Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - W T M Incentive Plan - Restricted Stock - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Restricted Shares
Beginning of period (in shares)	43,105	43,395	43,395	41,510
Issued (in shares)	0	0	14,055	15,600
Vested (in shares)	0	0	(14,345)	(13,715)
Forfeited (in shares)	0	0	0	0
End of period (in shares)	43,105	43,395	43,105	43,395
Unamortized Issue Date Fair Value
Beginning of period	$ 23.0	$ 21.9	$ 16.7	$ 12.5
Issued	0.0	0.0	15.1	14.5
Forfeited	0.0	0.0	0.0	0.0
Expense recognized	(3.6)	(2.6)	(12.4)	(7.7)
End of period	$ 19.4	$ 19.3	$ 19.4	$ 19.3