Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restricted Shares
Beginning of period (in shares)	37,595	38,350	38,350	37,850
Issued (in shares)	0	0	10,895	13,225
Vested (in shares)	0	0	(11,650)	(12,725)
Forfeited (in shares)	0	0	0	0
End of period (in shares)	37,595	38,350	37,595	38,350
Unamortized Issue Date Fair Value
Beginning of period	$ 24.3	$ 23.0	$ 15.5	$ 15.9
Issued	0.0	0.0	16.0	13.8
Vested	0.0	0.0	0.0	0.0
Forfeited	0.0	0.0	0.0	0.0
Expense recognized	(4.1)	(3.8)	(11.3)	(10.5)
End of period	$ 20.2	$ 19.2	$ 20.2	$ 19.2