reserve is being accreted up to the undiscounted liability through interest expense over the expected cash flow period, which is through 2065. Cash payments related to this liability were $2 million in the first nine months of both fiscal 2014 and 2013.
	Other
	The Company is subject to various other lawsuits, claims and contingent liabilities arising in the ordinary course of its business and with respect to the Company’s divested businesses. In the opinion of the Company, although final disposition of some or all of these other suits and claims may impact the Company’s consolidated financial statements in a particular period, they should not, in the aggregate, have a material adverse effect on the Company’s financial position.

Income_Tax_Uncertainties

Income Tax Uncertainties	9 Months Ended
	Jun. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Tax Uncertainties	'
	J. Income Tax Uncertainties
	Cabot files U.S. federal and state and non-U.S. income tax returns in jurisdictions with varying statutes of limitations. The 2007 through 2013 tax years remain subject to examination by the IRS and various tax years from 2005 through 2013 remain subject to examination by the respective state tax authorities. In significant non-U.S. jurisdictions, various tax years from 2004 through 2013 remain subject to examination by their respective tax authorities. Cabot’s significant non-U.S. jurisdictions include China, France, Germany, Italy, Japan, and the Netherlands.
	Certain Cabot subsidiaries are under audit in jurisdictions outside of the U.S. In addition, certain statutes of limitations are scheduled to expire in the near future. It is reasonably possible that a change in the unrecognized tax benefits may also occur within the next twelve months related to the settlement of one or more of these audits, however, an estimated range of the impact on the unrecognized tax benefits cannot be quantified at this time.
	During the nine months ended June 30, 2014, Cabot reached a bilateral agreement with the tax authorities in the U.S. and Canada that covered the fiscal years 2001 through 2012, resulting in a $6 million net benefit. In addition, Cabot expects to recover prepaid taxes of approximately $16 million from the Canadian government, of which $1 million has been received as of June 30, 2014.

Earnings_Per_Share

Earnings Per Share	9 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Earnings Per Share	'
	K. Earnings Per Share
	The following tables summarize the components of the basic and diluted earnings per common share computations:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars and shares in millions,
			except per share amounts)
	Basic EPS:
	Net income attributable to Cabot Corporation		$	52		$	59		$	168		$	106
	Less: Dividends and dividend equivalents to participating securities			—			—			—			—
	Less: Undistributed earnings allocated to participating securities(1)			1			1			2			1
	Earnings allocated to common shareholders (numerator)		$	51		$	58		$	166		$	105
	Weighted average common shares and participating securities outstanding			65.1			64.4			64.9			64.2
	Less: Participating securities(1)			0.6			0.6			0.6			0.6
	Adjusted weighted average common shares (denominator)			64.5			63.8			64.3			63.6
	Amounts per share - basic:
	Income from continuing operations attributable to Cabot Corporation		$	0.8		$	0.88		$	2.61		$	1.66
	(Loss) Income from discontinued operations			(0.01	)		0.04			(0.03	)		(0.01	)
	Net income attributable to Cabot Corporation		$	0.79		$	0.92		$	2.58		$	1.65
	Diluted EPS:
	Earnings allocated to common shareholders		$	51		$	58		$	166		$	105
	Plus: Earnings allocated to participating securities			1			1			2			1
	Less: Adjusted earnings allocated to participating securities(2)			(1	)		(1	)		(2	)		(1	)
	Earnings allocated to common shareholders (numerator)		$	51		$	58		$	166		$	105
	Adjusted weighted average common shares outstanding			64.5			63.8			64.3			63.6
	Effect of dilutive securities:
	Common shares issuable(3)			0.7			0.7			0.7			0.7
	Adjusted weighted average common shares (denominator)			65.2			64.5			65			64.3
	Amounts per share - diluted:
	Income from continuing operations attributable to Cabot Corporation		$	0.79		$	0.87		$	2.58		$	1.64
	(Loss) Income from discontinued operations			(0.01	)		0.03			(0.03	)		(0.01	)
	Net income attributable to Cabot Corporation		$	0.78		$	0.9		$	2.55		$	1.63
	(1)	Participating securities consist of shares of unvested restricted stock and unvested time-based restricted stock units.
	Undistributed earnings are the earnings which remain after dividends declared during the period are assumed to be distributed to the common and participating shareholders. Undistributed earnings are allocated to common and participating shareholders on the same basis as dividend distributions. The calculation of undistributed earnings is as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Calculation of undistributed earnings:
	Net income attributable to Cabot Corporation		$	52		$	59		$	168		$	106
	Less: Dividends declared on common stock			14			13			40			39
	Less: Dividends declared on participating securities			—			—			—			—
	Undistributed earnings		$	38		$	46		$	128		$	67
	Allocation of undistributed earnings:
	Undistributed earnings allocated to common shareholders		$	37		$	45		$	126		$	66
	Undistributed earnings allocated to participating shareholders			1			1			2			1
	Undistributed earnings		$	38		$	46		$	128		$	67
	(2)	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.
	(3)	Represents incremental shares of common stock from the (i) assumed exercise of stock options issued under Cabot’s equity incentive plans; (ii) assumed issuance of shares to employees pursuant to the Company’s Deferred Compensation and Supplemental Retirement Plan; and (iii) assumed issuance of shares under outstanding performance-based restricted stock unit awards issued under Cabot’s equity incentive plans. For the three and nine months ended June 30, 2014, 142,115 and 203,019 incremental shares of common stock, respectively, were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive. For the three and nine months ended June 30, 2013, 308,000 incremental shares of common stock were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive.

Restructuring

Restructuring	9 Months Ended
	Jun. 30, 2014
Restructuring And Related Activities [Abstract]	'
Restructuring	'
	L. Restructuring
	Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations as follows:
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
		(Dollars in millions)
	Cost of sales	$	2		$	3		$	11		$	24
	Selling and administrative expenses		2			1			13			4
	Research and Technical expenses		—			1			—			1
	Total	$	4		$	5		$	24		$	29
	Details of these restructuring activities and the related reserves during the three months ended June 30, 2014 are as follows:
		Severance			Environmental			Asset			Other			Total
		and Employee			Remediation			Impairment
		Benefits						and
								Accelerated
								Depreciation
		(Dollars in millions)
	Reserve at March 31, 2014	$	16		$	2		$	—		$	2		$	20
	Charges		2			—			1			1			4
	Costs charged against assets/liabilities		—			—			(1	)		—			(1	)
	Cash paid		(1	)		—			—			(1	)		(2	)
	Reserve at June 30, 2014	$	17		$	2		$	—		$	2		$	21
	Details of these restructuring activities and the related reserves during the nine months ended June 30, 2014 are as follows:
		Severance			Environmental			Asset			Asset			Other			Total
		and Employee			Remediation			Impairment			Sales
		Benefits						and
								Accelerated
								Depreciation
		(Dollars in millions)
	Reserve at September 30, 2013	$	7		$	2		$	—		$	—		$	1		$	10
	Charges		15			1			4			1			3			24
	Costs charged against assets/liabilities		—			—			(4	)		—			—			(4	)
	Cash paid		(5	)		(1	)		—			(1	)		(2	)		(9	)
	Reserve at June 30, 2014	$	17		$	2		$	—		$	—		$	2		$	21
	Shared Service Center Transition
	In January 2014, the Company announced its intention to open a new Europe, Middle East and Africa (“EMEA”) Shared Service Center in Riga, Latvia, and to close its Leuven, Belgium site, subject to the Belgian information and consultation process, which was successfully completed in June 2014. These proposed actions were developed following an extensive evaluation of the Company’s shared service capabilities in the EMEA region and a determination that the proposed future EMEA Shared Service Center will enable the Company to provide the highest quality of service at the most competitive cost.
	The Company has recorded $15 million and $1 million of cash charges during the nine and three months ended June 30, 2014, respectively, related to this plan, comprised primarily of employee severance. The Company expects that the majority of actions related to the transition of the shared service center will occur in calendar 2014 and result in total cash charges between $22 million and $26 million. Through June 30, 2014, the Company has made $1 million in cash payments related to this plan, mainly related to transition costs, and expects to make cash payments of $5 million in the fourth quarter of fiscal 2014 and the remainder (between $17 million and $21 million) in fiscal 2015.
	As of June 30, 2014, Cabot has $14 million of accrued restructuring costs in the Consolidated Balance Sheet related to this closure which is mainly comprised of accrued severance charges.
	Closure of Port Dickson, Malaysia Manufacturing Facility
	On April 26, 2013, the Company announced that the Board of its joint venture carbon black company, Cabot Malaysia Sdn. Bhd. (“CMSB”), decided to cease carbon black production at its Port Dickson, Malaysia facility. The facility ceased production in June 2013. The Company holds a 51 percent equity share in CMSB. The decision, which affected approximately 90 carbon black employees, was driven by the facility’s manufacturing inefficiencies and raw materials costs.
	The Company has recorded pre-tax charges of $2 million and $18 million in the nine months ended June 30, 2014 and 2013, respectively, related to this closure and less than $1 million and $2 million for the three months ended June 30, 2014 and 2013, respectively. Fiscal 2014 charges are comprised primarily of site demolition, clearing and environmental remediation whereas fiscal 2013 costs were comprised of asset impairment charges.
	Through June 30, 2014, the consolidated joint venture has recorded pre-tax restructuring costs of approximately $20 million comprised mainly of accelerated depreciation and asset write-offs of $15 million, severance charges of $2 million, site demolition, clearing and environmental remediation charges of $2 million, and other closure related charges of $1 million. CMSB’s net income or loss is attributable to Cabot Corporation and to the noncontrolling interest in the joint venture. The portion of the charges that are allocable to the noncontrolling interest in CMSB (49%) are recorded within Net income (loss) attributable to noncontrolling interests, net of tax, in the Consolidated Statements of Operations.
	The Company expects that the majority of actions related to closure of the plant will be completed in fiscal 2014 and result in total pre-tax charges to the consolidated joint venture of approximately $23 million. The expected charges are comprised of asset impairments and accelerated depreciation of $15 million, site demolition, clearing and environmental remediation of $5 million, severance charges of $2 million, and other closure related charges of $1 million.
	Cumulative net cash outlays related to this plan are expected to be approximately $8 million comprised primarily of $6 million for site demolition, clearing and environmental remediation, $1 million for severance, and $1 million for other closure related charges. Through June 30, 2014, CMSB has made approximately $3 million in cash payments related to this plan related mainly to severance and site demolition and clearing costs.
	CMSB expects to make net cash payments of $5 million during the remainder of fiscal 2014 and thereafter mainly comprised of site demolition, clearing and environmental remediation costs. These amounts exclude any proceeds that may be received from the sale of land or other manufacturing assets.
	As of June 30, 2014, Cabot has $1 million of accrued restructuring costs in the Consolidated Balance Sheets related to this closure which is mainly comprised of accrued severance and other charges.
	Other Activities
	The Company has recorded pre-tax charges of approximately $6 million and $7 million during the first nine months of fiscal 2014 and fiscal 2013, respectively, and $2 million for the each of the three months ended June 30, 2014 and 2013 related to restructuring activities at several other locations. Fiscal 2014 charges are comprised mainly of accelerated depreciation and asset write-offs whereas fiscal 2013 costs are comprised mainly of severance-related charges. The Company anticipates recording additional charges of $2 million during the remainder of fiscal 2014 and thereafter related to these actions.
	Through June 30, 2014, Cabot has made cash payments of $16 million related to these activities and expects to pay $3 million in the remainder of fiscal 2014 and thereafter mainly for severance and other closure related costs at the impacted locations.
	As of June 30, 2014, Cabot has $3 million of accrued severance and other closure related costs in the Consolidated Balance Sheets related to these activities.
	Previous Actions and Sites Pending Sale
	Beginning in fiscal 2009, the Company entered into several different restructuring plans which have been substantially completed, pending the sale of former manufacturing sites in Thane, India, Stanlow, U.K. and Hong Kong. The Company has incurred total cumulative pre-tax charges of approximately $163 million related to these plans through June 30, 2014 comprised of $67 million for severance charges, $65 million for accelerated depreciation and asset impairments, $10 million for environmental, demolition and site clearing costs, and $22 million of other closure related charges partially offset by gains on asset sales of $1 million. These amounts do not include any gain that may be recorded if the Company successfully sells its land rights and certain manufacturing related assets in India and Hong Kong or its land in the U.K.
	Pre-tax restructuring expenses related to these plans were approximately $1 million and $2 million during the first nine months of fiscal 2014 and 2013, respectively, and less than $1 million in each of the three months ended June 30, 2014 and 2013, respectively.
	Since fiscal 2009, Cabot has made net cash payments of $85 million related to these plans and expects to pay approximately $2 million in the remainder of fiscal 2014 and thereafter. The remaining payments consist mainly of environmental and other closure related costs. These amounts do not include any proceeds that may be received if the Company successfully sells its land rights and certain manufacturing related assets in India and Hong Kong or its land in the U.K.
	As of June 30, 2014, Cabot has $3 million of accrued environmental and other closure related costs in the Consolidated Balance Sheets related to these activities.

