Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,346.8	$ 404.2
Other comprehensive income	87.0	47.0
Balance at end of period	1,873.8	1,036.5
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	(1,614.2)	(1,488.0)
Other comprehensive income (loss) before reclassifications	56.2	22.7
Amounts reclassified from accumulated other comprehensive loss	30.7	24.0
Other comprehensive income	86.9	46.7
Balance at end of period	(1,527.3)	(1,584.3)
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)
Pension and other postretirement benefit plan adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,271.2)	(1,133.7)
Other comprehensive income (loss) before reclassifications	0.0	0.0
Amounts reclassified from accumulated other comprehensive loss	27.6	27.4
Other comprehensive income	27.6	27.4
Balance at end of period	(1,243.6)	(1,253.1)
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)
Accumulated currency translation adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(311.5)	(341.3)
Other comprehensive income (loss) before reclassifications	69.1	21.8
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Other comprehensive income	69.1	21.8
Balance at end of period	(242.4)	(315.7)
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
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	(31.5)	(13.0)
Other comprehensive income (loss) before reclassifications	(12.9)	0.9
Amounts reclassified from accumulated other comprehensive loss	3.1	(3.4)
Other comprehensive income	(9.8)	(2.5)
Balance at end of period	$ (41.3)	(15.5)
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