Retirement Benefits (Pension Plan Assets at Fair Value by Asset Class) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 254.1	$ 217.7	$ 49.2
Level 3 | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0	7.8
Level 3 | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		254.1	217.7	41.4
U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,832.4	2,684.9	2,869.2
U.S. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.7	13.8
U.S. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		401.1	397.9
U.S. | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		152.9	173.8
U.S. | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		524.8	545.2
U.S. | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,572.1	1,344.6
U.S. | US pension plans at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,664.6	2,475.3
U.S. | Real estate pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	167.8	209.6
U.S. | Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.7	13.8
U.S. | Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		401.1	397.9
U.S. | Level 1 | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		152.9	173.8
U.S. | Level 1 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 1 | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 1 | US pension plans at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		567.7	585.5
U.S. | Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 2 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 2 | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 2 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		524.8	545.2
U.S. | Level 2 | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,572.1	1,344.6
U.S. | Level 2 | US pension plans at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,096.9	1,889.8
U.S. | Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 3 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 3 | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 3 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 3 | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
U.S. | Level 3 | US pension plans at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,672.4	1,588.2	$ 1,540.0
International | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.4	15.8
International | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		711.3	727.9
International | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		679.9	615.2
International | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.7	11.6
International | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		254.1	217.7
International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.4	15.8
International | Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.4	15.8
International | Level 1 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 1 | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 1 | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 1 | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,404.9	1,354.7
International | Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 2 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		711.3	727.9
International | Level 2 | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		679.9	615.2
International | Level 2 | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.7	11.6
International | Level 2 | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		254.1	217.7
International | Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 3 | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 3 | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 3 | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0	0.0
International | Level 3 | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 254.1	$ 217.7

[1] Real estate pooled funds consist of funds that invest in properties. These funds generally allow for quarterly redemption with 30 days' notice. Timing for redemption could be delayed based on the priority of our request and the availability of funds. Interests in these funds are valued using the net asset value (NAV) per share practical expedient and are not classified in the fair value hierarchy.