Financial_Instruments_and_Fair

Financial Instruments and Fair Value Measurements	9 Months Ended
	Jun. 30, 2014
Fair Value Disclosures [Abstract]	'
Financial Instruments and Fair Value Measurements	'
	M. Financial Instruments and Fair Value Measurements
	The FASB authoritative guidance on fair value measurements defines fair value, provides a framework for measuring fair value in generally accepted accounting principles, and requires certain disclosures about fair value measurements. The disclosures focus on the inputs used to measure fair value. The guidance establishes the following hierarchy for categorizing these inputs:
	Level 1	—	Quoted market prices in active markets for identical assets or liabilities
	Level 2	—	Significant other observable inputs (e.g., quoted prices for similar items in active markets, quoted prices for identical or similar items in markets that are not active, inputs other than quoted prices that are observable such as interest rate and yield curves, and market-corroborated inputs)
	Level 3	—	Significant unobservable inputs
	There were no transfers of financial assets or liabilities measured at fair value between Level 1 and Level 2, or transfers into or out of Level 3, during the first nine months of both fiscal 2014 and 2013.
	At both June 30, 2014 and September 30, 2013, the fair value of Guaranteed investment contracts, included in Other assets in the Consolidated Balance Sheets, was $14 million. Guaranteed investment contracts were classified as Level 2 instruments within the fair value hierarchy as the fair value determination was based on the other observable inputs.
	At June 30, 2014 and September 30, 2013, the fair values of cash and cash equivalents, accounts and notes receivable, accounts payable and accrued liabilities, and notes payable approximated their carrying values due to the short-term nature of these instruments. The carrying value of the long-term fixed rate debt was $958 million and $971 million, at June 30, 2014 and September 30, 2013, respectively. The fair value of the long-term fixed rate debt was $1.01 billion at both June 30, 2014 and September 30, 2013. The fair values of Cabot’s fixed rate long-term debt are estimated based on comparable quoted market prices at the respective period ends. The carrying amounts of Cabot’s floating rate long-term debt and capital lease obligations approximate their fair values. All such measurements are based on observable inputs and are classified as Level 2 within the fair value hierarchy. The valuation technique used is the discounted cash flow model.

Derivatives

Derivatives	9 Months Ended
	Jun. 30, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Derivatives	'
	N. Derivatives
	Interest Rate Risk Management
	Cabot’s objective is to maintain a certain fixed-to-variable interest rate mix on the Company’s debt obligations. Cabot may enter into interest rate swaps as a hedge of the underlying debt instruments to effectively change the characteristics of the interest rate without changing the debt instrument. As of both June 30, 2014 and September 30, 2013, there were no derivatives held to manage interest rate risk.
	Foreign Currency Risk Management
	Cabot’s international operations are subject to certain risks, including currency exchange rate fluctuations and government actions. Cabot endeavors to match the currency in which debt is issued to the currency of the Company’s major, stable cash receipts. In some situations Cabot may issue debt denominated in U.S. dollars and enter into cross currency swaps that exchange the dollar principal and interest payments into a currency where the Company expects long-term, stable cash receipts.
	Additionally, the Company has foreign currency exposure arising from its net investments in foreign operations. Cabot may enter into cross-currency swaps to mitigate the impact of currency rate changes on the Company’s net investments.
	The Company also has foreign currency exposure arising from the denomination of assets and liabilities in foreign currencies other than the functional currency of a given subsidiary as well as the risk that currency fluctuations could affect the dollar value of future cash flows generated in foreign currencies. Accordingly, Cabot uses forward contracts to minimize the exposure to foreign currency risk.
	In certain situations where the Company has forecasted purchases under a long-term commitment or forecasted sales denominated in a foreign currency, Cabot may enter into appropriate financial instruments in accordance with the Company’s risk management policy to hedge future cash flow exposures. The following table provides details of the derivatives held as of June 30, 2014 and September 30, 2013 to manage foreign currency risk:
				Notional Amount
	Description		Borrowing	June 30, 2014	September 30, 2013	Hedge Designation
	Forward Foreign Currency Contracts (1)		N/A	USD 13 million	USD 31 million	No designation
	-1	Cabot’s forward foreign exchange contracts are denominated primarily in the Brazilian real, British pound sterling, Chinese renminbi, Czech koruna and Indian rupee.
	Accounting for Derivative Instruments and Hedging Activities
	The Company determines the fair value of derivative instruments using quoted market prices whenever available. When quoted market prices are not available for various types of derivative instruments (such as forwards, options and swaps), the Company uses standard models with market-based inputs, which take into account the present value of estimated future cash flows and the ability of the financial counterparty to perform. For interest rate and cross-currency swaps, the significant inputs to these models are interest rate curves for discounting future cash flows. For forward foreign currency contracts, the significant inputs are interest rate curves for discounting future cash flows, and exchange rate curves of the foreign currency for translating future cash flows.
	Fair Value Hedge
	For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivative as well as the offsetting gain or loss on the hedged item attributable to the hedged risk are recognized in current period earnings.
	Cash Flow Hedge
	For derivative instruments that are designated and qualify as cash flow hedges, the effective portion of the gain or loss on the derivative is recorded in Accumulated other comprehensive income and reclassified to earnings in the same period or periods during which the hedged transaction affects earnings. Gains and losses on the derivative representing either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in current period earnings.
	Other Derivative Instruments
	From time to time, the Company may enter into certain derivative instruments that may not be designated as hedges for accounting purposes, which include cross currency swaps, foreign currency forward contracts and commodity derivatives. Although these derivatives do not qualify for hedge accounting, Cabot believes that such instruments are closely correlated with the underlying exposure, thus managing the associated risk. The gains or losses from changes in the fair value of derivative instruments that are not accounted for as hedges are recognized in current period earnings.
	For the three and nine months ended June 30, 2014 there were no derivatives designated as hedges. For both the three and nine months ended June 30, 2013, for derivatives designated as hedges, the change in unrealized gains in Accumulated other comprehensive income, the hedge ineffectiveness recognized in earnings, the realized gains or losses reclassified from Accumulated other comprehensive income, and the losses reclassified from Accumulated other comprehensive income to earnings were immaterial.
	During the three and nine months ended June 30, 2014, Cabot recognized in earnings through Other income within the Consolidated Statements of Operations losses of $1 million and $5 million, respectively, related to its foreign currency forward contracts, which were not designated as hedges.
	For the three and nine months ended June 30, 2013, gains of $3 million and $1 million, respectively, were recognized in earnings as a result of the remeasurement to Euros of the $175 million bond issued by one of Cabot’s European subsidiaries. These gains, which were recognized in earnings through Other income within the Consolidated Statements of Operations, were offset by losses of $2 million and less than one million, for the three and nine months ended June 30, 2013, respectively, from Cabot’s cross currency swaps that are not designated as hedges, but which Cabot entered into to offset the foreign currency translation exposure on the debt. Additionally, during the three and nine months ended June 30, 2013, Cabot recognized in earnings through Other income within the Consolidated Statements of Operations a loss of $2 million and a gain of $7 million, respectively, related to its foreign currency forward contracts, which were not designated as hedges.
	For both June 30, 2014 and September 30, 2013, the fair value of derivative instruments were immaterial and were presented in Prepaid expenses and other current assets and Accounts payable and accrued liabilities on the Consolidated Balance Sheets.
	The net after-tax amounts to be reclassified from Accumulated other comprehensive income to earnings within the next 12 months are expected to be immaterial.

Venezuela

Venezuela	9 Months Ended
	Jun. 30, 2014
Equity Method Investments And Joint Ventures [Abstract]	'
Venezuela	'
	O. Venezuela
	Cabot owns 49% of an operating Carbon Black affiliate in Venezuela, which is accounted for as an equity affiliate, through wholly owned subsidiaries that carry the investment and receive its dividends. As of June 30, 2014, these subsidiaries carried the operating affiliate investment of $17 million and held 20 million bolivars (less than $1 million) in cash.
	During the nine months ended June 30, 2014 and 2013, the operating affiliate declared dividends in the amount of $4 million and $2 million, respectively, which were paid in U.S. dollars and repatriated to the Company’s wholly owned subsidiaries.
	During the second quarter of fiscal 2014, the Venezuelan government enacted several changes to Venezuela’s foreign exchange regime, introducing a multi-tier foreign exchange system whereby there are now three exchange rate mechanisms available to convert Venezuelan Bolivars to U.S. dollars. In March 2014, the Venezuelan government created a currency exchange mechanism referred to as SICAD 2 (Supplementary System for the Administration of Foreign Currency) and allowed its use by all entities for all transactions. The exchange rate on March 31, 2014 under SICAD 2 was 50.8 bolivars to the U.S. dollar (B/$) compared to the previously used official exchange rate of 6.3 B/$. A significant portion of the Company’s operating affiliate’s sales are exports denominated in U.S. dollars. The Venezuelan government mandates that a certain percentage of the dollars collected from these sales be converted into Bolivars. Since the exchange rate that was made available to the Company when converting these dollars into Bolivars was the SICAD 2 exchange rate, the operating affiliate remeasured its bolivar denominated monetary accounts at that rate. From a segment reporting perspective, the negative impact of the exchange rate devaluation on the operating affiliate’s results was $8 million, of which Cabot’s share was $4 million for the nine months ended June 30, 2014. The SICAD 2 rate at June 30, 2014 was 50.0 B/$.
	In addition, in the second quarter of fiscal 2014 the Company remeasured the bolivar denominated monetary accounts in its wholly owned subsidiaries at the SICAD 2 rate as it was determined that this exchange mechanism is applicable to these subsidiaries. This resulted in the recognition of a $2 million loss which was recorded within Other income within the Consolidated Statements of Operations. The Company also recognized a tax benefit of $2 million from a reduction in its bolivar denominated deferred tax liability due to the impact of the devaluation of the bolivar on unremitted earnings.
	The operating entity has been profitable historically and has significant export operations from which it is entitled to retain a certain percentage of the foreign currency that it collects, which is principally the U.S. dollar. The Company continues to closely monitor developments in Venezuela and their potential impact on the recoverability of its equity affiliate investment.
	The Company closely monitors its ability to convert its bolivar holdings into U.S. dollars, as the Company intends to convert substantially all bolivars held by its wholly owned subsidiaries in Venezuela to U.S. dollars as soon as practical. Any future change in the SICAD 2 rate or opening of additional parallel markets could lead the Company to change the exchange rate it uses and result in gains or losses on the bolivar denominated assets held by its wholly owned subsidiaries.

