Restructuring and Other Costs - Schedule of Reconciliation of Accruals and Charges (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Restructuring And Other Costs [Abstract]
Additional accruals and adjustments to accruals (see table above)			$ 23.3	$ 10.0
PP&E and related costs			338.8	0.2
Severance and other employee costs				0.4
Acquisition costs			1.9	0.4
Integration costs			0.4	1.1
Divestiture costs			0.8	0.6
Other restructuring costs			1.1	7.1
Restructuring and other costs	$ 0.6	$ 6.9	$ 366.3	$ 19.8

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=123586518&loc=d3e1043-128460

+ Details

Name:	us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Costs incurred to effect a business combination which have been expensed during the period. Such costs could include business integration costs, systems integration and conversion costs, and severance and other employee-related costs.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessCombinationIntegrationRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of other expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses associated with a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

+ Details

Name:	us-gaap_OtherRestructuringCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Additional accruals and adjustments to accruals.

+ References

No definition available.

+ Details

Name:	wrk_AdditionalAccrualsAndAdjustmentsToAccruals
Namespace Prefix:	wrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Divestiture related costs credits.

+ References

No definition available.

+ Details

Name:	wrk_DivestitureRelatedCostsCredits
Namespace Prefix:	wrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Restructuring and other costs.

+ References

No definition available.

+ Details

Name:	wrk_RestructuringAndOtherCostsAbstract
Namespace Prefix:	wrk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount charged against earnings in the period for incurred and estimated costs, excluding asset retirement obligations, associated with exit from or disposal of business activities or restructurings pursuant to a program that is planned and controlled by management, and materially changes either the scope of a business undertaken by an entity, or the manner in which that business is conducted and other costs including but not limited to asset impairments, acquisition and integration costs.

+ References

No definition available.

+ Details

Name:	wrk_RestructuringAndOtherCostsNet
Namespace Prefix:	wrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Severance and Other Employee Costs

+ References

No definition available.

+ Details

Name:	wrk_RestructuringAndOtherCostsNetSeveranceAndOtherEmployeeCosts
Namespace Prefix:	wrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Restructuring and other costs, property, plant and equipment and related costs.

+ References

No definition available.

+ Details

Name:	wrk_RestructuringAndOtherCostsPropertyPlantAndEquipmentAndRelatedCosts
Namespace Prefix:	wrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration