Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss	$ 0.0	$ 0.0	$ 5.0	$ 0.0	$ 43.7	$ 0.0	$ 0.0	$ 0.0	$ 5.0	$ 43.7
Shares of common stock in ESOP	2,197,262								2,197,262
Defined contribution plan cost recognized									$ 40.6	34.2	$ 33.7
Pension and postretirement benefits	$ 26.0				34.1				$ 26.0	34.1
U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									7.00%
U.S. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									7.80%
U.S. | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									4.70%
U.S. | Real estate/other
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									6.50%
International
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									5.30%
U.K.
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									5.80%
Percentage of plan assets as component of total international plan assets									80.00%
U.K. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									7.40%
U.K. | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets									2.70%
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss					43.7				$ (6.2)	(5.2)	10.5
ABO for all defined benefit pension plans	4,931.6				4,376.4				4,931.6	4,376.4
Company contributions									40.2
Defined Benefit Pension Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Anticipated contributions for next fiscal year	30.0								30.0
Defined Benefit Pension Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Anticipated contributions for next fiscal year	40.0								40.0
Defined Benefit Pension Plan | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
ABO for plans with obligation in excess plan assets	92.6								92.6
PBO for plans with obligation in excess of plan assets	99.2								99.2
Defined Benefit Pension Plan | U.S.
Defined Benefit Plan Disclosure [Line Items]
Prior service cost arising during the period									1.1	0.1
Pension settlement loss									$ 6.2	45.0	$ 10.5
Amortization period based on approximate average remaining service period									7 years
ABO for plans with obligation in excess plan assets	2,941.2				2,608.6				$ 2,941.2	2,608.6
PBO for plans with obligation in excess of plan assets	3,069.2				2,733.6				$ 3,069.2	$ 2,733.6
Expected return on plan assets									7.00%	7.50%	8.00%
Company contributions									$ 16.0	$ 14.6
Accumulated benefit obligations	3,281.6				2,922.8				3,281.6	2,922.8	$ 3,357.7
Pension and postretirement benefits	18.3				23.5				18.3	23.5
Gains arising during the period									262.9	(167.7)
Amortization of net actuarial loss									(71.5)	(132.4)
Net actuarial loss on pretax basis	871.8				680.4				871.8	680.4
Defined Benefit Pension Plan | International
Defined Benefit Plan Disclosure [Line Items]
Prior service cost arising during the period				$ 4.7					4.7	0.7
Pension settlement loss									0.2	3.5	$ 1.7
ABO for plans with obligation in excess plan assets	413.3				357.9				413.3	357.9
PBO for plans with obligation in excess of plan assets	521.1				452.6				$ 521.1	$ 452.6
Expected return on plan assets									5.30%	5.80%	6.10%
Company contributions									$ 24.2	$ 53.7
Accumulated benefit obligations	1,864.0				1,660.5				1,864.0	1,660.5	$ 1,749.5
Pension and postretirement benefits	0.0				1.2				0.0	1.2
Gains arising during the period									161.5	(64.6)
Amortization of net actuarial loss									(11.1)	(43.7)
Net actuarial loss on pretax basis	594.0				443.6				$ 594.0	443.6
Defined Benefit Pension Plan | U.K.
Defined Benefit Plan Disclosure [Line Items]
Amortization period based on approximate average remaining life expectancy									25 years
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	43.7				56.4				$ 43.7	56.4
Pension and postretirement benefits	7.7				9.4				7.7	9.4
Gains arising during the period									(6.1)	(3.1)
Amortization of net actuarial loss									0.0	(0.3)
Net actuarial loss on pretax basis	$ (1.7)				$ 4.4				$ (1.7)	$ 4.4