Pension and Other Postretirement Benefits - Change in Projected Benefit Obligation, Fair Value of Plan Assets, and Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Amounts recognized in assets or liabilities [Abstract]
Other liabilities	$ (53.3)	$ (83.5)
Other Benefits
Change in benefit obligation [roll forward]
Service cost	0.5	0.6	$ 1.0
Interest cost	2.1	2.0	4.0
North America | Pension Benefits
Change in benefit obligation [roll forward]
Benefit obligation at beginning of period	100.6	71.2
Service cost	4.2	4.1	4.0
Interest cost	3.6	2.5	2.5
Plan participants' contributions	0.6	0.6
Actuarial (gain) loss	(4.3)	(1.0)
Business combinations	0.0	25.5
Benefits Paid	(3.4)	(3.0)
Currency translation	(0.4)	0.7
Benefit obligation at end of period	100.9	100.6	71.2
Change in fair value of plan assets [roll forward]
Fair value of plan assets at beginning of period	80.9	54.3
Actual return on plan assets	3.9	5.4
Employer contributions	32.6	7.7
Business combinations	0.0	15.2
Benefits Paid	(3.4)	(3.0)
Currency translation	(0.5)	0.7
Fair value of plan assets at end of period	114.1	80.9	54.3
Defined benefit plan, funded status of plan [Abstract]
Funded status	13.2	(19.7)
Amounts recognized in assets or liabilities [Abstract]
Other assets	13.7	1.5
Other liabilities	(0.5)	(21.2)
Net amount recognized	13.2	(19.7)
Amounts recognized in accumulated other comprehensive income or loss [Abstract]
Net actuarial (gain) loss	9.5	14.3
Prior service cost (credit)	0.5	0.6
Total	10.0	14.9
Weighted-average assumptions used to determine benefit obligation [Abstract]
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	98.9	98.3
North America | Other Benefits
Change in benefit obligation [roll forward]
Benefit obligation at beginning of period	65.0	68.6
Service cost	0.5	0.6
Interest cost	2.1	2.0
Plan Amendment	0.0	(0.1)
Actuarial (gain) loss	(9.9)	(4.4)
Benefits Paid	(2.3)	(2.1)
Currency translation	(0.2)	0.4
Benefit obligation at end of period	55.2	65.0	68.6
Change in fair value of plan assets [roll forward]
Fair value of plan assets at beginning of period	0.0
Employer contributions	2.3	2.1
Benefits Paid	(2.3)	(2.1)
Fair value of plan assets at end of period	0.0	0.0
Defined benefit plan, funded status of plan [Abstract]
Funded status	(55.2)	(65.0)
Amounts recognized in assets or liabilities [Abstract]
Other current liabilities	(2.4)	(2.7)
Other liabilities	(52.8)	(62.3)
Net amount recognized	(55.2)	(65.0)
Amounts recognized in accumulated other comprehensive income or loss [Abstract]
Net actuarial (gain) loss	(1.2)	8.6
Prior service cost (credit)	(24.1)	(28.8)
Total	$ (25.3)	$ (20.2)
U.S. Plans, Pension Benefits | UNITED STATES
Weighted-average assumptions used to determine benefit obligation [Abstract]
Discount rate	4.27%	3.77%
U.S. Plans, Pension Benefits | UNITED STATES | Pension Benefits
Weighted-average assumptions used to determine benefit obligation [Abstract]
Discount rate	4.30%	3.86%
Rate of compensation increase	3.00%	3.00%
International Pension Plans | Other International
Weighted-average assumptions used to determine benefit obligation [Abstract]
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	$ 677.6	$ 690.4
International Pension Plans | Other International | Pension Benefits
Change in benefit obligation [roll forward]
Benefit obligation at beginning of period	723.6	0.0
Service cost	6.7	1.7	0.0
Interest cost	19.6	4.9	0.0
Plan participants' contributions	2.4	0.6
Actuarial (gain) loss	(14.0)	(46.4)
Business combinations	0.0	746.0
Benefits Paid	(27.6)	(6.2)
Currency translation	(19.5)	23.0
Benefit obligation at end of period	691.2	723.6	0.0
Change in fair value of plan assets [roll forward]
Fair value of plan assets at beginning of period	876.7	0.0
Actual return on plan assets	9.6	0.3
Employer contributions	7.2	2.4
Business combinations	0.0	852.2
Benefits Paid	(27.6)	(6.2)
Currency translation	(23.8)	27.4
Fair value of plan assets at end of period	844.5	876.7	$ 0.0
Defined benefit plan, funded status of plan [Abstract]
Funded status	153.3	153.1
Amounts recognized in assets or liabilities [Abstract]
Other assets	153.3	153.1
Other liabilities	0.0	0.0
Net amount recognized	153.3	153.1
Amounts recognized in accumulated other comprehensive income or loss [Abstract]
Net actuarial (gain) loss	(32.0)	(40.2)
Prior service cost (credit)	0.0	0.0
Total	$ (32.0)	$ (40.2)
Weighted-average assumptions used to determine benefit obligation [Abstract]
Discount rate	2.81%	2.72%
Rate of compensation increase	2.75%	2.70%
International Pension Plans | CANADA
Weighted-average assumptions used to determine benefit obligation [Abstract]
Discount rate	3.54%	3.69%
Rate of compensation increase	2.75%	2.75%
International Pension Plans | CANADA | Pension Benefits
Weighted-average assumptions used to determine benefit obligation [Abstract]
Discount rate	3.53%	3.63%
Rate of compensation increase	2.75%	2.69%