Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ 4.2	$ 7.2	$ 12.8	$ 22.0
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	0.7	(6.0)	6.7	(5.4)
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ (0.1)	$ (0.1)	$ (0.4)	$ 0.5