Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 2,663.6
Other comprehensive income (loss) before reclassifications	$ (81.4)	$ 21.0	(33.0)	$ 5.7
Amounts reclassified from accumulated other comprehensive loss	22.4	23.3	71.7	69.4
Other comprehensive income (loss)	(59.0)	44.3	38.7	75.1
Balance at end of period	1,444.2		1,444.2
Pension and other postretirement benefit plan adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(886.6)	(1,191.5)	(927.0)	(1,239.8)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0	0.7
Amounts reclassified from accumulated other comprehensive loss	20.2	23.8	60.6	71.4
Other comprehensive income (loss)	20.2	23.8	60.6	72.1
Balance at end of period	(866.4)	(1,167.7)	(866.4)	(1,167.7)
Accumulated currency translation adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(177.3)	(320.3)	(237.7)	(294.9)
Other comprehensive income (loss) before reclassifications	(95.7)	24.9	(35.3)	(0.5)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(95.7)	24.9	(35.3)	(0.5)
Balance at end of period	(273.0)	(295.4)	(273.0)	(295.4)
Net unrealized gains (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(14.9)	3.9	(14.4)	(4.1)
Other comprehensive income (loss) before reclassifications	14.2	(3.9)	4.8	5.6
Amounts reclassified from accumulated other comprehensive loss	2.2	(0.5)	11.1	(2.0)
Other comprehensive income (loss)	16.4	(4.4)	15.9	3.6
Balance at end of period	1.5	(0.5)	1.5	(0.5)
Net unrealized gains (losses) on available-for-sale investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2.7)	(0.1)	(0.1)	0.0
Other comprehensive income (loss) before reclassifications	0.1	0.0	(2.5)	(0.1)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.1	0.0	(2.5)	(0.1)
Balance at end of period	(2.6)	(0.1)	(2.6)	(0.1)
Total accumulated other comprehensive loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,081.5)	(1,508.0)	(1,179.2)	(1,538.8)
Balance at end of period	$ (1,140.5)	$ (1,463.7)	$ (1,140.5)	$ (1,463.7)