Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Oct. 01, 2019	Oct. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,036,500,000	$ 1,274,300,000	$ 404,200,000	$ 1,617,500,000
Other comprehensive (loss) income	(42,400,000)	36,600,000	4,600,000	1,600,000
Adoption of accounting standard/other					$ 2,200,000	$ 6,100,000
Balance at end of period	937,900,000	1,336,300,000	937,900,000	1,336,300,000
Total accumulated other comprehensive loss, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,584,300,000)	(976,900,000)	(1,488,000,000.0)	(941,900,000)
Other comprehensive income (loss) before reclassifications	(64,600,000)	25,100,000	(41,900,000)	(22,400,000)
Amounts reclassified from accumulated other comprehensive loss	23,100,000	11,500,000	47,100,000	24,000,000.0
Other comprehensive (loss) income	(41,500,000)	36,600,000	5,200,000	1,600,000
Adoption of accounting standard/other					(146,800,000)
Balance at end of period	(1,625,800,000)	(940,300,000)	(1,625,800,000)	(940,300,000)
Pension and other postretirement benefit plan adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,253,100,000)	(644,300,000)	(1,133,700,000)	(658,100,000)
Other comprehensive income (loss) before reclassifications	0	0	0	(300,000)
Amounts reclassified from accumulated other comprehensive loss	27,400,000	14,200,000	54,800,000	28,300,000
Other comprehensive (loss) income	27,400,000	14,200,000	54,800,000	28,000,000.0
Adoption of accounting standard/other					(146,800,000)
Balance at end of period	(1,225,700,000)	(630,100,000)	(1,225,700,000)	(630,100,000)
Accumulated currency translation adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(315,700,000)	(314,500,000)	(341,300,000)	(286,000,000.0)
Other comprehensive income (loss) before reclassifications	(73,400,000)	19,000,000.0	(51,600,000)	(9,500,000)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Other comprehensive (loss) income	(73,400,000)	19,000,000.0	(51,600,000)	(9,500,000)
Adoption of accounting standard/other					$ 3,800,000
Balance at end of period	(389,100,000)	(295,500,000)	(389,100,000)	(295,500,000)
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	(15,500,000)	(16,400,000)	(13,000,000.0)	4,400,000
Other comprehensive income (loss) before reclassifications	8,800,000	5,200,000	9,700,000	(14,000,000.0)
Amounts reclassified from accumulated other comprehensive loss	(4,300,000)	(2,700,000)	(7,700,000)	(4,300,000)
Other comprehensive (loss) income	4,500,000	2,500,000	2,000,000.0	(18,300,000)
Balance at end of period	(11,000,000.0)	(13,900,000)	(11,000,000.0)	(13,900,000)
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	(1,700,000)	0	(2,200,000)
Other comprehensive income (loss) before reclassifications	0	900,000	0	1,400,000
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Other comprehensive (loss) income	0	900,000	0	1,400,000
Balance at end of period	$ 0	$ (800,000)	$ 0	$ (800,000)