Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ 4.3	$ 7.4
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	(6.5)	(0.3)
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.1	$ 0.3