Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan adjustments tax expense	$ 4.3	$ 7.4	$ 8.6	$ 14.8
Net change in unrealized gains and losses on cash flow hedges tax expense (benefit)	0.5	(0.3)	(6.0)	(0.6)
Net change in unrealized gains and losses on available-for-sale investments net of tax benefit	$ 0.2	$ 0.3	$ 0.3	$ 0.6