Special Purpose Entities - Schedule of Restricted Assets and Non-recourse Liabilities Held by Special Purposes Entities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023
Debt Instrument [Line Items]
Other current assets	$ 0.0	$ 862.1
Other current liabilities	0.0	776.7
Rock-Tenn Company and MeadWestvaco Corporation's [Member] | Timber Notes [Member]
Debt Instrument [Line Items]
Other current assets		862.1
Other noncurrent assets	383.5	382.7
Other current liabilities		776.7
Other noncurrent liabilities	$ 330.7	$ 330.2