Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 937.9	$ 1,336.3	$ 404.2	$ 1,617.5
Other comprehensive income	56.0	10.1	60.6	11.7
Adoption of accounting standard/other					$ 2.2	$ 6.1
Balance at end of period	1,089.1	1,156.5	1,089.1	1,156.5
Total accumulated other comprehensive loss, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,625.8)	(940.3)	(1,488.0)	(941.9)
Other comprehensive income (loss) before reclassifications	32.4	(0.4)	(9.5)	(22.8)
Amounts reclassified from accumulated other comprehensive loss	23.5	10.5	70.6	34.5
Other comprehensive income	55.9	10.1	61.1	11.7
Adoption of accounting standard/other					(146.8)
Balance at end of period	(1,569.9)	(930.2)	(1,569.9)	(930.2)
Pension and other postretirement benefit plan adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,225.7)	(630.1)	(1,133.7)	(658.1)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0	(0.3)
Amounts reclassified from accumulated other comprehensive loss	27.2	14.1	82.0	42.4
Other comprehensive income	27.2	14.1	82.0	42.1
Adoption of accounting standard/other					(146.8)
Balance at end of period	(1,198.5)	(616.0)	(1,198.5)	(616.0)
Accumulated currency translation adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(389.1)	(295.5)	(341.3)	(286.0)
Other comprehensive income (loss) before reclassifications	40.1	(2.7)	(11.5)	(12.2)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income	40.1	(2.7)	(11.5)	(12.2)
Adoption of accounting standard/other					$ 3.8
Balance at end of period	(349.0)	(298.2)	(349.0)	(298.2)
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	(11.0)	(13.9)	(13.0)	4.4
Other comprehensive income (loss) before reclassifications	(7.7)	1.7	2.0	(12.3)
Amounts reclassified from accumulated other comprehensive loss	(3.7)	(3.6)	(11.4)	(7.9)
Other comprehensive income	(11.4)	(1.9)	(9.4)	(20.2)
Balance at end of period	(22.4)	(15.8)	(22.4)	(15.8)
Net unrealized gains (losses) on available-for-sale investments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	0.0	(0.8)	0.0	(2.2)
Other comprehensive income (loss) before reclassifications	0.0	0.6	0.0	2.0
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income	0.0	0.6	0.0	2.0
Balance at end of period	$ 0.0	$ (0.2)	$ 0.0	$ (0.2)