Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other items, net	$ (3.8)	$ (0.9)	$ 13.1	$ (2.2)
Interest expense	(77.2)	(16.5)	(125.4)	(29.9)
(Loss)/earnings before income taxes	(74.0)	17.8	16.0	136.8
Income tax (benefit)/provision	11.7	(10.0)	(7.5)	(68.6)
Net (loss)/earnings	(73.3)	7.8	(2.5)	68.2
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (loss)/earnings	0.7	(3.0)	1.7	(6.5)
Gains and losses on cash flow hedges | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other items, net	2.0	(1.9)	4.8	(4.3)
Interest expense	0.3	(0.3)	0.2	(0.8)
(Loss)/earnings before income taxes	2.3	(2.2)	5.0	(5.1)
Income tax (benefit)/provision	(0.6)	0.6	(1.2)	1.2
Net (loss)/earnings	1.7	(1.6)	3.8	(3.9)
Amortization of defined benefit pension items | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Actuarial loss	(1.2)	(1.8)	(2.5)	(3.3)
Settlement loss	0.0	0.0	0.0	(0.1)
(Loss)/earnings before income taxes	(1.2)	(1.8)	(2.5)	(3.4)
Income tax (benefit)/provision	0.2	0.4	0.4	0.8
Net (loss)/earnings	$ (1.0)	$ (1.4)	$ (2.1)	$ (2.6)