Acquisitions - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Nov. 02, 2018	Jun. 30, 2020	Sep. 30, 2019	Jul. 01, 2015
Business Acquisition [Line Items]
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Number of WRKCo Shares to WestRock Shares	1
WRKCo Inc. [Member]
Business Acquisition [Line Items]
Common Stock, Par or Stated Value Per Share	$ 0.01
KapStone Acquisition [Member]
Business Acquisition [Line Items]
Common Stock, Par or Stated Value Per Share	0.0001
Business Acquisition, Share Price	$ 35.00
Ratio of KapStone Shares to WestRock Shares	49.81%
Maximum percentage of issued and outstanding KapStone Shares to elect WestRock stock consideration	25.00%
Estimated Enterprise Value of Acquisition	$ 4,900.0
Consideration paid in cash	$ 3,300.0
Business acquisition, number of shares issued	1,600,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	$ 70.1
Business acquisition, percentage of equity interest on shares issued and outstanding	0.60%
Fair value of share-based awards issued in business combinations	$ 70.8
KapStone Acquisition [Member] | Minimum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Useful Life	1 year
KapStone Acquisition [Member] | Maximum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Useful Life	20 years