Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ (7.9)	$ (4.3)
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	1.1	6.5
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.0	$ (0.1)