Accumulated Other Comprehensive Loss - Reclassifications out of Other Comprehensive Loss (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]
Income before income taxes	$ 402.4	$ 299.6	$ 523.2	$ 597.4
Income tax provision	(56.4)	(72.2)	(96.9)	(606.4)
Sales	1,657.2	1,651.2	3,299.5	3,237.8
Cost of sales	(949.0)	(947.3)	(1,852.6)	(1,833.7)
Selling, general and administrative expenses	(385.0)	(386.6)	(771.7)	(773.2)
Treasury locks related to 2019 debt issuance	23.7	17.3	44.4	37.3
Net income (loss)	346.0	227.4	426.3	(9.0)
Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	11.5	25.2	24.0	49.3
Pension and other postretirement benefit plan adjustments [Member] | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	18.5	27.6	36.9	55.2
Income tax provision	(4.3)	(7.4)	(8.6)	(14.8)
Net income (loss)	14.2	20.2	28.3	40.4
Amortization of prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(1.0)	(1.3)	(2.1)	(2.4)
Amortization of net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	19.7	28.9	39.4	57.6
Settlements
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(0.2)	0.0	(0.4)	0.0
Net unrealized gains (losses) on cash flow hedges, net of tax | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(3.7)	6.8	(6.0)	12.0
Income tax provision	1.0	(1.8)	1.7	(3.1)
Net income (loss)	(2.7)	5.0	(4.3)	8.9
Net unrealized gains (losses) on cash flow hedges, net of tax | Forward exchange contracts | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Sales	(0.6)	(0.8)	(0.2)	(1.3)
Cost of sales	(3.3)	8.4	(6.5)	14.3
Selling, general and administrative expenses	0.0	(0.8)	0.5	(1.0)
Net unrealized gains (losses) on cash flow hedges, net of tax | Treasury locks related to 2019 debt issuance | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Treasury locks related to 2019 debt issuance	$ 0.2	$ 0.0	$ 0.2	$ 0.0