Financial_Information_by_Segme

Financial Information by Segment	9 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Financial Information by Segment	'
	P. Financial Information by Segment
	The Company identifies a business as an operating segment if: i) it engages in business activities from which it may earn revenues and incur expenses; ii) its operating results are regularly reviewed by the Chief Operating Decision Maker (“CODM”) to make decisions about resources to be allocated to the segment and assess its performance; and iii) it has available discrete financial information. The Company has determined that all of its businesses are operating segments. The CODM reviews financial information at the operating segment level to allocate resources and to assess the operating results and financial performance for each operating segment. Operating segments are aggregated into a reportable segment if the operating segments are determined to have similar economic characteristics and if the operating segments are similar in the following areas: i) nature of products and services; ii) nature of production processes; iii) type or class of customer for their products and services; iv) methods used to distribute the products or provide services; and v) if applicable, the nature of the regulatory environment.
	The Company has four reportable segments: Reinforcement Materials, Performance Materials, Advanced Technologies and Purification Solutions. Reinforcement Materials represents the Company’s Rubber Blacks business. Purification Solutions represents the Company’s Activated Carbon business. Performance Materials is an aggregation of the Specialty Carbons and Compounds and Fumed Metal Oxides businesses, which are similar in terms of economic characteristics, nature of products, processes, customer class and product distribution methods.
	The Company has combined and disclosed four of its operating segments (Specialty Fluids, Inkjet, Aerogel and Elastomer Composites) into an other reportable segment labeled “Advanced Technologies”. The Security Materials business was previously included in the Advanced Technologies reportable segment. During the second quarter of fiscal 2014, as discussed in Note D, the Company reached an agreement to sell its Security Materials business. Accordingly, results of the Security Materials business for all periods presented have been recast as discontinued operations.
	Reportable segment operating profit (loss) before interest and taxes (“Segment EBIT”) is presented for each reportable segment in the financial information by reportable segment table below on the line entitled Income (loss) from continuing operations before taxes. Segment EBIT excludes certain items, meaning items considered by management to be unusual and not representative of segment results. In addition, Segment EBIT includes Equity in (loss) earnings of affiliated companies, net of tax, the full operating results of a contractual joint venture in Purification Solutions, royalties, Net income (loss) attributable to noncontrolling interests, net of tax, and discounting charges for certain Notes receivable, but excludes Interest expense, foreign currency transaction gains and losses, interest income, dividend income, unearned revenue, the effects of LIFO accounting for inventory, general unallocated income (expense) and unallocated corporate costs.
	Prior year Purification Solutions Segment EBIT has been adjusted to include an allocation of corporate administrative and functional support costs, which were previously reflected in unallocated corporate costs and other segment results. Beginning in the second quarter of fiscal 2014, a reclassification has been made in the Purification Solutions segment information in order to align the presentation of shipping and handling costs on customer sales with the rest of the Company’s businesses. The reclassification was made between Purification Solutions and Unallocated and other to include the shipping and handling costs in Revenues from external customers. There is no impact on Segment EBIT as a result of the reclassification. Historical periods have been adjusted to reflect this reclassification.
	Financial information by reportable segment is as follows:
			Reinforcement			Performance			Advanced			Purification			Segment		Unallocated			Consolidated
			Materials			Materials			Technologies			Solutions			Total		and Other(1)			Total
			(Dollars in millions)
	Three Months Ended June 30, 2014
	Revenues from external customers(2)		$	534		$	243		$	47		$	78		$	902	$	38		$	940
	Income (loss) from continuing operations before taxes(3)		$	61		$	41		$	14		$	(7	)	$	109	$	(29	)	$	80
	Three Months Ended June 30, 2013
	Revenues from external customers(2)		$	486		$	233		$	67		$	81		$	867	$	34		$	901
	Income (loss) from continuing operations before taxes(3)		$	49		$	35		$	28		$	(1	)	$	111	$	(37	)	$	74
	Nine Months Ended June 30, 2014
	Revenues from external customers(2)		$	1,555		$	709		$	159		$	230		$	2,653	$	83		$	2,736
	Income (loss) from continuing operations before taxes(3)		$	186		$	122		$	51		$	(20	)	$	339	$	(102	)	$	237
	Nine Months Ended June 30, 2013
	Revenues from external customers(2)		$	1,420		$	672		$	143			244		$	2,479	$	81		$	2,560
	Income (loss) from continuing operations before taxes(3)		$	141		$	99		$	44			4		$	288	$	(134	)	$	154
	(1)	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the Chief Operating Decision Maker.
	(2)	Revenue from external customers that are categorized as Unallocated and Other reflects royalties, other operating revenues, external shipping and handling costs, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable. Details are provided in the table below:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Royalties, other operating revenues, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable.		$	10		$	9		$	(2	)	$	6
	Shipping and handling fees			28			25			85			75
	Total		$	38		$	34		$	83		$	81
	(3)	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Interest expense		$	(14	)	$	(15	)	$	(41	)	$	(47	)
	Total certain items, pre-tax(a)			(7	)		(4	)		(19	)		(43	)
	Less: Equity in loss (earnings) of affiliated companies, net of tax(b)			2			(3	)		2			(9	)
	Unallocated corporate costs(c)			(14	)		(12	)		(43	)		(37	)
	General unallocated income (expense)(d)			4			(3	)		(1	)		2
	Total		$	(29	)	$	(37	)	$	(102	)	$	(134	)
		(a)	Certain items are items that management does not consider to be representative of operating segment results and they are, therefore, excluded from Segment EBIT. Certain items, pre-tax, for the three months ended June 30, 2014 include $3 million related to global restructuring activities, $3 million of foreign currency loss on revaluations and $1 million related to legal and environmental matters and reserves. Certain items, pre-tax, for the nine months ended June 30, 2014 include $24 million related to global restructuring activities, $5 million for acquisition and integration-related charges (consisting of $3 million for certain other one-time integration costs and $2 million of additional charges related to acquisition accounting adjustments for the acquired inventory of NHUMO), $3 million of foreign currency loss on revaluations and $16 million for legal and environmental matters and reserves offset by a $29 million non-cash gain recognized on the Company’s pre-existing investment in NHUMO as a result of the NHUMO transaction. Certain items, pre-tax, for the three months ended June 30, 2013 include $5 million related to global restructuring activities and $2 million for acquisition and integration-related charges offset by $3 million of foreign currency gain on revaluation. Certain items, pre-tax, for the nine months ended June 30, 2013 include $29 million related to global restructuring activities and $18 million for acquisition and integration-related charges (consisting of $7 million for certain other one-time integration costs and $11 million of charges related to acquisition accounting adjustments for the acquired inventory) offset by $4 million of foreign currency gain on revaluation.
		(b)	Equity in loss (earnings) of affiliated companies, net of tax, is included in Segment EBIT and is removed from Unallocated and other to reconcile to income (loss) from continuing operations before taxes.
		(c)	Unallocated corporate costs are not controlled by the operating segments and primarily benefit corporate interests.
		(d)	General unallocated income (expense) consists of gains (losses) arising from foreign currency transactions, net of other foreign currency risk management activities, the impact of accounting for certain inventory on a LIFO basis, the profit or loss related to the corporate adjustment for unearned revenue, and the impact of including the full operating results of an equity affiliate in Purification Solutions Segment EBIT.
	Performance Materials is comprised of two businesses that sell the following products: specialty grades of carbon black and thermoplastic concentrates and compounds (the Specialty Carbons and Compounds business); and fumed silica, fumed alumina and dispersions thereof (the Fumed Metal Oxides business). The net sales from each of these businesses for the three and nine months ended June 30, 2014 and 2013 are as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Specialty Carbons and Compounds		$	165		$	159		$	485		$	464
	Fumed Metal Oxides			78			74			224			208
	Total Performance Materials		$	243		$	233		$	709		$	672
	The net sales from each of the Advanced Technologies businesses are as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Inkjet Colorants		$	17		$	18		$	46		$	46
	Aerogel			2			9			8			17
	Elastomer Composites			4			5			28			17
	Specialty Fluids			24			35			77			63
Total Advanced Technologies		$	47		$	67		$	159		$	143

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	9 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Revenue Recognition and Accounts Receivable	'
	Revenue Recognition and Accounts Receivable
	Cabot recognizes revenue when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price is fixed or determinable and collectability is reasonably assured. Cabot generally is able to ensure that products meet customer specifications prior to shipment. If the Company is unable to determine that the product has met the specified objective criteria prior to shipment or if title has not transferred because of sales terms, the revenue is considered “unearned” and is deferred until the revenue recognition criteria are met.
	Shipping and handling charges related to sales transactions are recorded as sales revenue when billed to customers or included in the sales price.
	The following table shows the relative size of the revenue recognized in each of the Company’s reportable segments for the periods presented. The revenue of the Advanced Technologies reportable segment excludes the Security Materials business from all periods as the Company began reporting the business under discontinued operations.
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
	Reinforcement Materials		59	%		56	%		58	%		57	%
	Performance Materials		27	%		27	%		27	%		27	%
	Purification Solutions		9	%		9	%		9	%		10	%
	Advanced Technologies		5	%		8	%		6	%		6	%
	Cabot derives the substantial majority of its revenues from the sale of products in Reinforcement Materials and Performance Materials. Revenue from these products is typically recognized when the product is shipped and title and risk of loss have passed to the customer. The Company offers certain of its customers cash discounts and volume rebates as sales incentives. The discounts and volume rebates are recorded as a reduction in sales at the time revenue is recognized and are estimated based on historical experience and contractual obligations. Cabot periodically reviews the assumptions underlying its estimates of discounts and volume rebates and adjusts its revenues accordingly.
	Revenue in Purification Solutions is typically recognized when the product is shipped and title and risk of loss have passed to the customer. For major activated carbon injection systems projects, revenue is recognized using the percentage-of-completion method.
	Revenue in Advanced Technologies, excluding the Specialty Fluids business, is typically recognized when the product is shipped and title and risk of loss have passed to the customer. Depending on the nature of the contract with the customer, a portion of the segment’s revenue may be recognized using proportional performance. Cabot has technology and licensing agreements with one customer that are accounted for as multiple element arrangements. Revenue is recognized ratably over the term of the agreements, limited by the cumulative amounts that become due, some of which are through 2022.
	A significant portion of the revenue in the Specialty Fluids business, included in Advanced Technologies, arises from the rental of cesium formate. This revenue is recognized throughout the rental period based on the contracted rental terms. Customers are also billed and revenue is recognized, typically at the end of the job, for cesium formate product that is not returned. The Company also generates revenue from cesium formate sold outside of a rental process and revenue is recognized upon delivery of the fluid.
	Cabot maintains allowances for doubtful accounts based on an assessment of the collectability of specific customer accounts, the aging of accounts receivable and other economic information on both a historical and prospective basis. Customer account balances are charged against the allowance when it is probable the receivable will not be recovered. There is no material off-balance sheet credit exposure related to customer receivable balances.
Property, Plant and Equipment	'
	Property, Plant and Equipment
	Property, plant and equipment are recorded at cost. Depreciation of property, plant and equipment is calculated using the straight-line method over the estimated useful lives. The depreciable lives for buildings, machinery and equipment and other fixed assets are twenty to twenty-five years, ten to twenty-five years, and three to twenty-five years, respectively. The cost and accumulated depreciation for property, plant and equipment sold, retired, or otherwise disposed of are removed from the Consolidated Balance Sheets and resulting gains or losses are included in earnings in the Consolidated Statements of Operations. Expenditures for repairs and maintenance are charged to expenses as incurred. Expenditures for major renewals and betterments, which significantly extend the useful lives of existing plant and equipment, are capitalized and depreciated.
Intangible Assets and Goodwill	'
	Intangible Assets and Goodwill
	The Company records tangible and intangible assets acquired and liabilities assumed in business combinations under the acquisition method of accounting. Amounts paid for an acquisition are allocated to the assets acquired and liabilities assumed based on their fair values at the date of acquisition. Goodwill is comprised of the purchase price of business acquisitions in excess of the fair value assigned to the net tangible and identifiable intangible assets acquired. Goodwill is not amortized but is reviewed for impairment annually, as of May 31, or when events or changes in the business environment indicate that the carrying value of the reporting unit may exceed its fair value. A reporting unit, for the purpose of the impairment test, is at or below the operating segment level, and constitutes a business for which discrete financial information is available and regularly reviewed by segment management. The separate businesses included within Performance Materials are considered separate reporting units. The goodwill balance relative to this segment is recorded in the Fumed Metal Oxides reporting unit within Performance Materials.
	For the purpose of the goodwill impairment test, the Company first assesses qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying value amount and as a basis for determining whether it is necessary to perform the two-step goodwill impairment test. Alternatively, the Company may elect to proceed directly to the two-step goodwill impairment test. If an initial qualitative assessment identifies that it is more likely than not that the carrying value of a reporting unit exceeds its estimated fair value, an additional quantitative evaluation is performed under the two-step impairment test. If based on the quantitative evaluation the fair value of the reporting unit is less than its carrying amount, the Company performs an analysis of the fair value of all assets and liabilities of the reporting unit. If the implied fair value of the reporting unit’s goodwill is determined to be less than its carrying amount, an impairment is recognized for the difference. The fair value of a reporting unit is based on discounted estimated future cash flows. The fair value is also benchmarked against a market approach using the guideline public companies method. The assumptions used to estimate fair value include management’s best estimates of future growth rates, operating cash flows, capital expenditures and discount rates over an estimate of the remaining operating period at the reporting unit level. Should the fair value of any of the Company’s reporting units decline because of reduced operating performance, market declines, as a result of changes in the discount rate, or other indicators of impairment, charges for impairment may be necessary. Based on the Company’s most recent annual goodwill impairment test performed as of May 31, 2014 the fair values of the Reinforcement Materials and Fumed Metal Oxides reporting units were substantially in excess of their carrying values. The fair value of the Purification Solutions reporting unit exceeded its carrying value by approximately 9%. At June 30, 2014, the Purification Solutions reporting unit had the most significant goodwill balance, in the amount of $468 million. The future growth in the Purification Solutions business is highly dependent on achieving the expected volumes and margins in the activated carbon based mercury removal business. These volumes and margins are highly dependent on the overall size of the mercury removal market and the Company’s successful realization of its anticipated market share. The size of the mercury removal market significantly depends on, among other factors, the adoption and enforcement of environmental laws and regulations, particularly those that would require U.S. based coal fired electrical utilities to reduce the quantity of air pollutants they release, including mercury, to comply with the Mercury and Air Toxics Standards that become effective in April 2015.
	The Company uses assumptions and estimates in determining the fair value of assets acquired and liabilities assumed in a business combination. The determination of the fair value of intangible assets requires the use of significant judgment with regard to (i) assumptions used in the valuation model; and (ii) determination of the intangible assets’ useful lives. The Company estimates the fair value of identifiable acquisition-related intangible assets principally based on projections of cash flows that will arise from these assets. The projected cash flows are discounted to determine the fair value of the assets at the dates of acquisition. The Company’s intangible assets are primarily comprised of trademarks that are considered indefinite-lived, customer relationships, patented and unpatented technology and other intellectual property, all of which are considered definite-lived intangible assets. The Company reviews definite-lived intangible assets for impairment when indication of potential impairment exists, such as a significant reduction in cash flows associated with the assets. Actual cash flows arising from a particular intangible asset could vary from projected cash flows which could imply different carrying values from those established at the dates of acquisition and which could result in impairment of such asset.
	The Company evaluates indefinite-lived intangible assets, which are comprised of the trademarks of Purification Solutions, for impairment annually or when events occur or circumstances change that may reduce the fair value of the asset below its carrying amount. The annual review is performed as of May 31. The Company may first perform a qualitative assessment to determine whether it is necessary to perform the quantitative impairment test or bypass the qualitative assessment and proceed directly to performing the quantitative impairment test. The quantitative impairment test is based on discounted estimated future cash flows. The assumptions used to estimate fair value include management’s best estimates of future growth rates and discount rates over an estimate of the remaining operating period at the unit of accounting level. These future growth rates depend on achieving the expected volumes and pricing levels of the products of Purification Solutions. Finite lived intangible assets are amortized over their estimated useful lives.
Long-lived Assets	'
	Long-lived Assets
	The Company’s long-lived assets primarily include property, plant and equipment, long-term investments and assets held for rent. The carrying values of long-lived assets are reviewed for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset may not be recoverable. An asset impairment is recognized when the carrying value of the asset is not recoverable based on the probability-weighted undiscounted estimated future cash flows to be generated by the asset. The Company’s estimates reflect management’s assumptions about selling prices, production and sales volumes, costs and market conditions over an estimate of the remaining operating period. If an impairment is indicated, the asset is written down to fair value. If the asset does not have a readily determinable market value, a discounted cash flow model may be used to determine the fair value of the asset. The key inputs to the discounted cash flow would be the same as the undiscounted cash flow noted above, with the addition of the discount rate used. In circumstances when an asset does not have separate identifiable cash flows, an impairment charge is recorded when the Company no longer intends to use the asset.
	To test for impairment of assets the Company generally uses a probability-weighted estimate of the future undiscounted net cash flows of the assets over their remaining lives to determine if the value of the asset is recoverable. Long-lived assets are grouped with other assets and liabilities at the lowest level for which independent identifiable cash flows are determinable.
Income Tax in Interim Periods	'
	Income Tax in Interim Periods
	The Company records its tax provision or benefit on an interim basis using an estimated annual effective tax rate. This rate is applied to the current period ordinary income or loss to determine the income tax provision or benefit allocated to the interim period. Losses from jurisdictions for which no benefit can be recognized and the income tax effects of unusual or infrequent items are excluded from the estimated annual effective tax rate and are recognized in the impacted interim period.
	Valuation allowances are provided against the future tax benefits that arise from the deferred tax assets in jurisdictions for which no benefit can be recognized. The estimated annual effective tax rate may be significantly impacted by nondeductible expenses and the Company’s projected earnings mix by tax jurisdiction. Adjustments to the estimated annual effective income tax rate are recognized in the period when such estimates are revised.
Inventory Valuation	'
	Inventory Valuation
	Inventories are stated at the lower of cost or market. The cost of all carbon black inventories in the U.S. is determined using the last-in, first-out (“LIFO”) method. Had the Company used the first-in, first-out (“FIFO”) method instead of the LIFO method for such inventories, the value of those inventories would have been $53 million and $55 million higher as of June 30, 2014 and September 30, 2013, respectively. The cost of Specialty Fluids inventories is determined using the average cost method. The cost of other U.S. and non-U.S. inventories is determined using the FIFO method.
	Cabot reviews inventory for both potential obsolescence and potential declines in anticipated selling prices. In this review, the Company makes assumptions about the future demand for and market value of the inventory, and based on these assumptions estimates the amount of any obsolete, unmarketable, slow moving or overvalued inventory. Cabot writes down the value of these inventories by an amount equal to the difference between the cost of the inventory and its estimated market value.
Pensions and Other Postretirement Benefits	'
	Pensions and Other Postretirement Benefits
	The Company recognizes the funded status of defined benefit pension and other postretirement benefit plans as an asset or liability. This amount is defined as the difference between the fair value of plan assets and the benefit obligation. The Company is required to recognize as a component of other comprehensive income, net of tax, the actuarial gains/losses and prior service costs/credits that arise but were not previously required to be recognized as components of net periodic benefit cost. Other comprehensive income is adjusted as these amounts are later recognized in income as components of net periodic benefit cost.
Accumulated Other Comprehensive Income	'
	Accumulated Other Comprehensive Income
	Accumulated other comprehensive income (“AOCI”), which is included as a component of stockholders’ equity, includes unrealized gains or losses on available-for-sale marketable securities, currency translation adjustments in foreign subsidiaries, translation adjustments on foreign equity securities and pension liability adjustments.
	At the beginning of fiscal 2014, the Company adopted new accounting guidance for the presentation of amounts reclassified from AOCI. The guidance specifically requires, either on the face of the financial statements or in the notes, presentation of significant amounts reclassified from AOCI to net income for each component of AOCI and the respective line items within the Consolidated Statements of Operations. These disclosures are included in Note H to the consolidated financial statements.
	Accumulated other comprehensive items in the accompanying Consolidated Balance Sheets consist of the following items net of tax:
		June 30,			September 30,
		2014			2013
	Foreign currency translation adjustments at beginning of period	$	154		$	167
	Net foreign currency translation adjustments during the period		(16	)		(13	)
	Balance at end of period		138			154
	Unrealized gain on investments at beginning of period		2			—
	Net unrealized gains during the period		2			2
	Balance at end of period		4			2
	Pension and other postretirement benefit plans at beginning of period		(53	)		(75	)
	Net change in pension and other postretirement benefit plans during the period		1			22
	Balance at end of period		(52	)		(53	)
	Total accumulated other comprehensive income	$	90		$	103
Recent Accounting Pronouncements	'
	Recent Accounting Pronouncements
	In July 2013, the FASB issued a new standard related to the “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists”. The standard requires, unless certain conditions exist, an unrecognized tax benefit or a portion of an unrecognized tax benefit be presented in the financial statements as a reduction to a deferred tax asset for a net operating loss carryforward, similar to a tax loss or a tax credit carryforward. This standard is applicable for fiscal years beginning after December 15, 2013, and for interim periods within those years. The Company will adopt this standard on October 1, 2014. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements and it does not expect the impact to be material.
	In April 2014, the FASB issued a new standard related to the “Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity”. The standard requires discontinued operations treatment for disposals of a component or group of components that represents a strategic shift that has or will have a major impact on an entity’s operations or financial results and requires additional disclosures for discontinued operations and new disclosures for individually material disposal transactions that do not meet the definition of a discontinued operation. This standard is applicable for fiscal years beginning after December 15, 2014 and for interim periods within those years with early adoption permitted but only for disposals that have not been reported in financial statements previously issued. The Company expects to adopt this standard beginning on October 1, 2015. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements.
	In May 2014, the FASB issued a new standard related to the “Revenue from Contracts with Customers” which amends the existing accounting standards for revenue recognition. The standard requires entities to recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration the entity expects to be entitled to in exchange for those goods or services. This standard is applicable for fiscal years beginning after December 15, 2016 and for interim periods within those years and early adoption is not permitted. The Company expects to adopt this standard on October 1, 2017. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements.

