Retirement Benefits - Defined Benefit Plan Change in Fair Value of Plan Assets Level Three (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 3,788.3
Plan assets at end of year	3,754.8	$ 3,788.3
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	77.2	85.4
Realized Gains (Losses)	0.0	0.0
Unrealized Gains (Losses)	(0.6)	(13.4)
Purchases, Sales, Issuances, and Settlements, Net	8.0	5.2
Plan assets at end of year	84.6	77.2
U.S. Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	2,890.1
Plan assets at end of year	2,827.3	2,890.1
U.S. Plans | Insurance contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	0.9
Plan assets at end of year	0.9	0.9
U.S. Plans | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	0.9
Plan assets at end of year	0.9	0.9
U.S. Plans | Level 3 | Insurance contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	0.9	0.9
Realized Gains (Losses)	0.0	0.0
Unrealized Gains (Losses)	0.0	0.0
Purchases, Sales, Issuances, and Settlements, Net	0.0	0.0
Plan assets at end of year	0.9	0.9
Non-US Plan | Insurance contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	71.5
Plan assets at end of year	79.1	71.5
Non-US Plan | Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	4.8
Plan assets at end of year	4.6	4.8
Non-US Plan | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	76.3
Plan assets at end of year	83.7	76.3
Non-US Plan | Level 3 | Insurance contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	71.5	79.7
Realized Gains (Losses)	0.0	0.0
Unrealized Gains (Losses)	(0.4)	(13.4)
Purchases, Sales, Issuances, and Settlements, Net	8.0	5.2
Plan assets at end of year	79.1	71.5
Non-US Plan | Level 3 | Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	4.8	4.8
Realized Gains (Losses)	0.0	0.0
Unrealized Gains (Losses)	(0.2)	0.0
Purchases, Sales, Issuances, and Settlements, Net	0.0
Plan assets at end of year	$ 4.6	$ 4.8