Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ (7.9)	$ (4.2)	$ (23.7)	$ (12.8)
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	5.0	0.7	3.5	6.7
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.0	$ (0.1)	$ 0.0	$ (0.4)