Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - W T M Incentive Plan - Restricted Stock - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Restricted Shares
Beginning of period (in shares)	37,850	43,105
Issued (in shares)	13,075	13,000
Vested (in shares)	(12,725)	(17,717)
Forfeited (in shares)	0	(583)
Expense recognized (in shares)	0	0
End of period (in shares)	38,200	37,805
Unamortized Issue Date Fair Value
Beginning of period	$ 15.9	$ 15.2
Issued	13.6	15.6
Vested	0.0	0.0
Forfeited	0.0	(0.6)
Expense recognized	(3.0)	(4.0)
End of period	$ 26.5	$ 26.2