Employee Benefit Plans - Schedule of Fair Value of Plan Assets by Major Category and Level Within Fair Value Hierarchy (Details) - Pension Benefits - USD ($) $ in Millions		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		$ 437.3	$ 408.9	$ 384.2
Level 1
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		171.4	160.3
Level 1 | U.S. companies
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[1]	77.4	75.9
Level 1 | Mutual Funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[1]	81.5	72.7
Level 1 | Money market funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[2]	12.5	11.7
Level 2
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		37.6	34.7
Level 2 | U.S. companies
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[1]	8.1	6.9
Level 2 | International companies
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[3]	13.5	13.3
Level 2 | Government and agency bonds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[4]	7.7	7.0
Level 2 | Corporate bonds and notes
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[5]	8.3	7.5
Level 3
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		0.8	0.7
Level 3 | Other
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		0.8	0.7
Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		209.8	195.7
Fair Value, Inputs, Level 1, 2 and 3 | U.S. companies
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[1]	85.5	82.8
Fair Value, Inputs, Level 1, 2 and 3 | International companies
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[3]	13.5	13.3
Fair Value, Inputs, Level 1, 2 and 3 | Mutual Funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[1]	81.5	72.7
Fair Value, Inputs, Level 1, 2 and 3 | Government and agency bonds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[4]	7.7	7.0
Fair Value, Inputs, Level 1, 2 and 3 | Corporate bonds and notes
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[5]	8.3	7.5
Fair Value, Inputs, Level 1, 2 and 3 | Money market funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[2]	12.5	11.7
Fair Value, Inputs, Level 1, 2 and 3 | Other
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value		0.8	0.7
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Total assets - at fair value	[6]	$ 227.5	$ 213.2

[1] Primarily valued using a market approach based on the quoted market prices of identical instruments that are actively traded on public exchanges.

[2] These investments largely consist of short-term investment funds and are valued using a market approach based on the quoted market prices of identical instruments.

[3] Valuation model looks at underlying security “best” price, exchange rate for underlying security’s currency against the U.S. dollar and ratio of underlying security to American depository receipt.

[4] These investments consist of debt securities issued by the U.S. Treasury, U.S. government agencies and U.S. government-sponsored enterprises and have a variety of structures, coupon rates and maturities. These investments are considered to have low default risk as they are guaranteed by the U.S. government. Fixed income securities are primarily valued using a market approach with inputs that include broker quotes, benchmark yields, base spreads and reported trades.

[5] These investments consist of debt obligations issued by a variety of private and public corporations. These are investment grade securities which historically have provided a steady stream of income. Fixed income securities are primarily valued using a market approach with inputs that include broker quotes, benchmark yields, base spreads and reported trades.

[6] These investments consist of privately placed funds that are valued based on NAV. NAV of the funds is based on the fair value of each funds underlying investments. In accordance with ASC Subtopic 820-10, certain investments that are measured at fair value using the NAV per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.