Significant_Accounting_Policie2

Significant Accounting Policies (Tables)	9 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Segment Reporting Revenue Percentage	'
	The following table shows the relative size of the revenue recognized in each of the Company’s reportable segments for the periods presented. The revenue of the Advanced Technologies reportable segment excludes the Security Materials business from all periods as the Company began reporting the business under discontinued operations.
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
	Reinforcement Materials		59	%		56	%		58	%		57	%
	Performance Materials		27	%		27	%		27	%		27	%
	Purification Solutions		9	%		9	%		9	%		10	%
	Advanced Technologies		5	%		8	%		6	%		6	%
Accumulated Other Comprehensive Items in Accompanying Consolidated Balance Sheets	'
	Accumulated other comprehensive items in the accompanying Consolidated Balance Sheets consist of the following items net of tax:
		June 30,			September 30,
		2014			2013
	Foreign currency translation adjustments at beginning of period	$	154		$	167
	Net foreign currency translation adjustments during the period		(16	)		(13	)
	Balance at end of period		138			154
	Unrealized gain on investments at beginning of period		2			—
	Net unrealized gains during the period		2			2
	Balance at end of period		4			2
	Pension and other postretirement benefit plans at beginning of period		(53	)		(75	)
	Net change in pension and other postretirement benefit plans during the period		1			22
	Balance at end of period		(52	)		(53	)
	Total accumulated other comprehensive income	$	90		$	103

Acquisition_of_NHUMO_Tables

Acquisition of NHUMO (Tables)	9 Months Ended
	Jun. 30, 2014
Business Combinations [Abstract]	'
Allocation of Purchase Price	'
	The allocation of the purchase price set forth below was based on preliminary estimates of the fair value of assets acquired, liabilities assumed, and Cabot’s previously held equity interest in NHUMO as of the acquisition date. The Company is continuing to obtain information to complete its valuation of these accounts and the associated tax accounting.
		(in millions)
	Assets
	Cash	$	7
	Accounts receivable		33
	Inventories		14
	Property, plant and equipment		48
	Other non-current assets		1
	Intangible assets		57
	Goodwill		51
	Total assets acquired		211
	Liabilities
	Current liabilities		(18	)
	Deferred tax liabilities		(31	)
	Total liabilities assumed		(49	)
	Net assets acquired	$	162
	Cash consideration paid		80
	Fair value of redeemable preferred stock		28
	Previously held equity interest in NHUMO		54
	Total	$	162

Discontinued_Operations_and_No1

Discontinued Operations and Notes Receivable from Sale of Business (Tables)	9 Months Ended
	Jun. 30, 2014
Discontinued Operations And Disposal Groups [Abstract]	'
Components of Discontinued Operations	'
	The following table summarizes the results from discontinued operations during the three and nine months ended June 30, 2014 and 2013:
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
		(Dollars in millions)
	Net sales and other operating revenues	$	2		$	1		$	5		$	4
	Loss from operations before income taxes		(1	)		(1	)		(3	)		(6	)
	Benefit from income taxes		—			2			1			4
	(Loss) Income from discontinued operations, net of tax	$	(1	)	$	1		$	(2	)	$	(2	)
Summary of Assets Held for Sale	'
	The following table summarizes the assets held for sale in the Company’s Consolidated Balance Sheets:
		June 30, 2014			September 30, 2013
		(Dollars in millions)
	Assets
	Inventories	$	3		$	3
	Other current assets		1			1
	Total current assets held for sale	$	4		$	4
	Property, plant and equipment, net	$	5		$	5
	Goodwill		2			2
	Intangible assets, net		2			2
	Total noncurrent assets held for sale	$	9		$	9

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	9 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs	'
	Net periodic defined benefit pension and other postretirement benefit costs include the following:
		Three Months Ended June 30
		2014						2013						2014					2013
		Pension Benefits												Postretirement Benefits
		U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
		(Dollars in millions)
	Service cost	$	1		$	2		$	1		$	2		$	—		$	—	$	—		$	—
	Interest cost		2			4			1			4			1			1		—			1
	Expected return on plan assets		(3	)		(5	)		(2	)		(4	)		—			—		—			—
	Amortization of prior service credit		—			—			—			—			(1	)		—		—			—
	Amortization of actuarial loss		—			1			1			1			—			—		—			—
	Net periodic benefit cost	$	—		$	2		$	1		$	3		$	—		$	1	$	—		$	1
		Nine Months Ended June 30
		2014						2013						2014					2013
		Pension Benefits												Postretirement Benefits
		U.S.			Foreign			U.S.			Foreign			U.S.			Foreign		U.S.			Foreign
		(Dollars in millions)
	Service cost	$	2		$	7		$	4		$	7		$	—		$	—	$	—		$	—
	Interest cost		6			12			4			11			2			1		1			1
	Expected return on plan assets		(8	)		(15	)		(6	)		(13	)		—			—		—			—
	Amortization of prior service credit		—			—			—			—			(2	)		—		(2	)		—
	Amortization of actuarial loss		—			3			2			3			—			—		—			—
	Net periodic benefit cost	$	—		$	7		$	4		$	8		$	—		$	1	$	(1	)	$	1

Goodwill_and_Other_Intangible_1

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Jun. 30, 2014
Goodwill And Intangible Assets Disclosure [Abstract]	'
Schedule of Goodwill Balances	'
	The carrying amount of goodwill attributable to each reportable segment with goodwill balances and the changes in those balances during the period ended June 30, 2014 are as follows:
			Reinforcement		Performance			Purification		Total
			Materials		Materials			Solutions
			(Dollars in millions)
	Balance at September 30, 2013		$	25	$	11		$	466	$	502
	Goodwill acquired(1)			51		—			—		51
	Foreign currency impact			—		—			2		2
	Balance at June 30, 2014		$	76	$	11		$	468	$	555
	(1)	Goodwill acquired relates to the NHUMO transaction as described in Note C.
Schedule of Intangible Assets	'
	The following table provides information regarding the Company’s intangible assets:
			June 30, 2014							September 30, 2013
			Gross		Accumulated			Net		Gross		Accumulated			Net
			Carrying		Amortization			Intangible		Carrying		Amortization			Intangible
			Value					Assets		Value					Assets
			(Dollars in millions)
	Intangible assets with finite lives
	Developed technology		$	155	$	(15	)	$	140	$	154	$	(9	)	$	145
	Customer relationships(1)			171		(14	)		157		113		(7	)		106
	Total intangible assets, finite lives		$	326	$	(29	)	$	297	$	267	$	(16	)	$	251
	Trademarks, indefinite lives			57		—			57		57		—			57
	Total intangible assets		$	383	$	(29	)	$	354	$	324	$	(16	)	$	308
	(1)	The change in the gross carrying value of the Customer relationships intangible asset is primarily due to the NHUMO transaction as described in Note C.

