Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Restricted Shares
End of period (in shares)	37,595	38,200
Unamortized Issue Date Fair Value
End of period	$ 28.3	$ 26.5
W T M Incentive Plan
Restricted Shares
Beginning of period (in shares)	38,350	37,850
Issued (in shares)	10,895	13,075
Vested (in shares)	(11,650)	(12,725)
Forfeited (in shares)	0	0
Expense recognized (in shares)	0	0
Unamortized Issue Date Fair Value
Beginning of period	$ 15.5	$ 15.9
Issued	16.0	13.6
Vested	0.0	0.0
Forfeited	0.0	0.0
Expense recognized	$ (3.2)	$ (3.0)