Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Expense related to defined contribution savings plans	$ 47.0	$ 41.5	$ 38.6
Recognized prior service credits	4.9
Recognized prior service credits, net of tax	3.8
Recognized net actuarial losses	115.1
Recognized net actuarial losses, net of tax	84.3
Accumulated benefit obligation for pension plans	3,962.3	4,252.2
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Voluntary contribution to U.S. qualified pension plan	50.3	254.9
Recognized prior service credits	(0.6)	3.7	2.9
Recognized net actuarial losses	113.4	152.9	124.5
Estimated future payments, during next fiscal year	32.0
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Voluntary contribution to U.S. qualified pension plan	10.1	10.0
Recognized prior service credits	5.5	6.1	11.1
Recognized net actuarial losses	1.7	$ 2.3	$ 2.3
Estimated future payments, during next fiscal year	$ 10.3
U.S. Plans | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.35%	3.90%	3.75%
U.S. Plans | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.15%	3.40%
Non-US Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.48%	2.30%
Non-US Plan | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.30%	3.20%
Qualified Plan | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Voluntary contribution to U.S. qualified pension plan		$ 200.0
Minimum
Defined Benefit Plan Disclosure [Line Items]
Company contribution to defined contribution plan (percentage)	3.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Company contribution to defined contribution plan (percentage)	7.00%