Debt - Schedule of Carrying Value of Individual Components of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Total debt	$ 10,795.5	$ 6,415.2
Less: current portion of debt	1,422.4	740.7
Long-term debt due after one year	9,373.1	5,674.5
Notes Due Fiscal 2019 to 2022 [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	1,114.1	1,470.9
Notes Due Fiscal 2023 to 2028 [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	3,271.3	2,534.4
Notes Due Fiscal 2029 to 2033 [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	1,707.8	964.1
Notes Due Fiscal 2037 to 2047 [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	179.1	178.5
Term Loan Facilities [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	2,905.8	599.4
Revolving Credit and Swing Facilities [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	527.2	355.0
Receivables Facility [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Total debt	150.0
Commercial Paper [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	588.3
Capital Lease Obligations [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Total debt	189.0	171.0
Supplier Financing and Commercial Card Programs [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total debt	130.1	105.1
International and Other Debt [Member]
Debt Instrument [Line Items]
Total debt	$ 32.8	$ 36.8

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtLongtermAndShorttermCombinedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_NotesDueFiscal2019To2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_NotesDueFiscal2023To2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_NotesDueFiscal2029To2033Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_NotesDueFiscal2037To2047Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_TermLoanFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_RevolvingCreditAndSwingFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_ReceivablesFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_CapitalLeaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_SupplierFinancingAndCommercialCardProgramsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wrk_InternationalAndOtherDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: