Earnings per Share - Reconciliation of Average Common Shares Outstanding (Details) - shares shares in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Basic average common shares outstanding (in shares)	72.4	73.4	72.3	75.8
Impact of restricted shares, restricted share units and performance share units (in shares)	1.0	1.1	1.0	1.3
Impact of convertible notes (in shares)	0.0	0.8	0.0	0.9
Diluted average common shares outstanding (in shares)	73.4	75.3	73.3	78.0