Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ (8.0)	$ (4.3)	$ (15.8)	$ (8.6)
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	(2.6)	(0.5)	(1.5)	6.0
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.0	$ (0.2)	$ 0.0	$ (0.3)