Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 346.0	$ 227.4	$ 426.3	$ (9.0)
Other comprehensive income (loss), net of tax:
Pension and other postretirement benefit plan adjustments (net of tax (expense) of ($4.3), ($7.4), ($8.6) and ($14.8))	14.2	20.2	28.0	40.4
Currency translation adjustments	19.0	76.5	(9.5)	60.4
Net change in unrealized gains and losses on cash flow hedges (net of tax (expense) benefit of ($0.5), $0.3, $6.0 and $0.6)	2.5	(1.0)	(18.3)	(0.5)
Net change in unrealized gains and losses on available-for-sale investments (net of tax (expense) benefit of ($0.2), $0.3, ($0.3) and $0.6)	0.9	(1.5)	1.4	(2.6)
Other comprehensive income	36.6	94.2	1.6	97.7
Comprehensive income	$ 382.6	$ 321.6	$ 427.9	$ 88.7