Condensed Consolidated Statement of Comprehensive (Loss) Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 227.4	$ 189.5	$ (9.0)	$ 404.2
Other comprehensive income, net of tax:
Pension and other postretirement benefit plan adjustments (net of tax expense of $7.4, $12.4, $14.8 and $25.0)	20.2	23.8	40.4	48.3
Currency translation adjustments	76.5	60.8	60.4	(25.4)
Net change in unrealized gains and losses on cash flow hedges (net of tax (benefit) expense of ($0.3), ($0.7), ($0.6) and $3.3)	(1.0)	(3.6)	(0.5)	8.0
Net change in unrealized gains and losses on available-for-sale investments (net of tax benefit of $0.3, $0.0, $0.6 and $0.0)	(1.5)	(0.1)	(2.6)	(0.1)
Other comprehensive income	94.2	80.9	97.7	30.8
Comprehensive income	$ 321.6	$ 270.4	$ 88.7	$ 435.0