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	9 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Schedule of Activity in Noncontrolling Interests	'
	The following table illustrates the noncontrolling interests activity for the periods presented:
		2014			2013
		(Dollars in millions)
	Balance at September 30	$	132		$	126
	Net income attributable to noncontrolling interests		14			3
	Noncontrolling interest foreign currency translation adjustment		(2	)		1
	Contribution from noncontrolling interests		—			13
	Noncontrolling interest dividends		(25	)		(17	)
	Balance at June 30	$	119		$	126

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Changes in Each Component of Accumulated Other Comprehensive Income, Net of Tax	'
	Changes in each component of Accumulated other comprehensive income, net of tax, are as follows:
		Currency				Unrealized				Pension and Other			Total
		Translation				Gains on				Postretirement
		Adjustment				Investments				Benefit Liability
										Adjustments
		(Dollars in millions)
	Balance at September 30, 2013, attributable to Cabot Corporation	$	154			$	2			$	(53	)	$	103
	Other comprehensive loss before reclassifications		(18	)			—				—			(18	)
	Amounts reclassified from accumulated other comprehensive income		—				2				1			3
	Net other comprehensive items		(18	)			2				1			(15	)
	Less: Noncontrolling interest		(2	)			—				—			(2	)
	Balance at June 30, 2014, attributable to Cabot Corporation	$	138			$	4			$	(52	)	$	90
Amounts Reclassified Out of Accumulated Other Comprehensive Income	'
	The amounts reclassified out of Accumulated other comprehensive income and into the Statement of Operations for the nine months ended June 30, 2014 and 2013 are as follows:
				Nine Months Ended June 30
		Affected Line Item in the Consolidated		2014				2013
		Statements of Operations
				(Dollars in millions)
	Pension and other postretirement benefit liability adjustment
	Amortization of actuarial losses	Net Periodic Benefit Cost - see Note E for details		$	3			$	5
	Amortization of prior service cost	Net Periodic Benefit Cost - see Note E for details			(2	)			(2	)
	Total before tax				1				3
	Tax impact	Provision for income taxes			—				(2	)
	Total after tax			$	1			$	1

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	9 Months Ended
	Jun. 30, 2014
Commitments And Contingencies Disclosure [Abstract]	'
Schedule of Components of Purchase Commitments	'
	Purchase Commitments
	Cabot has entered into long-term purchase agreements primarily for the purchase of raw materials. Under certain of these agreements the quantity of material being purchased is fixed, but the price paid changes as market prices change. For those commitments, the amounts included in the table below are based on market prices at June 30, 2014.
		Payments Due by Fiscal Year
		Remainder of		2015		2016		2017		2018		Thereafter		Total
		Fiscal 2014
		(Dollars in millions)
	Reinforcement Materials	$	102	$	318	$	280	$	189	$	181	$	2,628	$	3,698
	Performance Materials		12		39		33		29		30		236		379
	Advanced Technologies		1		2		1		1		1		—		6
	Purification Solutions		12		20		11		10		9		17		79
	Total	$	127	$	379	$	325	$	229	$	221	$	2,881	$	4,162

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	9 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Components of Basic and Diluted Earnings Per Common Share	'
	The following tables summarize the components of the basic and diluted earnings per common share computations:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars and shares in millions,
			except per share amounts)
	Basic EPS:
	Net income attributable to Cabot Corporation		$	52		$	59		$	168		$	106
	Less: Dividends and dividend equivalents to participating securities			—			—			—			—
	Less: Undistributed earnings allocated to participating securities(1)			1			1			2			1
	Earnings allocated to common shareholders (numerator)		$	51		$	58		$	166		$	105
	Weighted average common shares and participating securities outstanding			65.1			64.4			64.9			64.2
	Less: Participating securities(1)			0.6			0.6			0.6			0.6
	Adjusted weighted average common shares (denominator)			64.5			63.8			64.3			63.6
	Amounts per share - basic:
	Income from continuing operations attributable to Cabot Corporation		$	0.8		$	0.88		$	2.61		$	1.66
	(Loss) Income from discontinued operations			(0.01	)		0.04			(0.03	)		(0.01	)
	Net income attributable to Cabot Corporation		$	0.79		$	0.92		$	2.58		$	1.65
	Diluted EPS:
	Earnings allocated to common shareholders		$	51		$	58		$	166		$	105
	Plus: Earnings allocated to participating securities			1			1			2			1
	Less: Adjusted earnings allocated to participating securities(2)			(1	)		(1	)		(2	)		(1	)
	Earnings allocated to common shareholders (numerator)		$	51		$	58		$	166		$	105
	Adjusted weighted average common shares outstanding			64.5			63.8			64.3			63.6
	Effect of dilutive securities:
	Common shares issuable(3)			0.7			0.7			0.7			0.7
	Adjusted weighted average common shares (denominator)			65.2			64.5			65			64.3
	Amounts per share - diluted:
	Income from continuing operations attributable to Cabot Corporation		$	0.79		$	0.87		$	2.58		$	1.64
	(Loss) Income from discontinued operations			(0.01	)		0.03			(0.03	)		(0.01	)
	Net income attributable to Cabot Corporation		$	0.78		$	0.9		$	2.55		$	1.63
	(1)	Participating securities consist of shares of unvested restricted stock and unvested time-based restricted stock units.
Calculation of Undistributed Earnings	'
	The calculation of undistributed earnings is as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Calculation of undistributed earnings:
	Net income attributable to Cabot Corporation		$	52		$	59		$	168		$	106
	Less: Dividends declared on common stock			14			13			40			39
	Less: Dividends declared on participating securities			—			—			—			—
	Undistributed earnings		$	38		$	46		$	128		$	67
	Allocation of undistributed earnings:
	Undistributed earnings allocated to common shareholders		$	37		$	45		$	126		$	66
	Undistributed earnings allocated to participating shareholders			1			1			2			1
	Undistributed earnings		$	38		$	46		$	128		$	67
	(2)	Undistributed earnings are adjusted for the assumed distribution of dividends to the dilutive securities, which are described in (3) below, and then reallocated to participating securities.
	(3)	Represents incremental shares of common stock from the (i) assumed exercise of stock options issued under Cabot’s equity incentive plans; (ii) assumed issuance of shares to employees pursuant to the Company’s Deferred Compensation and Supplemental Retirement Plan; and (iii) assumed issuance of shares under outstanding performance-based restricted stock unit awards issued under Cabot’s equity incentive plans. For the three and nine months ended June 30, 2014, 142,115 and 203,019 incremental shares of common stock, respectively, were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive. For the three and nine months ended June 30, 2013, 308,000 incremental shares of common stock were not included in the calculation of diluted earnings per share because the inclusion of these shares would have been antidilutive.

Restructuring_Tables

Restructuring (Tables)	9 Months Ended
	Jun. 30, 2014
Restructuring And Related Activities [Abstract]	'
Recorded Restructuring Activities	'
	Cabot’s restructuring activities were recorded in the Consolidated Statements of Operations as follows:
		Three Months Ended						Nine Months Ended
		June 30						June 30
		2014			2013			2014			2013
		(Dollars in millions)
	Cost of sales	$	2		$	3		$	11		$	24
	Selling and administrative expenses		2			1			13			4
	Research and Technical expenses		—			1			—			1
	Total	$	4		$	5		$	24		$	29
Restructuring Activities and Related Reserves	'
	Details of these restructuring activities and the related reserves during the three months ended June 30, 2014 are as follows:
		Severance			Environmental			Asset			Other			Total
		and Employee			Remediation			Impairment
		Benefits						and
								Accelerated
								Depreciation
		(Dollars in millions)
	Reserve at March 31, 2014	$	16		$	2		$	—		$	2		$	20
	Charges		2			—			1			1			4
	Costs charged against assets/liabilities		—			—			(1	)		—			(1	)
	Cash paid		(1	)		—			—			(1	)		(2	)
	Reserve at June 30, 2014	$	17		$	2		$	—		$	2		$	21
	Details of these restructuring activities and the related reserves during the nine months ended June 30, 2014 are as follows:
		Severance			Environmental			Asset			Asset			Other			Total
		and Employee			Remediation			Impairment			Sales
		Benefits						and
								Accelerated
								Depreciation
		(Dollars in millions)
	Reserve at September 30, 2013	$	7		$	2		$	—		$	—		$	1		$	10
	Charges		15			1			4			1			3			24
	Costs charged against assets/liabilities		—			—			(4	)		—			—			(4	)
	Cash paid		(5	)		(1	)		—			(1	)		(2	)		(9	)
	Reserve at June 30, 2014	$	17		$	2		$	—		$	—		$	2		$	21

Derivatives_Tables

Derivatives (Tables) (Forward Foreign Currency Contracts [Member])	9 Months Ended
	Jun. 30, 2014
Forward Foreign Currency Contracts [Member]	'
Schedule of Derivatives Foreign Currency	'
	The following table provides details of the derivatives held as of June 30, 2014 and September 30, 2013 to manage foreign currency risk:
				Notional Amount
	Description		Borrowing	June 30, 2014	September 30, 2013	Hedge Designation
	Forward Foreign Currency Contracts (1)		N/A	USD 13 million	USD 31 million	No designation
	-1	Cabot’s forward foreign exchange contracts are denominated primarily in the Brazilian real, British pound sterling, Chinese renminbi, Czech koruna and Indian rupee.

Financial_Information_by_Segme1

Financial Information by Segment (Tables)	9 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Financial Information by Reportable Segment	'
	Financial information by reportable segment is as follows:
			Reinforcement			Performance			Advanced			Purification			Segment		Unallocated			Consolidated
			Materials			Materials			Technologies			Solutions			Total		and Other(1)			Total
			(Dollars in millions)
	Three Months Ended June 30, 2014
	Revenues from external customers(2)		$	534		$	243		$	47		$	78		$	902	$	38		$	940
	Income (loss) from continuing operations before taxes(3)		$	61		$	41		$	14		$	(7	)	$	109	$	(29	)	$	80
	Three Months Ended June 30, 2013
	Revenues from external customers(2)		$	486		$	233		$	67		$	81		$	867	$	34		$	901
	Income (loss) from continuing operations before taxes(3)		$	49		$	35		$	28		$	(1	)	$	111	$	(37	)	$	74
	Nine Months Ended June 30, 2014
	Revenues from external customers(2)		$	1,555		$	709		$	159		$	230		$	2,653	$	83		$	2,736
	Income (loss) from continuing operations before taxes(3)		$	186		$	122		$	51		$	(20	)	$	339	$	(102	)	$	237
	Nine Months Ended June 30, 2013
	Revenues from external customers(2)		$	1,420		$	672		$	143			244		$	2,479	$	81		$	2,560
	Income (loss) from continuing operations before taxes(3)		$	141		$	99		$	44			4		$	288	$	(134	)	$	154
	(1)	Unallocated and Other includes certain items and eliminations necessary to reflect management’s reporting of operating segment results. These items are reflective of the segment reporting presented to the Chief Operating Decision Maker.
	(2)	Revenue from external customers that are categorized as Unallocated and Other reflects royalties, other operating revenues, external shipping and handling costs, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable. Details are provided in the table below:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Royalties, other operating revenues, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable.		$	10		$	9		$	(2	)	$	6
	Shipping and handling fees			28			25			85			75
	Total		$	38		$	34		$	83		$	81
	(3)	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Interest expense		$	(14	)	$	(15	)	$	(41	)	$	(47	)
	Total certain items, pre-tax(a)			(7	)		(4	)		(19	)		(43	)
	Less: Equity in loss (earnings) of affiliated companies, net of tax(b)			2			(3	)		2			(9	)
	Unallocated corporate costs(c)			(14	)		(12	)		(43	)		(37	)
	General unallocated income (expense)(d)			4			(3	)		(1	)		2
	Total		$	(29	)	$	(37	)	$	(102	)	$	(134	)
		(a)	Certain items are items that management does not consider to be representative of operating segment results and they are, therefore, excluded from Segment EBIT. Certain items, pre-tax, for the three months ended June 30, 2014 include $3 million related to global restructuring activities, $3 million of foreign currency loss on revaluations and $1 million related to legal and environmental matters and reserves. Certain items, pre-tax, for the nine months ended June 30, 2014 include $24 million related to global restructuring activities, $5 million for acquisition and integration-related charges (consisting of $3 million for certain other one-time integration costs and $2 million of additional charges related to acquisition accounting adjustments for the acquired inventory of NHUMO), $3 million of foreign currency loss on revaluations and $16 million for legal and environmental matters and reserves offset by a $29 million non-cash gain recognized on the Company’s pre-existing investment in NHUMO as a result of the NHUMO transaction. Certain items, pre-tax, for the three months ended June 30, 2013 include $5 million related to global restructuring activities and $2 million for acquisition and integration-related charges offset by $3 million of foreign currency gain on revaluation. Certain items, pre-tax, for the nine months ended June 30, 2013 include $29 million related to global restructuring activities and $18 million for acquisition and integration-related charges (consisting of $7 million for certain other one-time integration costs and $11 million of charges related to acquisition accounting adjustments for the acquired inventory) offset by $4 million of foreign currency gain on revaluation.
		(b)	Equity in loss (earnings) of affiliated companies, net of tax, is included in Segment EBIT and is removed from Unallocated and other to reconcile to income (loss) from continuing operations before taxes.
		(c)	Unallocated corporate costs are not controlled by the operating segments and primarily benefit corporate interests.
		(d)	General unallocated income (expense) consists of gains (losses) arising from foreign currency transactions, net of other foreign currency risk management activities, the impact of accounting for certain inventory on a LIFO basis, the profit or loss related to the corporate adjustment for unearned revenue, and the impact of including the full operating results of an equity affiliate in Purification Solutions Segment EBIT.
Schedule of Performance Segment	'
	The net sales from each of these businesses for the three and nine months ended June 30, 2014 and 2013 are as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Specialty Carbons and Compounds		$	165		$	159		$	485		$	464
	Fumed Metal Oxides			78			74			224			208
	Total Performance Materials		$	243		$	233		$	709		$	672
Schedule of Advanced Technologies Business Segment	'
	The net sales from each of the Advanced Technologies businesses are as follows:
			Three Months Ended						Nine Months Ended
			June 30						June 30
			2014			2013			2014			2013
			(Dollars in millions)
	Inkjet Colorants		$	17		$	18		$	46		$	46
	Aerogel			2			9			8			17
	Elastomer Composites			4			5			28			17
	Specialty Fluids			24			35			77			63
	Total Advanced Technologies		$	47		$	67		$	159		$	143

