Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 2,663.6
Other comprehensive loss before reclassifications	(20.6)	$ (73.7)
Amounts reclassified from accumulated other comprehensive loss	24.1	23.6
Other comprehensive income (loss)	3.5	(50.1)
Balance at end of period	2,156.2
Pension and other postretirement benefit plan adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(927.0)	(1,239.8)
Other comprehensive loss before reclassifications	0.0	0.7
Amounts reclassified from accumulated other comprehensive loss	20.2	23.8
Other comprehensive income (loss)	20.2	24.5
Balance at end of period	(906.8)	(1,215.3)
Accumulated currency translation adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(237.7)	(294.9)
Other comprehensive loss before reclassifications	(16.1)	(86.2)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Other comprehensive income (loss)	(16.1)	(86.2)
Balance at end of period	(253.8)	(381.1)
Net unrealized gains (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(14.4)	(4.1)
Other comprehensive loss before reclassifications	(3.4)	11.8
Amounts reclassified from accumulated other comprehensive loss	3.9	(0.2)
Other comprehensive income (loss)	0.5	11.6
Balance at end of period	(13.9)	7.5
Net unrealized gains (losses) on available-for-sale investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(0.1)	0.0
Other comprehensive loss before reclassifications	(1.1)	0.0
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Other comprehensive income (loss)	(1.1)	0.0
Balance at end of period	(1.2)	0.0
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,179.2)	(1,538.8)
Balance at end of period	$ (1,175.7)	$ (1,588.9)