Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 2,663.6
Other comprehensive income (loss) before reclassifications	$ 69.0	$ 58.4	48.4	$ (15.3)
Amounts reclassified from accumulated other comprehensive loss	25.2	22.5	49.3	46.1
Other comprehensive income (loss)	94.2	80.9	97.7	30.8
Balance at end of period	1,947.2		1,947.2
Pension and other postretirement benefit plan adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(906.8)	(1,215.3)	(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	40.4	47.6
Other comprehensive income (loss)	20.2	23.8	40.4	48.3
Balance at end of period	(886.6)	(1,191.5)	(886.6)	(1,191.5)
Accumulated currency translation adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(253.8)	(381.1)	(237.7)	(294.9)
Other comprehensive income (loss) before reclassifications	76.5	60.8	60.4	(25.4)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	76.5	60.8	60.4	(25.4)
Balance at end of period	(177.3)	(320.3)	(177.3)	(320.3)
Net unrealized gains (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(13.9)	7.5	(14.4)	(4.1)
Other comprehensive income (loss) before reclassifications	(6.0)	(2.3)	(9.4)	9.5
Amounts reclassified from accumulated other comprehensive loss	5.0	(1.3)	8.9	(1.5)
Other comprehensive income (loss)	(1.0)	(3.6)	(0.5)	8.0
Balance at end of period	(14.9)	3.9	(14.9)	3.9
Net unrealized gains (losses) on available-for-sale investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1.2)	0.0	(0.1)	0.0
Other comprehensive income (loss) before reclassifications	(1.5)	(0.1)	(2.6)	(0.1)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(1.5)	(0.1)	(2.6)	(0.1)
Balance at end of period	(2.7)	(0.1)	(2.7)	(0.1)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,175.7)	(1,588.9)	(1,179.2)	(1,538.8)
Balance at end of period	$ (1,081.5)	$ (1,508.0)	$ (1,081.5)	$ (1,508.0)