Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 2,128.7	$ 937.9	$ 1,346.8	$ 404.2
Other comprehensive income	450.3	56.0	538.6	60.6
Balance at end of period	2,578.1	1,089.1	2,578.1	1,089.1
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				2.2
Total accumulated other comprehensive loss, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,526.0)	(1,625.8)	(1,614.2)	(1,488.0)
Other comprehensive income (loss) before reclassifications	396.5	32.4	420.3	(9.5)
Amounts reclassified from accumulated other comprehensive loss	54.5	23.5	118.9	70.6
Other comprehensive income	451.0	55.9	539.2	61.1
Balance at end of period	(1,075.0)	(1,569.9)	(1,075.0)	(1,569.9)
Total accumulated other comprehensive loss, net of tax | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				(146.8)
Balance at end of period		(143.0)		(143.0)
Pension and other postretirement benefit plan adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,215.6)	(1,225.7)	(1,271.2)	(1,133.7)
Other comprehensive income (loss) before reclassifications	379.1	0.0	379.7	0.0
Amounts reclassified from accumulated other comprehensive loss	48.1	27.2	103.1	82.0
Other comprehensive income	427.2	27.2	482.8	82.0
Balance at end of period	(788.4)	(1,198.5)	(788.4)	(1,198.5)
Pension and other postretirement benefit plan adjustments, net of tax | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at end of period		(146.8)		(146.8)
Accumulated currency translation adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(275.7)	(389.1)	(311.5)	(341.3)
Other comprehensive income (loss) before reclassifications	23.0	40.1	58.8	(11.5)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income	23.0	40.1	58.8	(11.5)
Balance at end of period	(252.7)	(349.0)	(252.7)	(349.0)
Accumulated currency translation adjustments, net of tax | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at end of period		3.8		3.8
Net unrealized gains (losses) on cash flow hedges, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(34.7)	(11.0)	(31.5)	(13.0)
Other comprehensive income (loss) before reclassifications	(5.6)	(7.7)	(18.2)	2.0
Amounts reclassified from accumulated other comprehensive loss	6.4	(3.7)	15.8	(11.4)
Other comprehensive income	0.8	(11.4)	(2.4)	(9.4)
Balance at end of period	$ (33.9)	(22.4)	$ (33.9)	(22.4)
Net unrealized gains (losses) on cash flow hedges, net of tax | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at end of period		$ 0.0		$ 0.0