Basis_of_Presentation_Addition

Basis of Presentation - Additional Information (Detail) (NHUMO [Member])	Nov. 30, 2013
NHUMO [Member]	'
Basis Of Presentation [Line Items]	'
Percentage acquisition	60.00%
Ownership percentage prior to acquisition	40.00%

Significant_Accounting_Policie3

Significant Accounting Policies - Segment Reporting Revenue Percentage (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reinforcement Materials [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenue, by segment	59.00%	56.00%	58.00%	57.00%
Performance Materials [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenue, by segment	27.00%	27.00%	27.00%	27.00%
Purification Solutions [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenue, by segment	9.00%	9.00%	9.00%	10.00%
Advanced Technologies [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenue, by segment	5.00%	8.00%	6.00%	6.00%

Significant_Accounting_Policie4

Significant Accounting Policies - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Sep. 30, 2013
	Customer
Significant Accounting Policies [Line Items]	'	'
Revenue recognized due	'2022	'
Number of customer having licensing agreements	1	'
Goodwill	$555	$502
Value of inventories under FIFO method	53	55
Purification Solutions [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Goodwill	$468	$466
Approximate percentage of fair value of reporting unit exceeds carrying value	9.00%	'
Successful realization of anticipated market share, years	'3 years	'
Buildings [Member] | Minimum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'20 years	'
Buildings [Member] | Maximum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'25 years	'
Machinery and Equipment [Member] | Minimum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'10 years	'
Machinery and Equipment [Member] | Maximum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'25 years	'
Other Fixed Assets [Member] | Minimum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'3 years	'
Other Fixed Assets [Member] | Maximum [Member]	'	'
Significant Accounting Policies [Line Items]	'	'
Useful life of property, plant and equipment	'25 years	'

Significant_Accounting_Policie5

Significant Accounting Policies - Accumulated Other Comprehensive Items in Accompanying Consolidated Balance Sheets (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]	'	'	'
Foreign currency translation adjustments at beginning of period	$154	$167	$167
Net foreign currency translation adjustments during the period	-16	'	-13
Foreign currency translation adjustments at end of period	138	'	154
Unrealized gain on investments at beginning of period	2	'	'
Net unrealized gains during the period	2	'	2
Unrealized gain on investments at end of period	4	'	2
Pension and other postretirement benefit plans at beginning of period	-53	-75	-75
Net change in pension and other postretirement benefit plans during the period	1	1	22
Pension and other postretirement benefit plans at end of period	-52	'	-53
Total accumulated other comprehensive income	$90	'	$103

Acquisition_of_NHUMO_Additiona

Acquisition of NHUMO - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Nov. 30, 2013	Jun. 30, 2014
Business Acquisition [Line Items]	'	'
Preferred stock redemption amount	'	$29
Acquisition costs	'	1
Gain on acquisition	'	29
Customer Relationships [Member]	'	'
Business Acquisition [Line Items]	'	'
Intangible asset	'	57
Amortized period	'	'17 years
NHUMO [Member]	'	'
Business Acquisition [Line Items]	'	'
Percentage acquisition	60.00%	'
Ownership percentage prior to acquisition	40.00%	'
Cash paid on acquisition	80	80
Preferred stock redemption amount	25	28
Dividend percentage of preferred stock	6.00%	'
Preferred stock issued on acquisition	25	'
Dividend received from NHUMO	14	'
Gain on acquisition	'	29
Previously held equity interest in NHUMO	'	$54

Acquisition_of_NHUMO_Allocatio

Acquisition of NHUMO - Allocation of Purchase Price (Detail) (USD $)	Jun. 30, 2014	Sep. 30, 2013	Nov. 30, 2013	Jun. 30, 2014
In Millions, unless otherwise specified			NHUMO [Member]	NHUMO [Member]
Assets	'	'	'	'
Cash	'	'	'	$7
Accounts receivable	'	'	'	33
Inventories	'	'	'	14
Property, plant and equipment	'	'	'	48
Other non-current assets	'	'	'	1
Intangible assets	'	'	'	57
Goodwill	555	502	'	51
Total assets acquired	'	'	'	211
Liabilities	'	'	'	'
Current liabilities	'	'	'	-18
Deferred tax liabilities	'	'	'	-31
Total liabilities assumed	'	'	'	-49
Net assets acquired	'	'	'	162
Cash consideration paid	'	'	80	80
Fair value of redeemable preferred stock	29	'	25	28
Previously held equity interest in NHUMO	'	'	'	54
Total	'	'	'	$162

Discontinued_Operations_and_No2

Discontinued Operations and Notes Receivable from Sale of Business - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended			3 Months Ended
In Millions, unless otherwise specified	Jan. 31, 2012	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014	Jun. 30, 2014
					GAM Notes [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'	'
Total consideration from sale of business	$452	'	'	'	'
Cash payment, transactions closes	175	'	'	'	'
Notes receivable received on sale of discontinued operations	277	'	'	'	'
GAM promissory notes and inventory note, final maturity date	'2014-03	'	'	'	'
Cabot received final payment on GAM Notes	'	215	39	'	215
Approximate cash on sale of Security Materials business to SICPA	'	'	'	$20	'

Discontinued_Operations_and_No3

Discontinued Operations and Notes Receivable from Sale of Business - Components of Discontinued Operations (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Discontinued Operations And Disposal Groups [Abstract]	'	'	'	'
Net sales and other operating revenues	$2	$1	$5	$4
Loss from operations before income taxes	-1	-1	-3	-6
Benefit from income taxes	'	2	1	4
(Loss) Income from discontinued operations, net of tax	($1)	$1	($2)	($2)

Discontinued_Operations_and_No4

Discontinued Operations and Notes Receivable from Sale of Business - Summary of Assets Held for Sale (Detail) (USD $)	Jun. 30, 2014	Sep. 30, 2013
In Millions, unless otherwise specified
Long Lived Assets Held-for-sale [Line Items]	'	'
Current assets held for sale	$4	$4
Noncurrent assets held for sale	9	9
Inventories [Member]	'	'
Long Lived Assets Held-for-sale [Line Items]	'	'
Current assets held for sale	3	3
Other Current Assets [Member]	'	'
Long Lived Assets Held-for-sale [Line Items]	'	'
Current assets held for sale	1	1
Property, Plant and Equipment, Net [Member]	'	'
Long Lived Assets Held-for-sale [Line Items]	'	'
Noncurrent assets held for sale	5	5
Goodwill [Member]	'	'
Long Lived Assets Held-for-sale [Line Items]	'	'
Noncurrent assets held for sale	2	2
Intangible Assets, Net [Member]	'	'
Long Lived Assets Held-for-sale [Line Items]	'	'
Noncurrent assets held for sale	$2	$2

Employee_Benefit_Plans_Net_Per

Employee Benefit Plans - Net Periodic Defined Benefit Pension and Other Postretirement Benefit Costs (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
U.S. Pension Benefits [Member]	'	'	'	'
Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	$1	$1	$2	$4
Interest cost	2	1	6	4
Expected return on plan assets	-3	-2	-8	-6
Amortization of prior service credit	'	'	'	'
Amortization of actuarial loss	'	1	'	2
Net periodic benefit cost	'	1	'	4
Foreign Pension Benefits [Member]	'	'	'	'
Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	2	2	7	7
Interest cost	4	4	12	11
Expected return on plan assets	-5	-4	-15	-13
Amortization of prior service credit	'	'	'	'
Amortization of actuarial loss	1	1	3	3
Net periodic benefit cost	2	3	7	8
U. S. Postretirement Benefits [Member]	'	'	'	'
Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	'	'	'	'
Interest cost	1	'	2	1
Expected return on plan assets	'	'	'	'
Amortization of prior service credit	-1	'	-2	-2
Amortization of actuarial loss	'	'	'	'
Net periodic benefit cost	'	'	'	-1
Foreign Postretirement Benefits [Member]	'	'	'	'
Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	0	0	'	'
Interest cost	1	1	1	1
Expected return on plan assets	0	0	'	'
Amortization of prior service credit	0	0	'	'
Amortization of actuarial loss	0	0	'	'
Net periodic benefit cost	$1	$1	$1	$1

Goodwill_and_Other_Intangible_2

Goodwill and Other Intangible Assets - Additional Information (Detail) (USD $)	0 Months Ended			3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	31-May-14	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Sep. 30, 2013
				Cost of Sales and Selling and Administrative Expenses [Member]	Cost of Sales and Selling and Administrative Expenses [Member]	Cost of Sales and Selling and Administrative Expenses [Member]	Cost of Sales and Selling and Administrative Expenses [Member]	Minimum [Member]	Maximum [Member]	Weighted Average [Member]	Advanced Technologies [Member]
Goodwill And Intangible Assets [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Goodwill	'	$555	$502	'	'	'	'	'	'	'	$2
Goodwill annual impairment	0	'	'	'	'	'	'	'	'	'	'
Net intangible assets	'	'	2	'	'	'	'	'	'	'	'
Useful life of intangible assets	'	'	'	'	'	'	'	'16 years	'20 years	'19 years	'
Amortization of intangible assets	'	'	'	4	4	13	10	'	'	'	'
Amortization expense estimated for year one	'	17	'	'	'	'	'	'	'	'	'
Amortization expense estimated for year two	'	17	'	'	'	'	'	'	'	'	'
Amortization expense estimated for year three	'	17	'	'	'	'	'	'	'	'	'
Amortization expense estimated for year four	'	17	'	'	'	'	'	'	'	'	'
Amortization expense estimated for year five	'	17	'	'	'	'	'	'	'	'	'
Asset impairment charges	$0	'	'	'	'	'	'	'	'	'	'

Goodwill_and_Other_Intangible_3

Goodwill and Other Intangible Assets - Schedule of Goodwill Balances (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014
Goodwill And Other Intangible Asset [Line Items]	'
Beginning balance	$502
Goodwill acquired	51
Foreign currency impact	2
Ending balance	555
Reinforcement Materials [Member]	'
Goodwill And Other Intangible Asset [Line Items]	'
Beginning balance	25
Goodwill acquired	51
Foreign currency impact	'
Ending balance	76
Performance Materials [Member]	'
Goodwill And Other Intangible Asset [Line Items]	'
Beginning balance	11
Goodwill acquired	'
Foreign currency impact	'
Ending balance	11
Purification Solutions [Member]	'
Goodwill And Other Intangible Asset [Line Items]	'
Beginning balance	466
Goodwill acquired	'
Foreign currency impact	2
Ending balance	$468

Goodwill_and_Other_Intangible_4

Goodwill and Other Intangible Assets - Schedule of Intangible Assets (Detail) (USD $)	Jun. 30, 2014	Sep. 30, 2013
In Millions, unless otherwise specified
Finite-Lived Intangible Assets [Line Items]	'	'
Gross Carrying Value, finite lives	$326	$267
Gross Carrying Value	383	324
Accumulated Amortization	-29	-16
Net Intangible Assets, finite lives	297	251
Net Intangible Assets	354	308
Trademarks, Indefinite Lives [Member]	'	'
Finite-Lived Intangible Assets [Line Items]	'	'
Net Intangible Assets, Trademarks, indefinite lives	57	57
Developed Technology [Member]	'	'
Finite-Lived Intangible Assets [Line Items]	'	'
Gross Carrying Value, finite lives	155	154
Accumulated Amortization	-15	-9
Net Intangible Assets, finite lives	140	145
Customer Relationships [Member]	'	'
Finite-Lived Intangible Assets [Line Items]	'	'
Gross Carrying Value, finite lives	171	113
Accumulated Amortization	-14	-7
Net Intangible Assets, finite lives	$157	$106

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, except Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2007
Equity [Abstract]	'	'	'
Repurchase authorization, shares	379	379	10,000,000
Shares remaining available for repurchase under the current authorization	1,600,000	'	'
Cash dividends paid to common stockholders	$40	$39	'

Stockholders_Equity_Schedule_o

Stockholders' Equity - Schedule of Activity in Noncontrolling Interests (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Equity [Abstract]	'	'	'	'
Balance at September 30	'	'	$132	$126
Net income attributable to noncontrolling interests	5	3	14	3
Noncontrolling interest foreign currency translation adjustment	'	2	-2	1
Contribution from noncontrolling interests	'	'	'	13
Noncontrolling interest dividends	'	'	-25	-17
Balance at June 30	$119	$126	$119	$126

