Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended				6 Months Ended
	May 03, 2021	Feb. 01, 2021	Nov. 12, 2020	Oct. 15, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Jul. 31, 2015
Class of Stock [Line Items]
Common stock purchased (in shares)					500,000		980,136
Common stock purchased					$ 21.3	$ 21.3	$ 45.0
Average price (in dollars per share)					$ 42.61		$ 45.93
Dividends to common shareholders					$ 20.9	$ 21.0	$ 21.4	$ 21.4	$ 41.9	$ 42.8
Dividend declared (in dollars per share)	$ (0.30)	$ (0.30)	$ (0.30)			$ (0.30)	$ (0.30)	$ (0.30)
Dividends paid on common stock									43.3	43.7
Preferred stock dividend declared (in dollars per share)	$ 1.875	$ 1.875	$ 1.875	$ 1.875	$ 1.88	$ 1.875	$ 1.875	$ 1.83
Payments of Ordinary Dividends, Preferred Stock and Preference Stock									$ 8.1	$ 8.1
Common Stock
Class of Stock [Line Items]
Number of shares authorized to be acquired (shares)											7,500,000
Common stock purchased (in shares)						500,000	980,000
Retained Earnings
Class of Stock [Line Items]
Dividends to common shareholders					$ 20.9	$ 21.0	$ 21.4	$ 21.4