Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year	$ 2,599.8
Total other comprehensive income (loss), net of tax	3.2	$ (85.7)	$ 18.2
Balance at end of year	2,850.7	2,599.8
Employee Pension and Postretirement Benefits, Net of Tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year	(69.4)	(10.9)	(46.2)
Other comprehensive income (loss) before reclassifications	(29.3)	(49.5)	33.1
Amounts reclassified from accumulated other comprehensive income (loss)	2.8	0.1	2.2
Total other comprehensive income (loss), net of tax	(26.5)	(49.4)	35.3
Balance at end of year	(95.9)	(69.4)	(10.9)
Employee Pension and Postretirement Benefits, Net of Tax | Revision of Prior Period, Accounting Standards Update, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		(20.0)
Balance at end of year			(20.0)
Employee Pension and Postretirement Benefits, Net of Tax | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		(9.1)
Balance at end of year			(9.1)
Cumulative Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year	(132.5)	(96.2)	(78.6)
Other comprehensive income (loss) before reclassifications	30.4	(36.3)	(17.6)
Total other comprehensive income (loss), net of tax	30.4	(36.3)	(17.6)
Balance at end of year	(102.1)	(132.5)	(96.2)
Cumulative Translation Adjustments | Revision of Prior Period, Accounting Standards Update, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		(96.2)
Balance at end of year			(96.2)
Derivative Instruments, Net of Tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year	0.3	0.3	(0.2)
Other comprehensive income (loss) before reclassifications	(0.5)		0.6
Amounts reclassified from accumulated other comprehensive income (loss)	(0.2)		(0.1)
Total other comprehensive income (loss), net of tax	(0.7)		0.5
Balance at end of year	(0.4)	0.3	0.3
Derivative Instruments, Net of Tax | Revision of Prior Period, Accounting Standards Update, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		0.3
Balance at end of year			0.3
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year	(201.6)	(106.8)	(125.0)
Other comprehensive income (loss) before reclassifications	0.6	(85.8)	16.1
Amounts reclassified from accumulated other comprehensive income (loss)	2.6	0.1	2.1
Total other comprehensive income (loss), net of tax	3.2	(85.7)	18.2
Balance at end of year	$ (198.4)	(201.6)	(106.8)
Accumulated Other Comprehensive Income (Loss) | Revision of Prior Period, Accounting Standards Update, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		(115.9)
Balance at end of year			(115.9)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of year		$ (9.1)
Balance at end of year			$ (9.1)