Derivative Financial Instruments - Fair Value of Outstanding Derivative Contracts in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Derivatives, Fair Value [Line Items]
Document Period End Date	Mar. 31, 2018
Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Total asset derivatives	$ 3.7		$ 1.2
Total liability derivatives			0.4	$ 3.3
Derivatives designated as hedging instruments | Interest Rate Swap [Member] | Receivables, net
Derivatives, Fair Value [Line Items]
Total asset derivatives		$ 1.4	0.0
Derivatives designated as hedging instruments | Interest Rate Swap [Member] | Other assets
Derivatives, Fair Value [Line Items]
Total asset derivatives	1.9		0.4
Derivatives designated as hedging instruments | Interest Rate Contract [Member] | Accounts payable and other current liabilities
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0		0.5
Derivatives designated as hedging instruments | Interest Rate Contract [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0		0.2
Derivatives designated as hedging instruments | Commodity swap and option agreements | Receivables, net
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.3		0.6
Derivatives designated as hedging instruments | Foreign exchange contracts | Receivables, net
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.1		0.2
Derivatives designated as hedging instruments | Foreign exchange contracts | Accounts payable and other current liabilities
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.3		2.3
Derivatives designated as hedging instruments | Foreign exchange contracts | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0		0.3
Derivatives designated as hedging instruments | Commodity contracts | Accounts payable and other current liabilities
Derivatives, Fair Value [Line Items]
Total liability derivatives			0.1	0.0
Derivatives not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Total asset derivatives	3.7		1.5
Total liability derivatives	1.1		3.3
Derivatives not designated as hedging instrument | Foreign exchange forward agreements | Accounts payable and other current liabilities
Derivatives, Fair Value [Line Items]
Total liability derivatives			0.7	$ 0.0
Derivatives not designated as hedging instrument | Foreign exchange contracts | Receivables, net
Derivatives, Fair Value [Line Items]
Total asset derivatives	$ 0.0		$ 0.3