Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,617.5
Other comprehensive income (loss) before reclassifications	(47.5)	$ (20.6)
Amounts reclassified from accumulated other comprehensive loss	(12.5)	(24.1)
Other comprehensive income (loss)	(35.0)	3.5
Balance at end of period	1,274.3	2,156.2
Pension and other postretirement benefit plan adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(658.1)	(927.0)
Other comprehensive income (loss) before reclassifications	(0.3)	0.0
Amounts reclassified from accumulated other comprehensive loss	(14.1)	(20.2)
Other comprehensive income (loss)	13.8	20.2
Balance at end of period	(644.3)	(906.8)
Accumulated currency translation adjustments, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(286.0)	(237.7)
Other comprehensive income (loss) before reclassifications	(28.5)	(16.1)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Other comprehensive income (loss)	(28.5)	(16.1)
Balance at end of period	(314.5)	(253.8)
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	4.4	(14.4)
Other comprehensive income (loss) before reclassifications	(19.2)	(3.4)
Amounts reclassified from accumulated other comprehensive loss	1.6	(3.9)
Other comprehensive income (loss)	(20.8)	0.5
Balance at end of period	(16.4)	(13.9)
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	(2.2)	(0.1)
Other comprehensive income (loss) before reclassifications	0.5	(1.1)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Other comprehensive income (loss)	0.5	(1.1)
Balance at end of period	(1.7)	(1.2)
Total accumulated other comprehensive loss, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(941.9)	(1,179.2)
Balance at end of period	$ (976.9)	$ (1,175.7)