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income, Net of Tax (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'
Balance at September 30, 2013, attributable to Cabot Corporation	'	'	$103	'
Other comprehensive loss before reclassifications	'	'	-18	'
Amounts reclassified from accumulated other comprehensive income	'	'	3	'
Other comprehensive income (loss)	13	-2	-15	-21
Less: Noncontrolling interest	'	'	-2	'
Balance at June 30, 2014, attributable to Cabot Corporation	90	'	90	'
Currency Translation Adjustment [Member]	'	'	'	'
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'
Balance at September 30, 2013, attributable to Cabot Corporation	'	'	154	'
Other comprehensive loss before reclassifications	'	'	-18	'
Amounts reclassified from accumulated other comprehensive income	'	'	'	'
Other comprehensive income (loss)	'	'	-18	'
Less: Noncontrolling interest	'	'	-2	'
Balance at June 30, 2014, attributable to Cabot Corporation	138	'	138	'
Unrealized Gains on Investments [Member]	'	'	'	'
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'
Balance at September 30, 2013, attributable to Cabot Corporation	'	'	2	'
Other comprehensive loss before reclassifications	'	'	'	'
Amounts reclassified from accumulated other comprehensive income	'	'	2	'
Other comprehensive income (loss)	'	'	2	'
Less: Noncontrolling interest	'	'	'	'
Balance at June 30, 2014, attributable to Cabot Corporation	4	'	4	'
Pension and Other Postretirement Benefit Liability Adjustments [Member]	'	'	'	'
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'
Balance at September 30, 2013, attributable to Cabot Corporation	'	'	-53	'
Other comprehensive loss before reclassifications	'	'	'	'
Amounts reclassified from accumulated other comprehensive income	'	'	1	'
Other comprehensive income (loss)	'	'	1	'
Less: Noncontrolling interest	'	'	'	'
Balance at June 30, 2014, attributable to Cabot Corporation	($52)	'	($52)	'

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income - Amounts Reclassified Out of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Total before tax	$80	[1]	$74	[1]	$237	[1]	$154	[1]
Tax impact	-20		-16		-51		-52
Income from continuing operations	58		61		184		111
Reclassification out of Accumulated Other Comprehensive Income [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Total before tax	'		'		1		3
Tax impact	'		'		'		-2
Income from continuing operations	'		'		1		1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension and Other Postretirement Benefit Liability Adjustment [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Amortization of actuarial losses	'		'		3		5
Amortization of prior service cost	'		'		($2)		($2)
[1]	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:

Commitments_and_Contingencies_1

Commitments and Contingencies - Schedule of Components of Purchase Commitments (Detail) (USD $)	Jun. 30, 2014
In Millions, unless otherwise specified
Long-term Purchase Commitment [Line Items]	'
Payments Due by Remainder of Fiscal Year, 2014	$127
Payments Due by Fiscal Year, 2015	379
Payments Due by Fiscal Year, 2016	325
Payments Due by Fiscal Year, 2017	229
Payments Due by Fiscal Year, 2018	221
Payments Due, Thereafter	2,881
Payments Due, Total	4,162
Reinforcement Materials [Member]	'
Long-term Purchase Commitment [Line Items]	'
Payments Due by Remainder of Fiscal Year, 2014	102
Payments Due by Fiscal Year, 2015	318
Payments Due by Fiscal Year, 2016	280
Payments Due by Fiscal Year, 2017	189
Payments Due by Fiscal Year, 2018	181
Payments Due, Thereafter	2,628
Payments Due, Total	3,698
Performance Materials [Member]	'
Long-term Purchase Commitment [Line Items]	'
Payments Due by Remainder of Fiscal Year, 2014	12
Payments Due by Fiscal Year, 2015	39
Payments Due by Fiscal Year, 2016	33
Payments Due by Fiscal Year, 2017	29
Payments Due by Fiscal Year, 2018	30
Payments Due, Thereafter	236
Payments Due, Total	379
Advanced Technologies [Member]	'
Long-term Purchase Commitment [Line Items]	'
Payments Due by Remainder of Fiscal Year, 2014	1
Payments Due by Fiscal Year, 2015	2
Payments Due by Fiscal Year, 2016	1
Payments Due by Fiscal Year, 2017	1
Payments Due by Fiscal Year, 2018	1
Payments Due, Thereafter	'
Payments Due, Total	6
Purification Solutions [Member]	'
Long-term Purchase Commitment [Line Items]	'
Payments Due by Remainder of Fiscal Year, 2014	12
Payments Due by Fiscal Year, 2015	20
Payments Due by Fiscal Year, 2016	11
Payments Due by Fiscal Year, 2017	10
Payments Due by Fiscal Year, 2018	9
Payments Due, Thereafter	17
Payments Due, Total	$79

Commitments_and_Contingencies_2

Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2013
Respirator Liabilities [Member]	'	'	'	'
Loss Contingencies [Line Items]	'	'	'	'
Number of claimants	'	41,000	'	42,000
Respirator reserve on discounted basis	'	$9	'	$11
Respirator reserve on undiscounted basis	'	13	'	15
Cash payments for respirator reserves	'	2	2	'
Environmental Matters [Member]	'	'	'	'
Loss Contingencies [Line Items]	'	'	'	'
Reserved for environmental matters, on an undiscounted basis	'	17	'	5
Charge related to environmental matters	13	'	'	'
Cash payments for environmental reserves	'	$2	$2	'

Income_Tax_Uncertainties_Addit

Income Tax Uncertainties - Additional Information (Detail) (USD $)	0 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014
			Internal Revenue Service (IRS) [Member]	State Tax Authorities [Member]	Non-U.S. Jurisdictions [Member]	Minimum [Member]	Minimum [Member]	Minimum [Member]	Maximum [Member]	Maximum [Member]	Maximum [Member]
						Internal Revenue Service (IRS) [Member]	State Tax Authorities [Member]	Non-U.S. Jurisdictions [Member]	Internal Revenue Service (IRS) [Member]	State Tax Authorities [Member]	Non-U.S. Jurisdictions [Member]
Income Tax [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Tax years remain subject to examination, description	'	'	'2007 through 2013 tax years remain subject to examination by the IRS	'2005 through 2013 remain subject to examination by the respective state tax authorities	'2004 through 2013 remain subject to examination by their respective tax authorities	'	'	'	'	'	'
Tax years remain subject to examination	'	'	'	'	'	'2007	'2005	'2004	'2013	'2013	'2013
Unrecognized tax benefits	'	$6	'	'	'	'	'	'	'	'	'
Prepaid taxes recoverable from Canadian government	'	16	'	'	'	'	'	'	'	'	'
Amount received from Canadian government	$1	'	'	'	'	'	'	'	'	'	'

Earnings_Per_Share_Components_

Earnings Per Share - Components of Basic and Diluted Earnings Per Common Share (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Basic EPS:	'	'	'	'
Net income attributable to Cabot Corporation	$52	$59	$168	$106
Less: Dividends and dividend equivalents to participating securities	'	'	'	'
Less: Undistributed earnings allocated to participating securities	1	1	2	1
Earnings allocated to common shareholders (numerator)	51	58	166	105
Weighted average common shares and participating securities outstanding	65.1	64.4	64.9	64.2
Less: Participating securities	0.6	0.6	0.6	0.6
Adjusted weighted average common shares (denominator)	64.5	63.8	64.3	63.6
Income from continuing operations attributable to Cabot Corporation	$0.80	$0.88	$2.61	$1.66
(Loss) income from discontinued operations	($0.01)	$0.04	($0.03)	($0.01)
Net income attributable to Cabot Corporation	$0.79	$0.92	$2.58	$1.65
Diluted EPS:	'	'	'	'
Earnings allocated to common shareholders	51	58	166	105
Plus: Earnings allocated to participating securities	1	1	2	1
Less: Adjusted earnings allocated to participating securities	-1	-1	-2	-1
Earnings allocated to common shareholders (numerator)	$51	$58	$166	$105
Adjusted weighted average common shares outstanding	64.5	63.8	64.3	63.6
Common shares issuable	0.7	0.7	0.7	0.7
Adjusted weighted average common shares (denominator)	65.2	64.5	65	64.3
Income from continuing operations attributable to Cabot Corporation	$0.79	$0.87	$2.58	$1.64
(Loss) income from discontinued operations	($0.01)	$0.03	($0.03)	($0.01)
Net income attributable to Cabot Corporation	$0.78	$0.90	$2.55	$1.63

Earnings_Per_Share_Calculation

Earnings Per Share - Calculation of Undistributed Earnings (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]	'	'	'	'
Net income attributable to Cabot Corporation	$52	$59	$168	$106
Less: Dividends declared on common stock	14	13	40	39
Less: Dividends declared on participating securities	'	'	'	'
Undistributed earnings	38	46	128	67
Undistributed earnings allocated to common shareholders	37	45	126	66
Undistributed earnings allocated to participating shareholders	$1	$1	$2	$1

Earnings_Per_Share_Calculation1

Earnings Per Share - Calculation of Undistributed Earnings (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]	'	'	'	'
Incremental shares of common stock	142,115	308,000	203,019	308,000

Restructuring_Recorded_Restruc

Restructuring - Recorded Restructuring Activities (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Restructuring reserve, period expense	$4	$5	$24	$29
Cost of Sales [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Restructuring reserve, period expense	2	3	11	24
Selling and Administrative Expenses [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Restructuring reserve, period expense	2	1	13	4
Research and Technical Expenses [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Restructuring reserve, period expense	'	$1	'	$1

Restructuring_Restructuring_Ac

Restructuring - Restructuring Activities and Related Reserves (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	$20	'	$10	'
Charges	4	5	24	29
Costs charged against assets/liabilities	-1	'	-4	'
Cash paid	-2	'	-9	'
Reserve balance	21	'	21	'
Severance and Employee Benefits [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	16	'	7	'
Charges	2	'	15	'
Costs charged against assets/liabilities	0	'	0	'
Cash paid	-1	'	-5	'
Reserve balance	17	'	17	'
Environmental Remediation [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	2	'	2	'
Charges	0	'	1	'
Costs charged against assets/liabilities	0	'	0	'
Cash paid	0	'	-1	'
Reserve balance	2	'	2	'
Asset Sales [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	'	'	0	'
Charges	'	'	1	'
Costs charged against assets/liabilities	'	'	0	'
Cash paid	'	'	-1	'
Reserve balance	0	'	0	'
Other [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	2	'	1	'
Charges	1	'	3	'
Costs charged against assets/liabilities	0	'	0	'
Cash paid	-1	'	-2	'
Reserve balance	2	'	2	'
Asset Impairment and Accelerated Depreciation [Member]	'	'	'	'
Restructuring Cost and Reserve [Line Items]	'	'	'	'
Reserve balance	0	'	0	'
Charges	1	'	4	'
Costs charged against assets/liabilities	-1	'	-4	'
Cash paid	0	'	0	'
Reserve balance	$0	'	$0	'

Restructuring_Additional_Infor

Restructuring - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

0 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Mar. 31, 2014

Sep. 30, 2013

Jun. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Sep. 30, 2013

Jun. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Sep. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Sep. 30, 2009

Jun. 30, 2014

Jun. 30, 2013

Apr. 26, 2013

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2013

Sep. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Sep. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Environmental Remediation [Member]

Environmental Remediation [Member]

Environmental Remediation [Member]

Environmental Remediation [Member]

Severance and Employee Benefits [Member]

Severance and Employee Benefits [Member]

Severance and Employee Benefits [Member]

Severance and Employee Benefits [Member]

Previous Actions and Sites Pending Sale [Member]

Previous Actions and Sites Pending Sale [Member]

Previous Actions and Sites Pending Sale [Member]

Previous Actions and Sites Pending Sale [Member]

Previous Actions and Sites Pending Sale [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Closure of Port Dickson [Member]

Restructuring Activities Other [Member]

Restructuring Activities Other [Member]

Restructuring Activities Other [Member]

Restructuring Activities Other [Member]

Restructuring Activities Other [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Shared Service Center Transition [Member]

Maximum [Member]

Maximum [Member]

Employees

Environmental Remediation [Member]

Severance and Employee Benefits [Member]

Other Closure Charges [Member]

Severance Costs [Member]

Maximum [Member]

Maximum [Member]

Scenario, Forecast [Member]

Severance Costs [Member]

Severance Costs [Member]

Severance Costs [Member]

Severance Costs [Member]

Minimum [Member]

Maximum [Member]

Remainder of Fiscal 2015 [Member]

Remainder of Fiscal 2015 [Member]

Minimum [Member]

Maximum [Member]

Restructuring Cost and Reserve [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Pre-tax charge to earnings

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$163

'

'

'

'

'

$2

$18

'

'

'

'

$1

$2

$23

'

'

'

'

'

$5

$1

$15

$22

$26

$17

$21

Cash payments

2

'

9

'

'

'

0

1

'

'

1

5

'

'

'

'

85

'

'

'

'

'

'

'

'

3

'

'

'

'

'

'

16

'

'

'

1

'

'

'

'

Restructuring costs in accrued expenses

21

'

21

'

20

10

2

2

2

2

17

17

16

7

3

'

'

'

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

14

14

'

'

'

'

Equity share in CMSB

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

51.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Affected employees

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

90

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Restructuring charges

4

5

24

29

'

'

0

1

'

'

2

15

'

'

1

2

'

1

1

'

20

'

'

'

'

'

'

'

'

'

2

2

6

7

'

'

'

'

'

'

'

Accelerated depreciation and asset write-offs

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Severance charges

'

'

'

'

'

'

'

'

'

'

'

'

'

'

67

'

'

'

'

'

2

'

'

'

'

