Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Amount of insurance proceeds for an event or transaction that is unusual in nature or infrequent in occurrence, or both.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=114868883&loc=SL114871943-224233

+ Details

Name:	us-gaap_UnusualOrInfrequentItemInsuranceProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase in discount rate to estimate fair value of reporting unit.

+ References

No definition available.

+ Details

Name:	wrk_IncreaseInDiscountRateToEstimateFairValueOfReportingUnit
Namespace Prefix:	wrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reporting unit, discount rate.

+ References

No definition available.

+ Details

Name:	wrk_ReportingUnitDiscountRate
Namespace Prefix:	wrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reporting unit, growth rate.

+ References

No definition available.

+ Details

Name:	wrk_ReportingUnitGrowthRate
Namespace Prefix:	wrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reporting unit percentage of fair value in excess of carrying amount by less than ten percent.

+ References

No definition available.

+ Details

Name:	wrk_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmountByLessThanTenPercent
Namespace Prefix:	wrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ReportingUnitAxis=wrk_VictoryPackagingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=wrk_CorrugatedPackagingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=us-gaap_HurricaneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_CostOfSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=wrk_ConsumerPackagingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Segment Information - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment		3
Increase in discount rate to estimate fair value of reporting unit		1.00%
Goodwill	$ 7,321.9	$ 7,321.9	$ 5,577.6
Victory Packaging [Member]
Segment Reporting Information [Line Items]
Reporting unit, percentage of fair value in excess of carrying value	10.00%
Minimum [Member]
Segment Reporting Information [Line Items]
Reporting unit, discount rate	8.50%
Reporting unit, growth rate	0.00%
Maximum [Member]
Segment Reporting Information [Line Items]
Reporting unit, discount rate	14.00%
Reporting unit, growth rate	1.00%
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Goodwill	$ 3,705.8	3,705.8	1,966.7
Corrugated Packaging [Member] \| Hurricane Michael [Member] \| Cost of Goods Sold [Member]
Segment Reporting Information [Line Items]
Insurance proceeds recieved	$ 30.0	110.0
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Reporting unit, percentage of fair value in excess of carrying value	10.00%
Goodwill	$ 3,616.1	$ 3,616.1	$ 3,610.9