Retirement Benefits (Pension Plan Assets at Fair Value by Asset Class) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 256.5	$ 254.1	$ 217.7
U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,048.3	2,832.4	2,684.9
U.S. | Total U.S. Qualified Pension Plans at Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,878.6	2,664.6
U.S. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.9	13.7
U.S. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	573.9	401.1
U.S. | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	213.1	152.9
U.S. | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	762.0	524.8
U.S. | Bonds (government and corporate)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,312.7	1,572.1
U.S. | Real estate pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	169.7	167.8
U.S. | Level 1 | Total U.S. Qualified Pension Plans at Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	803.9	567.7
U.S. | Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.9	13.7
U.S. | Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	573.9	401.1
U.S. | Level 1 | Equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	213.1	152.9
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 | Bonds (government and corporate)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
U.S. | Level 2 | Total U.S. Qualified Pension Plans at Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,074.7	2,096.9
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	762.0	524.8
U.S. | Level 2 | Bonds (government and corporate)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,312.7	1,572.1
U.S. | Level 3 | Total U.S. Qualified Pension Plans at Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
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 | Bonds (government and corporate)
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	1,726.8	1,672.4	$ 1,588.2
International | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.9	13.4
International | Equity pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	746.8	711.3
International | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	694.1	679.9
International | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15.5	13.7
International | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	256.5	254.1
International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.9	13.4
International | Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.9	13.4
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	1,456.4	1,404.9
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	746.8	711.3
International | Level 2 | Fixed income pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	694.1	679.9
International | Level 2 | Other pooled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15.5	13.7
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	256.5	254.1
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	$ 256.5	$ 254.1