Fair Value - Summary of Activity Under A/R Sales Agreement (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Receivables sold to the financial institution and derecognized	$ 1,210.2	$ 943.0
Receivables collected by financial institution	(1,219.6)	(956.4)
Cash paid to financial institution	$ 9.4	$ 13.4

Cash Flows between a transferor and a transferee attributable to collections reinvested in revolving period transfers related to either a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107207-111719

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the initial fair value recorded for assets received on transfer of financial assets in a securitization, asset-backed financing arrangement, or a similar transfer which transaction is recognized as a sale of the transferred financial assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)(2)(i)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107207-111719

+ Details

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

- Definition

Increase decrease in accounts receivable from transfers net.

+ References

No definition available.

+ Details

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