Condensed Consolidated Statement of Comprehensive (Loss) Income (Parenthetical) - 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]
Pension and other postretirement benefit plan adjustments tax expense	$ 7.4	$ 12.4	$ 14.8	$ 25.0
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	(0.3)	(0.7)	(0.6)	3.3
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.3	$ 0.0	$ 0.6	$ 0.0