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

Portion of the charges that are allocable to the noncontrolling interest

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

49.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Site demolition, clearing and environmental remediation costs

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10

'

'

'

'

'

2

'

'

'

'

'

'

'

5

'

'

'

'

'

'

'

'

'

'

'

'

Other charges

'

'

'

'

'

'

'

'

'

'

'

'

'

'

22

'

'

'

'

'

1

'

'

'

'

'

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

Restructuring accelerated depreciation and asset impairments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

65

'

'

'

'

'

'

'

'

'

'

'

'

'

15

'

'

'

'

'

'

'

'

'

'

'

'

Expected cumulative net cash outlays related to plan

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

8

'

6

1

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected cash outlays remainder fiscal year

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5

'

'

'

'

'

'

'

'

'

3

'

3

'

'

'

'

'

'

'

'

Additional charges during fiscal 2014 and thereafter

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

Accrued severance charges

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3

'

3

'

'

'

'

'

'

'

'

Gain on sale of asset

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Charges during fiscal 2014 and thereafter

'

'

$2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Financial_Instruments_and_Fair1

Financial Instruments and Fair Value Measurements - Additional Information (Detail) (USD $)	9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Sep. 30, 2013
			Fixed Rate Debt [Member]	Fixed Rate Debt [Member]	Significant Observable Inputs (Level 2) [Member]	Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'	'	'
Fair value, liabilities, Level 2 to Level 1 transfers, amount	$0	$0	'	'	'	'
Fair value, assets, Level 2 to Level 1 transfers, amount	0	0	'	'	'	'
Fair value, liabilities, Level 1 to Level 2 transfers, amount	0	0	'	'	'	'
Fair value, assets, Level 1 to Level 2 transfers, amount	0	0	'	'	'	'
Fair value, assets, transfers into Level 3, amount	0	0	'	'	'	'
Fair value, assets, transfers out of Level 3, amount	0	0	'	'	'	'
Fair value, liabilities, transfers into Level 3, amount	0	0	'	'	'	'
Fair value, liabilities, transfers out of Level 3, amount	0	0	'	'	'	'
Other assets in the Consolidated Balance Sheets	'	'	'	'	14,000,000	14,000,000
Carrying value of long-term debt	'	'	958,000,000	971,000,000	'	'
Fair value of long-term debt	'	'	$1,010,000,000	$1,010,000,000	'	'

Derivatives_Additional_Informa

Derivatives - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2014	Sep. 30, 2013
				Forward Foreign Currency Contracts [Member]	Forward Foreign Currency Contracts [Member]	Forward Foreign Currency Contracts [Member]	Forward Foreign Currency Contracts [Member]	Currency Swaps [Member]	Maximum [Member]	Fair Value Hedging [Member]	Fair Value Hedging [Member]
									Currency Swaps [Member]	Interest Rate Swap-Fixed to Variable [Member]	Interest Rate Swap-Fixed to Variable [Member]
Derivative [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Derivatives held to manage interest rate risk	'	'	'	'	'	'	'	'	'	$0	$0
Derivatives designated as hedges	'	'	0	'	'	'	'	'	'	'	'
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	'	'	'	-1	-2	-5	7	-2	-1	'	'
Gain (loss) on foreign currency derivatives recorded in earnings	3	1	'	'	'	'	'	'	'	'	'
Bond held by one of Cabot's European subsidiaries	$175	$175	'	'	'	'	'	'	'	'	'

Derivatives_Schedule_of_Deriva

Derivatives - Schedule of Derivatives Foreign Currency (Detail) (Not Designated as Hedging Instrument [Member], Forward Foreign Currency Contracts [Member], USD $)	Jun. 30, 2014		Sep. 30, 2013
In Millions, unless otherwise specified
Not Designated as Hedging Instrument [Member] | Forward Foreign Currency Contracts [Member]	'		'
Derivative [Line Items]	'		'
Notional amount of forward foreign currency contract no designation hedge derivatives	$13	[1]	$31	[1]
[1]	Cabot's forward foreign exchange contracts are denominated primarily in the Brazilian real, British pound sterling, , Chinese renminbi, Czech koruna and Indian rupee.

Venezuela_Additional_Informati

Venezuela - Additional Information (Detail)

9 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Mar. 31, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

USD ($)

USD ($)

VEF

VEF

Maximum [Member]

Operating Affiliate, Venezuela [Member]

SICAD 2 Currency Exchange Mechanism [Member]

SICAD 2 Currency Exchange Mechanism [Member]

Subsidiaries [Member]

Subsidiaries [Member]

Operating Affiliate [Member]

Operating Affiliate [Member]

Operating Affiliate [Member]

USD ($)

VEF

VEF

USD ($)

USD ($)

USD ($)

USD ($)

Operating Affiliate, Venezuela [Member]

USD ($)

Nature Of Operations [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Equity method investment, ownership percentage

'

'

'

'

'

49.00%

'

'

'

'

'

'

'

Operating affiliate investment

$17

'

'

'

'

'

'

'

'

'

'

'

'

Due from affiliates, dividends cash

'

'

20

'

1

'

'

'

'

'

'

'

'

Cash dividends received from subsidiary

'

'

'

'

'

'

'

'

4

2

'

'

'

Conversion of bolivars to per USD

'

'

'

6.3

'

'

50

50.8

'

'

'

'

'

Negative impact of exchange rate devaluation

4

'

'

'

'

'

'

'

'

'

'

'

8

Currency denomination, remeasurement loss

'

'

'

'

'

'

'

'

'

'

2

'

'

Tax benefit due to reduction of deferred tax liability

($16)

$7

'

'

'

'

'

'

'

'

'

$2

'

Financial_Information_by_Segme2

Financial Information by Segment - Additional Information (Detail)	9 Months Ended
	Jun. 30, 2014
	Segment
Segment Reporting Information [Line Items]	'
Number of reportable segment	4
Number of operating segments	4
Performance Materials [Member]	'
Segment Reporting Information [Line Items]	'
Number of business activity	2

Financial_Information_by_Segme3

Financial Information by Segment - Financial Information by Reportable Segment (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	$940	[1]	$901	[1]	$2,736	[1]	$2,560	[1]
Income (loss) from continuing operations before taxes	80	[2]	74	[2]	237	[2]	154	[2]
Operating Segments [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	902	[1]	867	[1]	2,653	[1]	2,479	[1]
Income (loss) from continuing operations before taxes	109	[2]	111	[2]	339	[2]	288	[2]
Unallocated and Other [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	38	[1]	34	[1]	83	[1]	81	[1]
Income (loss) from continuing operations before taxes	-29	[2]	-37	[2]	-102	[2]	-134	[2]
Reinforcement Materials [Member] | Operating Segments [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	534	[1]	486	[1]	1,555	[1]	1,420	[1]
Income (loss) from continuing operations before taxes	61	[2]	49	[2]	186	[2]	141	[2]
Performance Materials [Member] | Operating Segments [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	243	[1]	233	[1]	709	[1]	672	[1]
Income (loss) from continuing operations before taxes	41	[2]	35	[2]	122	[2]	99	[2]
Advanced Technologies [Member] | Operating Segments [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	47	[1]	67	[1]	159	[1]	143	[1]
Income (loss) from continuing operations before taxes	14	[2]	28	[2]	51	[2]	44	[2]
Purification Solutions [Member] | Operating Segments [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Revenues from external customers	78	[1]	81	[1]	230	[1]	244	[1]
Income (loss) from continuing operations before taxes	($7)	[2]	($1)	[2]	($20)	[2]	$4	[2]
[1]	Revenue from external customers that are categorized as Unallocated and Other reflects royalties, other operating revenues, external shipping and handling costs, the impact of unearned revenue, the removal of 100% of the sales of an equity method affiliate and discounting charges for certain Notes receivable. Details are provided in the table below:
[2]	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:

Financial_Information_by_Segme4

Financial Information by Segment - Financial Information by Reportable Segment (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting [Abstract]	'	'	'	'
Percentage of sale of an equity method affiliate	100.00%	100.00%	100.00%	100.00%

Financial_Information_by_Segme5

Financial Information by Segment - Royalties Paid by Equity Affiliates, External Shipping and Handling Fees, and the Impact of Corporate Adjustment for Unearned Revenue and Notes Receivable Reflected by Unallocated and Other (Detail) (Unallocated and Other [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$38	$34	$83	$81
Royalties, Other Operating Revenues, the Impact of Unearned Revenue, the Removal of 100% of the Sales of an Equity Method Affiliate and Discounting Charges for Certain Notes Receivable [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	10	9	-2	6
Shipping and Handling Costs [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$28	$25	$85	$75

Financial_Information_by_Segme6

Financial Information by Segment - Royalties Paid by Equity Affiliates, External Shipping and Handling Fees, and the Impact of Corporate Adjustment for Unearned Revenue and Notes Receivable Reflected by Unallocated and Other (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting [Abstract]	'	'	'	'
Percentage of sale of an equity method affiliate	100.00%	100.00%	100.00%	100.00%

Financial_Information_by_Segme7

Financial Information by Segment - Schedule of Income (Loss) from Continuing Operations before Taxes for Unallocated and Other (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]	'		'		'		'
Interest expense	($14)		($15)		($41)		($47)
Less: Equity in loss (earnings) of affiliated companies, net of tax	-2		3		-2		9
Income from continuing operations before income taxes and equity in (loss) earnings of affiliated companies	80	[1]	74	[1]	237	[1]	154	[1]
Unallocated and Other [Member]	'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'
Interest expense	-14		-15		-41		-47
Total certain items, pre-tax	-7	[2]	-4	[2]	-19	[2]	-43	[2]
Less: Equity in loss (earnings) of affiliated companies, net of tax	2	[3]	-3	[3]	2	[3]	-9	[3]
Unallocated corporate costs	-14	[4]	-12	[4]	-43	[4]	-37	[4]
General unallocated income (expense)	4	[5]	-3	[5]	-1	[5]	2	[5]
Income from continuing operations before income taxes and equity in (loss) earnings of affiliated companies	($29)	[1]	($37)	[1]	($102)	[1]	($134)	[1]
[1]	Income (loss) from continuing operations before taxes that are categorized as Unallocated and Other includes:
[2]	Certain items are items that management does not consider to be representative of operating segment results and they are, therefore, excluded from Segment EBIT. Certain items, pre-tax, for the three months ended June 30, 2014 include $3 million related to global restructuring activities, $3 million of foreign currency loss on revaluations and $1 million related to legal and environmental matters and reserves. Certain items, pre-tax, for the nine months ended June 30, 2014 include $24 million related to global restructuring activities, $5 million for acquisition and integration-related charges (consisting of $3 million for certain other one-time integration costs and $2 million of additional charges related to acquisition accounting adjustments for the acquired inventory of NHUMO), $3 million of foreign currency loss on revaluations and $16 million for legal and environmental matters and reserves offset by a $29 million non-cash gain recognized on the Company's pre-existing investment in NHUMO as a result of the NHUMO transaction. Certain items, pre-tax, for the three months ended June 30, 2013 include $5 million related to global restructuring activities and $2 million for acquisition and integration-related charges offset by $3 million of foreign currency gain on revaluation. Certain items, pre-tax, for the nine months ended June 30, 2013 include $29 million related to global restructuring activities and $18 million for acquisition and integration-related charges (consisting of $7 million for certain other one-time integration costs and $11 million of charges related to acquisition accounting adjustments for the acquired inventory) offset by $4 million of foreign currency gain on revaluation.
[3]	Equity in loss (earnings) of affiliated companies, net of tax, is included in Segment EBIT and is removed from Unallocated and other to reconcile to income (loss) from continuing operations before taxes.
[4]	Unallocated corporate costs are not controlled by the operating segments and primarily benefit corporate interests.
[5]	General unallocated income (expense) consists of gains (losses) arising from foreign currency transactions, net of other foreign currency risk management activities, the impact of accounting for certain inventory on a LIFO basis, the profit or loss related to the corporate adjustment for unearned revenue, and the impact of including the full operating results of an equity affiliate in Purification Solutions Segment EBIT.

Financial_Information_by_Segme8

Financial Information by Segment - Schedule of Income (Loss) from Continuing Operations before Taxes for Unallocated and Other (Parenthetical) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Restructuring charges	$4	$5	$24	$29
Acquisition costs	'	'	1	'
Certain Item [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Restructuring charges	3	5	24	29
Acquisition costs	'	2	5	18
One-time integration costs	'	'	3	7
Acquisition of accounting adjustments for acquired inventory	'	'	2	11
Foreign currency exchange rate, re measurement income (loss)	-3	3	-3	4
Reserve for legal and environmental matters	1	'	16	'
Non-cash gain on acquisition	$29	'	$29	'

Financial_Information_by_Segme9

Financial Information by Segment - Schedule of Performance Segment (Detail) (Performance Materials [Member], Operating Segments [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$243	$233	$709	$672
Specialty Carbons and Compounds [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	165	159	485	464
Fumed Metal Oxides [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$78	$74	$224	$208

Recovered_Sheet1

Financial Information by Segment - Schedule of Advanced Technologies Business Segment (Detail) (Operating Segments [Member], Advanced Technologies [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$47	$67	$159	$143
Inkjet Colorants [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	17	18	46	46
Aerogel [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	2	9	8	17
Elastomer Composites [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	4	5	28	17
Specialty Fluids [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net sales	$24	$35	$77	$63